ForkJoinPool.java 143.4 KB
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/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
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 * published by the Free Software Foundation.  Oracle designates this
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 * particular file as subject to the "Classpath" exception as provided
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 * by Oracle in the LICENSE file that accompanied this code.
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 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
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 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
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 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
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 * http://creativecommons.org/publicdomain/zero/1.0/
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 */

package java.util.concurrent;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
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import java.util.concurrent.AbstractExecutorService;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.TimeUnit;
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/**
 * An {@link ExecutorService} for running {@link ForkJoinTask}s.
 * A {@code ForkJoinPool} provides the entry point for submissions
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 * from non-{@code ForkJoinTask} clients, as well as management and
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 * monitoring operations.
 *
 * <p>A {@code ForkJoinPool} differs from other kinds of {@link
 * ExecutorService} mainly by virtue of employing
 * <em>work-stealing</em>: all threads in the pool attempt to find and
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 * execute tasks submitted to the pool and/or created by other active
 * tasks (eventually blocking waiting for work if none exist). This
 * enables efficient processing when most tasks spawn other subtasks
 * (as do most {@code ForkJoinTask}s), as well as when many small
 * tasks are submitted to the pool from external clients.  Especially
 * when setting <em>asyncMode</em> to true in constructors, {@code
 * ForkJoinPool}s may also be appropriate for use with event-style
 * tasks that are never joined.
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 *
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 * <p>A static {@link #commonPool()} is available and appropriate for
 * most applications. The common pool is used by any ForkJoinTask that
 * is not explicitly submitted to a specified pool. Using the common
 * pool normally reduces resource usage (its threads are slowly
 * reclaimed during periods of non-use, and reinstated upon subsequent
 * use).
 *
 * <p>For applications that require separate or custom pools, a {@code
 * ForkJoinPool} may be constructed with a given target parallelism
 * level; by default, equal to the number of available processors. The
 * pool attempts to maintain enough active (or available) threads by
 * dynamically adding, suspending, or resuming internal worker
 * threads, even if some tasks are stalled waiting to join
 * others. However, no such adjustments are guaranteed in the face of
 * blocked I/O or other unmanaged synchronization. The nested {@link
 * ManagedBlocker} interface enables extension of the kinds of
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 * synchronization accommodated.
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 *
 * <p>In addition to execution and lifecycle control methods, this
 * class provides status check methods (for example
 * {@link #getStealCount}) that are intended to aid in developing,
 * tuning, and monitoring fork/join applications. Also, method
 * {@link #toString} returns indications of pool state in a
 * convenient form for informal monitoring.
 *
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 * <p>As is the case with other ExecutorServices, there are three
 * main task execution methods summarized in the following table.
 * These are designed to be used primarily by clients not already
 * engaged in fork/join computations in the current pool.  The main
 * forms of these methods accept instances of {@code ForkJoinTask},
 * but overloaded forms also allow mixed execution of plain {@code
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 * Runnable}- or {@code Callable}- based activities as well.  However,
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 * tasks that are already executing in a pool should normally instead
 * use the within-computation forms listed in the table unless using
 * async event-style tasks that are not usually joined, in which case
 * there is little difference among choice of methods.
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 *
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 *  <tr>
 *    <td></td>
 *    <td ALIGN=CENTER> <b>Call from non-fork/join clients</b></td>
 *    <td ALIGN=CENTER> <b>Call from within fork/join computations</b></td>
 *  </tr>
 *  <tr>
 *    <td> <b>Arrange async execution</td>
 *    <td> {@link #execute(ForkJoinTask)}</td>
 *    <td> {@link ForkJoinTask#fork}</td>
 *  </tr>
 *  <tr>
 *    <td> <b>Await and obtain result</td>
 *    <td> {@link #invoke(ForkJoinTask)}</td>
 *    <td> {@link ForkJoinTask#invoke}</td>
 *  </tr>
 *  <tr>
 *    <td> <b>Arrange exec and obtain Future</td>
 *    <td> {@link #submit(ForkJoinTask)}</td>
 *    <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
 *  </tr>
 * </table>
 *
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 * <p>The common pool is by default constructed with default
 * parameters, but these may be controlled by setting three {@link
 * System#getProperty system properties} with prefix {@code
 * java.util.concurrent.ForkJoinPool.common}: {@code parallelism} --
 * an integer greater than zero, {@code threadFactory} -- the class
 * name of a {@link ForkJoinWorkerThreadFactory}, and {@code
 * exceptionHandler} -- the class name of a {@link
 * java.lang.Thread.UncaughtExceptionHandler
 * Thread.UncaughtExceptionHandler}. Upon any error in establishing
 * these settings, default parameters are used.
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 *
 * <p><b>Implementation notes</b>: This implementation restricts the
 * maximum number of running threads to 32767. Attempts to create
 * pools with greater than the maximum number result in
 * {@code IllegalArgumentException}.
 *
 * <p>This implementation rejects submitted tasks (that is, by throwing
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 * {@link RejectedExecutionException}) only when the pool is shut down
 * or internal resources have been exhausted.
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 *
 * @since 1.7
 * @author Doug Lea
 */
public class ForkJoinPool extends AbstractExecutorService {

    /*
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     * Implementation Overview
     *
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     * This class and its nested classes provide the main
     * functionality and control for a set of worker threads:
     * Submissions from non-FJ threads enter into submission queues.
     * Workers take these tasks and typically split them into subtasks
     * that may be stolen by other workers.  Preference rules give
     * first priority to processing tasks from their own queues (LIFO
     * or FIFO, depending on mode), then to randomized FIFO steals of
     * tasks in other queues.
     *
     * WorkQueues
     * ==========
     *
     * Most operations occur within work-stealing queues (in nested
     * class WorkQueue).  These are special forms of Deques that
     * support only three of the four possible end-operations -- push,
     * pop, and poll (aka steal), under the further constraints that
     * push and pop are called only from the owning thread (or, as
     * extended here, under a lock), while poll may be called from
     * other threads.  (If you are unfamiliar with them, you probably
     * want to read Herlihy and Shavit's book "The Art of
     * Multiprocessor programming", chapter 16 describing these in
     * more detail before proceeding.)  The main work-stealing queue
     * design is roughly similar to those in the papers "Dynamic
     * Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005
     * (http://research.sun.com/scalable/pubs/index.html) and
     * "Idempotent work stealing" by Michael, Saraswat, and Vechev,
     * PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
     * The main differences ultimately stem from GC requirements that
     * we null out taken slots as soon as we can, to maintain as small
     * a footprint as possible even in programs generating huge
     * numbers of tasks. To accomplish this, we shift the CAS
     * arbitrating pop vs poll (steal) from being on the indices
     * ("base" and "top") to the slots themselves.  So, both a
     * successful pop and poll mainly entail a CAS of a slot from
     * non-null to null.  Because we rely on CASes of references, we
     * do not need tag bits on base or top.  They are simple ints as
     * used in any circular array-based queue (see for example
     * ArrayDeque).  Updates to the indices must still be ordered in a
     * way that guarantees that top == base means the queue is empty,
     * but otherwise may err on the side of possibly making the queue
     * appear nonempty when a push, pop, or poll have not fully
     * committed. Note that this means that the poll operation,
     * considered individually, is not wait-free. One thief cannot
     * successfully continue until another in-progress one (or, if
     * previously empty, a push) completes.  However, in the
     * aggregate, we ensure at least probabilistic non-blockingness.
     * If an attempted steal fails, a thief always chooses a different
     * random victim target to try next. So, in order for one thief to
     * progress, it suffices for any in-progress poll or new push on
     * any empty queue to complete. (This is why we normally use
     * method pollAt and its variants that try once at the apparent
     * base index, else consider alternative actions, rather than
     * method poll.)
     *
     * This approach also enables support of a user mode in which local
     * task processing is in FIFO, not LIFO order, simply by using
     * poll rather than pop.  This can be useful in message-passing
     * frameworks in which tasks are never joined.  However neither
     * mode considers affinities, loads, cache localities, etc, so
     * rarely provide the best possible performance on a given
     * machine, but portably provide good throughput by averaging over
     * these factors.  (Further, even if we did try to use such
     * information, we do not usually have a basis for exploiting it.
     * For example, some sets of tasks profit from cache affinities,
     * but others are harmed by cache pollution effects.)
     *
     * WorkQueues are also used in a similar way for tasks submitted
     * to the pool. We cannot mix these tasks in the same queues used
     * for work-stealing (this would contaminate lifo/fifo
     * processing). Instead, we randomly associate submission queues
     * with submitting threads, using a form of hashing.  The
     * ThreadLocal Submitter class contains a value initially used as
     * a hash code for choosing existing queues, but may be randomly
     * repositioned upon contention with other submitters.  In
     * essence, submitters act like workers except that they are
     * restricted to executing local tasks that they submitted (or in
     * the case of CountedCompleters, others with the same root task).
     * However, because most shared/external queue operations are more
     * expensive than internal, and because, at steady state, external
     * submitters will compete for CPU with workers, ForkJoinTask.join
     * and related methods disable them from repeatedly helping to
     * process tasks if all workers are active.  Insertion of tasks in
     * shared mode requires a lock (mainly to protect in the case of
     * resizing) but we use only a simple spinlock (using bits in
     * field qlock), because submitters encountering a busy queue move
     * on to try or create other queues -- they block only when
     * creating and registering new queues.
     *
     * Management
     * ==========
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     *
     * The main throughput advantages of work-stealing stem from
     * decentralized control -- workers mostly take tasks from
     * themselves or each other. We cannot negate this in the
     * implementation of other management responsibilities. The main
     * tactic for avoiding bottlenecks is packing nearly all
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     * essentially atomic control state into two volatile variables
     * that are by far most often read (not written) as status and
     * consistency checks.
     *
     * Field "ctl" contains 64 bits holding all the information needed
     * to atomically decide to add, inactivate, enqueue (on an event
     * queue), dequeue, and/or re-activate workers.  To enable this
     * packing, we restrict maximum parallelism to (1<<15)-1 (which is
     * far in excess of normal operating range) to allow ids, counts,
     * and their negations (used for thresholding) to fit into 16bit
     * fields.
     *
     * Field "plock" is a form of sequence lock with a saturating
     * shutdown bit (similarly for per-queue "qlocks"), mainly
     * protecting updates to the workQueues array, as well as to
     * enable shutdown.  When used as a lock, it is normally only very
     * briefly held, so is nearly always available after at most a
     * brief spin, but we use a monitor-based backup strategy to
     * block when needed.
     *
     * Recording WorkQueues.  WorkQueues are recorded in the
     * "workQueues" array that is created upon first use and expanded
     * if necessary.  Updates to the array while recording new workers
     * and unrecording terminated ones are protected from each other
     * by a lock but the array is otherwise concurrently readable, and
     * accessed directly.  To simplify index-based operations, the
     * array size is always a power of two, and all readers must
     * tolerate null slots. Worker queues are at odd indices. Shared
     * (submission) queues are at even indices, up to a maximum of 64
     * slots, to limit growth even if array needs to expand to add
     * more workers. Grouping them together in this way simplifies and
     * speeds up task scanning.
     *
     * All worker thread creation is on-demand, triggered by task
     * submissions, replacement of terminated workers, and/or
     * compensation for blocked workers. However, all other support
     * code is set up to work with other policies.  To ensure that we
     * do not hold on to worker references that would prevent GC, ALL
     * accesses to workQueues are via indices into the workQueues
     * array (which is one source of some of the messy code
     * constructions here). In essence, the workQueues array serves as
     * a weak reference mechanism. Thus for example the wait queue
     * field of ctl stores indices, not references.  Access to the
     * workQueues in associated methods (for example signalWork) must
     * both index-check and null-check the IDs. All such accesses
     * ignore bad IDs by returning out early from what they are doing,
     * since this can only be associated with termination, in which
     * case it is OK to give up.  All uses of the workQueues array
     * also check that it is non-null (even if previously
     * non-null). This allows nulling during termination, which is
     * currently not necessary, but remains an option for
     * resource-revocation-based shutdown schemes. It also helps
     * reduce JIT issuance of uncommon-trap code, which tends to
     * unnecessarily complicate control flow in some methods.
     *
     * Event Queuing. Unlike HPC work-stealing frameworks, we cannot
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     * let workers spin indefinitely scanning for tasks when none can
     * be found immediately, and we cannot start/resume workers unless
     * there appear to be tasks available.  On the other hand, we must
     * quickly prod them into action when new tasks are submitted or
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     * generated. In many usages, ramp-up time to activate workers is
     * the main limiting factor in overall performance (this is
     * compounded at program start-up by JIT compilation and
     * allocation). So we try to streamline this as much as possible.
     * We park/unpark workers after placing in an event wait queue
     * when they cannot find work. This "queue" is actually a simple
     * Treiber stack, headed by the "id" field of ctl, plus a 15bit
     * counter value (that reflects the number of times a worker has
     * been inactivated) to avoid ABA effects (we need only as many
     * version numbers as worker threads). Successors are held in
     * field WorkQueue.nextWait.  Queuing deals with several intrinsic
     * races, mainly that a task-producing thread can miss seeing (and
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     * signalling) another thread that gave up looking for work but
     * has not yet entered the wait queue. We solve this by requiring
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     * a full sweep of all workers (via repeated calls to method
     * scan()) both before and after a newly waiting worker is added
     * to the wait queue. During a rescan, the worker might release
     * some other queued worker rather than itself, which has the same
     * net effect. Because enqueued workers may actually be rescanning
     * rather than waiting, we set and clear the "parker" field of
     * WorkQueues to reduce unnecessary calls to unpark.  (This
     * requires a secondary recheck to avoid missed signals.)  Note
     * the unusual conventions about Thread.interrupts surrounding
     * parking and other blocking: Because interrupts are used solely
     * to alert threads to check termination, which is checked anyway
     * upon blocking, we clear status (using Thread.interrupted)
     * before any call to park, so that park does not immediately
     * return due to status being set via some other unrelated call to
     * interrupt in user code.
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     *
     * Signalling.  We create or wake up workers only when there
     * appears to be at least one task they might be able to find and
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     * execute. However, many other threads may notice the same task
     * and each signal to wake up a thread that might take it. So in
     * general, pools will be over-signalled.  When a submission is
     * added or another worker adds a task to a queue that has fewer
     * than two tasks, they signal waiting workers (or trigger
     * creation of new ones if fewer than the given parallelism level
     * -- signalWork), and may leave a hint to the unparked worker to
     * help signal others upon wakeup).  These primary signals are
     * buttressed by others (see method helpSignal) whenever other
     * threads scan for work or do not have a task to process.  On
     * most platforms, signalling (unpark) overhead time is noticeably
     * long, and the time between signalling a thread and it actually
     * making progress can be very noticeably long, so it is worth
     * offloading these delays from critical paths as much as
     * possible.
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     *
     * Trimming workers. To release resources after periods of lack of
     * use, a worker starting to wait when the pool is quiescent will
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     * time out and terminate if the pool has remained quiescent for a
     * given period -- a short period if there are more threads than
     * parallelism, longer as the number of threads decreases. This
     * will slowly propagate, eventually terminating all workers after
     * periods of non-use.
     *
     * Shutdown and Termination. A call to shutdownNow atomically sets
     * a plock bit and then (non-atomically) sets each worker's
     * qlock status, cancels all unprocessed tasks, and wakes up
     * all waiting workers.  Detecting whether termination should
     * commence after a non-abrupt shutdown() call requires more work
     * and bookkeeping. We need consensus about quiescence (i.e., that
     * there is no more work). The active count provides a primary
     * indication but non-abrupt shutdown still requires a rechecking
     * scan for any workers that are inactive but not queued.
     *
     * Joining Tasks
     * =============
     *
     * Any of several actions may be taken when one worker is waiting
     * to join a task stolen (or always held) by another.  Because we
     * are multiplexing many tasks on to a pool of workers, we can't
     * just let them block (as in Thread.join).  We also cannot just
     * reassign the joiner's run-time stack with another and replace
     * it later, which would be a form of "continuation", that even if
     * possible is not necessarily a good idea since we sometimes need
     * both an unblocked task and its continuation to progress.
     * Instead we combine two tactics:
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     *
     *   Helping: Arranging for the joiner to execute some task that it
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     *      would be running if the steal had not occurred.
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     *
     *   Compensating: Unless there are already enough live threads,
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     *      method tryCompensate() may create or re-activate a spare
     *      thread to compensate for blocked joiners until they unblock.
     *
     * A third form (implemented in tryRemoveAndExec) amounts to
     * helping a hypothetical compensator: If we can readily tell that
     * a possible action of a compensator is to steal and execute the
     * task being joined, the joining thread can do so directly,
     * without the need for a compensation thread (although at the
     * expense of larger run-time stacks, but the tradeoff is
     * typically worthwhile).
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     *
     * The ManagedBlocker extension API can't use helping so relies
     * only on compensation in method awaitBlocker.
     *
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     * The algorithm in tryHelpStealer entails a form of "linear"
     * helping: Each worker records (in field currentSteal) the most
     * recent task it stole from some other worker. Plus, it records
     * (in field currentJoin) the task it is currently actively
     * joining. Method tryHelpStealer uses these markers to try to
     * find a worker to help (i.e., steal back a task from and execute
     * it) that could hasten completion of the actively joined task.
     * In essence, the joiner executes a task that would be on its own
     * local deque had the to-be-joined task not been stolen. This may
     * be seen as a conservative variant of the approach in Wagner &
     * Calder "Leapfrogging: a portable technique for implementing
     * efficient futures" SIGPLAN Notices, 1993
     * (http://portal.acm.org/citation.cfm?id=155354). It differs in
     * that: (1) We only maintain dependency links across workers upon
     * steals, rather than use per-task bookkeeping.  This sometimes
     * requires a linear scan of workQueues array to locate stealers,
     * but often doesn't because stealers leave hints (that may become
     * stale/wrong) of where to locate them.  It is only a hint
     * because a worker might have had multiple steals and the hint
     * records only one of them (usually the most current).  Hinting
     * isolates cost to when it is needed, rather than adding to
     * per-task overhead.  (2) It is "shallow", ignoring nesting and
     * potentially cyclic mutual steals.  (3) It is intentionally
     * racy: field currentJoin is updated only while actively joining,
     * which means that we miss links in the chain during long-lived
     * tasks, GC stalls etc (which is OK since blocking in such cases
     * is usually a good idea).  (4) We bound the number of attempts
     * to find work (see MAX_HELP) and fall back to suspending the
     * worker and if necessary replacing it with another.
     *
     * Helping actions for CountedCompleters are much simpler: Method
     * helpComplete can take and execute any task with the same root
     * as the task being waited on. However, this still entails some
     * traversal of completer chains, so is less efficient than using
     * CountedCompleters without explicit joins.
     *
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     * It is impossible to keep exactly the target parallelism number
     * of threads running at any given time.  Determining the
     * existence of conservatively safe helping targets, the
     * availability of already-created spares, and the apparent need
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     * to create new spares are all racy, so we rely on multiple
     * retries of each.  Compensation in the apparent absence of
     * helping opportunities is challenging to control on JVMs, where
     * GC and other activities can stall progress of tasks that in
     * turn stall out many other dependent tasks, without us being
     * able to determine whether they will ever require compensation.
     * Even though work-stealing otherwise encounters little
     * degradation in the presence of more threads than cores,
     * aggressively adding new threads in such cases entails risk of
     * unwanted positive feedback control loops in which more threads
     * cause more dependent stalls (as well as delayed progress of
     * unblocked threads to the point that we know they are available)
     * leading to more situations requiring more threads, and so
     * on. This aspect of control can be seen as an (analytically
     * intractable) game with an opponent that may choose the worst
     * (for us) active thread to stall at any time.  We take several
     * precautions to bound losses (and thus bound gains), mainly in
     * methods tryCompensate and awaitJoin.
     *
     * Common Pool
     * ===========
     *
     * The static commonPool always exists after static
     * initialization.  Since it (or any other created pool) need
     * never be used, we minimize initial construction overhead and
     * footprint to the setup of about a dozen fields, with no nested
     * allocation. Most bootstrapping occurs within method
     * fullExternalPush during the first submission to the pool.
     *
     * When external threads submit to the common pool, they can
     * perform some subtask processing (see externalHelpJoin and
     * related methods).  We do not need to record whether these
     * submissions are to the common pool -- if not, externalHelpJoin
     * returns quickly (at the most helping to signal some common pool
     * workers). These submitters would otherwise be blocked waiting
     * for completion, so the extra effort (with liberally sprinkled
     * task status checks) in inapplicable cases amounts to an odd
     * form of limited spin-wait before blocking in ForkJoinTask.join.
     *
     * Style notes
     * ===========
     *
     * There is a lot of representation-level coupling among classes
     * ForkJoinPool, ForkJoinWorkerThread, and ForkJoinTask.  The
     * fields of WorkQueue maintain data structures managed by
     * ForkJoinPool, so are directly accessed.  There is little point
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     * trying to reduce this, since any associated future changes in
     * representations will need to be accompanied by algorithmic
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     * changes anyway. Several methods intrinsically sprawl because
     * they must accumulate sets of consistent reads of volatiles held
     * in local variables.  Methods signalWork() and scan() are the
     * main bottlenecks, so are especially heavily
     * micro-optimized/mangled.  There are lots of inline assignments
     * (of form "while ((local = field) != 0)") which are usually the
     * simplest way to ensure the required read orderings (which are
     * sometimes critical). This leads to a "C"-like style of listing
     * declarations of these locals at the heads of methods or blocks.
     * There are several occurrences of the unusual "do {} while
     * (!cas...)"  which is the simplest way to force an update of a
     * CAS'ed variable. There are also other coding oddities (including
     * several unnecessary-looking hoisted null checks) that help
     * some methods perform reasonably even when interpreted (not
     * compiled).
     *
     * The order of declarations in this file is:
     * (1) Static utility functions
     * (2) Nested (static) classes
     * (3) Static fields
     * (4) Fields, along with constants used when unpacking some of them
     * (5) Internal control methods
     * (6) Callbacks and other support for ForkJoinTask methods
     * (7) Exported methods
     * (8) Static block initializing statics in minimally dependent order
     */

    // Static utilities

    /**
     * If there is a security manager, makes sure caller has
     * permission to modify threads.
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     */
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    private static void checkPermission() {
        SecurityManager security = System.getSecurityManager();
        if (security != null)
            security.checkPermission(modifyThreadPermission);
    }

    // Nested classes
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    /**
     * Factory for creating new {@link ForkJoinWorkerThread}s.
     * A {@code ForkJoinWorkerThreadFactory} must be defined and used
     * for {@code ForkJoinWorkerThread} subclasses that extend base
     * functionality or initialize threads with different contexts.
     */
    public static interface ForkJoinWorkerThreadFactory {
        /**
         * Returns a new worker thread operating in the given pool.
         *
         * @param pool the pool this thread works in
         * @throws NullPointerException if the pool is null
         */
        public ForkJoinWorkerThread newThread(ForkJoinPool pool);
    }

    /**
     * Default ForkJoinWorkerThreadFactory implementation; creates a
     * new ForkJoinWorkerThread.
     */
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    static final class DefaultForkJoinWorkerThreadFactory
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        implements ForkJoinWorkerThreadFactory {
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        public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
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            return new ForkJoinWorkerThread(pool);
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        }
    }

    /**
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     * Per-thread records for threads that submit to pools. Currently
     * holds only pseudo-random seed / index that is used to choose
     * submission queues in method externalPush. In the future, this may
     * also incorporate a means to implement different task rejection
     * and resubmission policies.
     *
     * Seeds for submitters and workers/workQueues work in basically
     * the same way but are initialized and updated using slightly
     * different mechanics. Both are initialized using the same
     * approach as in class ThreadLocal, where successive values are
     * unlikely to collide with previous values. Seeds are then
     * randomly modified upon collisions using xorshifts, which
     * requires a non-zero seed.
     */
    static final class Submitter {
        int seed;
        Submitter(int s) { seed = s; }
    }

    /**
     * Class for artificial tasks that are used to replace the target
     * of local joins if they are removed from an interior queue slot
     * in WorkQueue.tryRemoveAndExec. We don't need the proxy to
     * actually do anything beyond having a unique identity.
     */
    static final class EmptyTask extends ForkJoinTask<Void> {
        private static final long serialVersionUID = -7721805057305804111L;
        EmptyTask() { status = ForkJoinTask.NORMAL; } // force done
        public final Void getRawResult() { return null; }
        public final void setRawResult(Void x) {}
        public final boolean exec() { return true; }
    }

    /**
     * Queues supporting work-stealing as well as external task
     * submission. See above for main rationale and algorithms.
     * Implementation relies heavily on "Unsafe" intrinsics
     * and selective use of "volatile":
     *
     * Field "base" is the index (mod array.length) of the least valid
     * queue slot, which is always the next position to steal (poll)
     * from if nonempty. Reads and writes require volatile orderings
     * but not CAS, because updates are only performed after slot
     * CASes.
     *
     * Field "top" is the index (mod array.length) of the next queue
     * slot to push to or pop from. It is written only by owner thread
     * for push, or under lock for external/shared push, and accessed
     * by other threads only after reading (volatile) base.  Both top
     * and base are allowed to wrap around on overflow, but (top -
     * base) (or more commonly -(base - top) to force volatile read of
     * base before top) still estimates size. The lock ("qlock") is
     * forced to -1 on termination, causing all further lock attempts
     * to fail. (Note: we don't need CAS for termination state because
     * upon pool shutdown, all shared-queues will stop being used
     * anyway.)  Nearly all lock bodies are set up so that exceptions
     * within lock bodies are "impossible" (modulo JVM errors that
     * would cause failure anyway.)
     *
     * The array slots are read and written using the emulation of
     * volatiles/atomics provided by Unsafe. Insertions must in
     * general use putOrderedObject as a form of releasing store to
     * ensure that all writes to the task object are ordered before
     * its publication in the queue.  All removals entail a CAS to
     * null.  The array is always a power of two. To ensure safety of
     * Unsafe array operations, all accesses perform explicit null
     * checks and implicit bounds checks via power-of-two masking.
     *
     * In addition to basic queuing support, this class contains
     * fields described elsewhere to control execution. It turns out
     * to work better memory-layout-wise to include them in this class
     * rather than a separate class.
     *
     * Performance on most platforms is very sensitive to placement of
     * instances of both WorkQueues and their arrays -- we absolutely
     * do not want multiple WorkQueue instances or multiple queue
     * arrays sharing cache lines. (It would be best for queue objects
     * and their arrays to share, but there is nothing available to
     * help arrange that).  Unfortunately, because they are recorded
     * in a common array, WorkQueue instances are often moved to be
     * adjacent by garbage collectors. To reduce impact, we use field
     * padding that works OK on common platforms; this effectively
     * trades off slightly slower average field access for the sake of
     * avoiding really bad worst-case access. (Until better JVM
     * support is in place, this padding is dependent on transient
     * properties of JVM field layout rules.) We also take care in
     * allocating, sizing and resizing the array. Non-shared queue
     * arrays are initialized by workers before use. Others are
     * allocated on first use.
     */
    static final class WorkQueue {
        /**
         * Capacity of work-stealing queue array upon initialization.
         * Must be a power of two; at least 4, but should be larger to
         * reduce or eliminate cacheline sharing among queues.
         * Currently, it is much larger, as a partial workaround for
         * the fact that JVMs often place arrays in locations that
         * share GC bookkeeping (especially cardmarks) such that
         * per-write accesses encounter serious memory contention.
         */
        static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
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        /**
         * Maximum size for queue arrays. Must be a power of two less
         * than or equal to 1 << (31 - width of array entry) to ensure
         * lack of wraparound of index calculations, but defined to a
         * value a bit less than this to help users trap runaway
         * programs before saturating systems.
         */
        static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M

        // Heuristic padding to ameliorate unfortunate memory placements
        volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06;

        int seed;                  // for random scanning; initialize nonzero
        volatile int eventCount;   // encoded inactivation count; < 0 if inactive
        int nextWait;              // encoded record of next event waiter
        int hint;                  // steal or signal hint (index)
        int poolIndex;             // index of this queue in pool (or 0)
        final int mode;            // 0: lifo, > 0: fifo, < 0: shared
        int nsteals;               // number of steals
        volatile int qlock;        // 1: locked, -1: terminate; else 0
        volatile int base;         // index of next slot for poll
        int top;                   // index of next slot for push
        ForkJoinTask<?>[] array;   // the elements (initially unallocated)
        final ForkJoinPool pool;   // the containing pool (may be null)
        final ForkJoinWorkerThread owner; // owning thread or null if shared
        volatile Thread parker;    // == owner during call to park; else null
        volatile ForkJoinTask<?> currentJoin;  // task being joined in awaitJoin
        ForkJoinTask<?> currentSteal; // current non-local task being executed

        volatile Object pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17;
        volatile Object pad18, pad19, pad1a, pad1b, pad1c, pad1d;

        WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner, int mode,
                  int seed) {
            this.pool = pool;
            this.owner = owner;
            this.mode = mode;
            this.seed = seed;
            // Place indices in the center of array (that is not yet allocated)
            base = top = INITIAL_QUEUE_CAPACITY >>> 1;
        }
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        /**
         * Returns the approximate number of tasks in the queue.
         */
        final int queueSize() {
            int n = base - top;       // non-owner callers must read base first
            return (n >= 0) ? 0 : -n; // ignore transient negative
        }

       /**
         * Provides a more accurate estimate of whether this queue has
         * any tasks than does queueSize, by checking whether a
         * near-empty queue has at least one unclaimed task.
         */
        final boolean isEmpty() {
            ForkJoinTask<?>[] a; int m, s;
            int n = base - (s = top);
            return (n >= 0 ||
                    (n == -1 &&
                     ((a = array) == null ||
                      (m = a.length - 1) < 0 ||
                      U.getObject
                      (a, (long)((m & (s - 1)) << ASHIFT) + ABASE) == null)));
        }

        /**
         * Pushes a task. Call only by owner in unshared queues.  (The
         * shared-queue version is embedded in method externalPush.)
         *
         * @param task the task. Caller must ensure non-null.
         * @throw RejectedExecutionException if array cannot be resized
         */
        final void push(ForkJoinTask<?> task) {
            ForkJoinTask<?>[] a; ForkJoinPool p;
            int s = top, m, n;
            if ((a = array) != null) {    // ignore if queue removed
                int j = (((m = a.length - 1) & s) << ASHIFT) + ABASE;
                U.putOrderedObject(a, j, task);
                if ((n = (top = s + 1) - base) <= 2) {
                    if ((p = pool) != null)
                        p.signalWork(this);
                }
                else if (n >= m)
                    growArray();
            }
        }

       /**
         * Initializes or doubles the capacity of array. Call either
         * by owner or with lock held -- it is OK for base, but not
         * top, to move while resizings are in progress.
         */
        final ForkJoinTask<?>[] growArray() {
            ForkJoinTask<?>[] oldA = array;
            int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;
            if (size > MAXIMUM_QUEUE_CAPACITY)
                throw new RejectedExecutionException("Queue capacity exceeded");
            int oldMask, t, b;
            ForkJoinTask<?>[] a = array = new ForkJoinTask<?>[size];
            if (oldA != null && (oldMask = oldA.length - 1) >= 0 &&
                (t = top) - (b = base) > 0) {
                int mask = size - 1;
                do {
                    ForkJoinTask<?> x;
                    int oldj = ((b & oldMask) << ASHIFT) + ABASE;
                    int j    = ((b &    mask) << ASHIFT) + ABASE;
                    x = (ForkJoinTask<?>)U.getObjectVolatile(oldA, oldj);
                    if (x != null &&
                        U.compareAndSwapObject(oldA, oldj, x, null))
                        U.putObjectVolatile(a, j, x);
                } while (++b != t);
            }
            return a;
        }

        /**
         * Takes next task, if one exists, in LIFO order.  Call only
         * by owner in unshared queues.
         */
        final ForkJoinTask<?> pop() {
            ForkJoinTask<?>[] a; ForkJoinTask<?> t; int m;
            if ((a = array) != null && (m = a.length - 1) >= 0) {
                for (int s; (s = top - 1) - base >= 0;) {
                    long j = ((m & s) << ASHIFT) + ABASE;
                    if ((t = (ForkJoinTask<?>)U.getObject(a, j)) == null)
                        break;
                    if (U.compareAndSwapObject(a, j, t, null)) {
                        top = s;
                        return t;
                    }
                }
            }
            return null;
        }

        /**
         * Takes a task in FIFO order if b is base of queue and a task
         * can be claimed without contention. Specialized versions
         * appear in ForkJoinPool methods scan and tryHelpStealer.
         */
        final ForkJoinTask<?> pollAt(int b) {
            ForkJoinTask<?> t; ForkJoinTask<?>[] a;
            if ((a = array) != null) {
                int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
                if ((t = (ForkJoinTask<?>)U.getObjectVolatile(a, j)) != null &&
                    base == b &&
                    U.compareAndSwapObject(a, j, t, null)) {
                    base = b + 1;
                    return t;
                }
            }
            return null;
        }

        /**
         * Takes next task, if one exists, in FIFO order.
         */
        final ForkJoinTask<?> poll() {
            ForkJoinTask<?>[] a; int b; ForkJoinTask<?> t;
            while ((b = base) - top < 0 && (a = array) != null) {
                int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
                t = (ForkJoinTask<?>)U.getObjectVolatile(a, j);
                if (t != null) {
                    if (base == b &&
                        U.compareAndSwapObject(a, j, t, null)) {
                        base = b + 1;
                        return t;
                    }
                }
                else if (base == b) {
                    if (b + 1 == top)
                        break;
                    Thread.yield(); // wait for lagging update (very rare)
                }
            }
            return null;
        }

        /**
         * Takes next task, if one exists, in order specified by mode.
         */
        final ForkJoinTask<?> nextLocalTask() {
            return mode == 0 ? pop() : poll();
        }

        /**
         * Returns next task, if one exists, in order specified by mode.
         */
        final ForkJoinTask<?> peek() {
            ForkJoinTask<?>[] a = array; int m;
            if (a == null || (m = a.length - 1) < 0)
                return null;
            int i = mode == 0 ? top - 1 : base;
            int j = ((i & m) << ASHIFT) + ABASE;
            return (ForkJoinTask<?>)U.getObjectVolatile(a, j);
        }

        /**
         * Pops the given task only if it is at the current top.
         * (A shared version is available only via FJP.tryExternalUnpush)
         */
        final boolean tryUnpush(ForkJoinTask<?> t) {
            ForkJoinTask<?>[] a; int s;
            if ((a = array) != null && (s = top) != base &&
                U.compareAndSwapObject
                (a, (((a.length - 1) & --s) << ASHIFT) + ABASE, t, null)) {
                top = s;
                return true;
            }
            return false;
        }

        /**
         * Removes and cancels all known tasks, ignoring any exceptions.
         */
        final void cancelAll() {
            ForkJoinTask.cancelIgnoringExceptions(currentJoin);
            ForkJoinTask.cancelIgnoringExceptions(currentSteal);
            for (ForkJoinTask<?> t; (t = poll()) != null; )
                ForkJoinTask.cancelIgnoringExceptions(t);
        }

        /**
         * Computes next value for random probes.  Scans don't require
         * a very high quality generator, but also not a crummy one.
         * Marsaglia xor-shift is cheap and works well enough.  Note:
         * This is manually inlined in its usages in ForkJoinPool to
         * avoid writes inside busy scan loops.
         */
        final int nextSeed() {
            int r = seed;
            r ^= r << 13;
            r ^= r >>> 17;
            return seed = r ^= r << 5;
        }

        // Specialized execution methods

        /**
         * Pops and runs tasks until empty.
         */
        private void popAndExecAll() {
            // A bit faster than repeated pop calls
            ForkJoinTask<?>[] a; int m, s; long j; ForkJoinTask<?> t;
            while ((a = array) != null && (m = a.length - 1) >= 0 &&
                   (s = top - 1) - base >= 0 &&
                   (t = ((ForkJoinTask<?>)
                         U.getObject(a, j = ((m & s) << ASHIFT) + ABASE)))
                   != null) {
                if (U.compareAndSwapObject(a, j, t, null)) {
                    top = s;
                    t.doExec();
                }
            }
        }

        /**
         * Polls and runs tasks until empty.
         */
        private void pollAndExecAll() {
            for (ForkJoinTask<?> t; (t = poll()) != null;)
                t.doExec();
        }

        /**
         * If present, removes from queue and executes the given task,
         * or any other cancelled task. Returns (true) on any CAS
         * or consistency check failure so caller can retry.
         *
         * @return false if no progress can be made, else true;
         */
        final boolean tryRemoveAndExec(ForkJoinTask<?> task) {
            boolean stat = true, removed = false, empty = true;
            ForkJoinTask<?>[] a; int m, s, b, n;
            if ((a = array) != null && (m = a.length - 1) >= 0 &&
                (n = (s = top) - (b = base)) > 0) {
                for (ForkJoinTask<?> t;;) {           // traverse from s to b
                    int j = ((--s & m) << ASHIFT) + ABASE;
                    t = (ForkJoinTask<?>)U.getObjectVolatile(a, j);
                    if (t == null)                    // inconsistent length
                        break;
                    else if (t == task) {
                        if (s + 1 == top) {           // pop
                            if (!U.compareAndSwapObject(a, j, task, null))
                                break;
                            top = s;
                            removed = true;
                        }
                        else if (base == b)           // replace with proxy
                            removed = U.compareAndSwapObject(a, j, task,
                                                             new EmptyTask());
                        break;
                    }
                    else if (t.status >= 0)
                        empty = false;
                    else if (s + 1 == top) {          // pop and throw away
                        if (U.compareAndSwapObject(a, j, t, null))
                            top = s;
                        break;
                    }
                    if (--n == 0) {
                        if (!empty && base == b)
                            stat = false;
                        break;
                    }
                }
            }
            if (removed)
                task.doExec();
            return stat;
        }

        /**
         * Polls for and executes the given task or any other task in
         * its CountedCompleter computation
         */
        final boolean pollAndExecCC(ForkJoinTask<?> root) {
            ForkJoinTask<?>[] a; int b; Object o;
            outer: while ((b = base) - top < 0 && (a = array) != null) {
                long j = (((a.length - 1) & b) << ASHIFT) + ABASE;
                if ((o = U.getObject(a, j)) == null ||
                    !(o instanceof CountedCompleter))
                    break;
                for (CountedCompleter<?> t = (CountedCompleter<?>)o, r = t;;) {
                    if (r == root) {
                        if (base == b &&
                            U.compareAndSwapObject(a, j, t, null)) {
                            base = b + 1;
                            t.doExec();
                            return true;
                        }
                        else
                            break; // restart
                    }
                    if ((r = r.completer) == null)
                        break outer; // not part of root computation
                }
            }
            return false;
        }

        /**
         * Executes a top-level task and any local tasks remaining
         * after execution.
         */
        final void runTask(ForkJoinTask<?> t) {
            if (t != null) {
                (currentSteal = t).doExec();
                currentSteal = null;
                ++nsteals;
                if (base - top < 0) {       // process remaining local tasks
                    if (mode == 0)
                        popAndExecAll();
                    else
                        pollAndExecAll();
                }
            }
        }

        /**
         * Executes a non-top-level (stolen) task.
         */
        final void runSubtask(ForkJoinTask<?> t) {
            if (t != null) {
                ForkJoinTask<?> ps = currentSteal;
                (currentSteal = t).doExec();
                currentSteal = ps;
            }
        }

        /**
         * Returns true if owned and not known to be blocked.
         */
        final boolean isApparentlyUnblocked() {
            Thread wt; Thread.State s;
            return (eventCount >= 0 &&
                    (wt = owner) != null &&
                    (s = wt.getState()) != Thread.State.BLOCKED &&
                    s != Thread.State.WAITING &&
                    s != Thread.State.TIMED_WAITING);
        }

        // Unsafe mechanics
        private static final sun.misc.Unsafe U;
        private static final long QLOCK;
        private static final int ABASE;
        private static final int ASHIFT;
        static {
            int s;
            try {
                U = sun.misc.Unsafe.getUnsafe();
                Class<?> k = WorkQueue.class;
                Class<?> ak = ForkJoinTask[].class;
                QLOCK = U.objectFieldOffset
                    (k.getDeclaredField("qlock"));
                ABASE = U.arrayBaseOffset(ak);
                s = U.arrayIndexScale(ak);
            } catch (Exception e) {
                throw new Error(e);
            }
            if ((s & (s-1)) != 0)
                throw new Error("data type scale not a power of two");
            ASHIFT = 31 - Integer.numberOfLeadingZeros(s);
        }
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    }

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    // static fields (initialized in static initializer below)

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    /**
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     * Creates a new ForkJoinWorkerThread. This factory is used unless
     * overridden in ForkJoinPool constructors.
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     */
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    public static final ForkJoinWorkerThreadFactory
        defaultForkJoinWorkerThreadFactory;
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    /**
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     * Per-thread submission bookkeeping. Shared across all pools
     * to reduce ThreadLocal pollution and because random motion
     * to avoid contention in one pool is likely to hold for others.
     * Lazily initialized on first submission (but null-checked
     * in other contexts to avoid unnecessary initialization).
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     */
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    static final ThreadLocal<Submitter> submitters;
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    /**
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     * Permission required for callers of methods that may start or
     * kill threads.
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     */
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    private static final RuntimePermission modifyThreadPermission;
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    /**
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     * Common (static) pool. Non-null for public use unless a static
     * construction exception, but internal usages null-check on use
     * to paranoically avoid potential initialization circularities
     * as well as to simplify generated code.
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     */
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    static final ForkJoinPool commonPool;
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    /**
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     * Common pool parallelism. Must equal commonPool.parallelism.
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     */
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    static final int commonPoolParallelism;
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    /**
1114
     * Sequence number for creating workerNamePrefix.
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     */
1116
    private static int poolNumberSequence;
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    /**
1119 1120
     * Return the next sequence number. We don't expect this to
     * ever contend so use simple builtin sync.
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     */
1122 1123 1124 1125 1126
    private static final synchronized int nextPoolId() {
        return ++poolNumberSequence;
    }

    // static constants
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    /**
1129 1130 1131 1132 1133 1134
     * Initial timeout value (in nanoseconds) for the thread
     * triggering quiescence to park waiting for new work. On timeout,
     * the thread will instead try to shrink the number of
     * workers. The value should be large enough to avoid overly
     * aggressive shrinkage during most transient stalls (long GCs
     * etc).
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     */
1136
    private static final long IDLE_TIMEOUT      = 2000L * 1000L * 1000L; // 2sec
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    /**
1139
     * Timeout value when there are more threads than parallelism level
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     */
1141
    private static final long FAST_IDLE_TIMEOUT =  200L * 1000L * 1000L;
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    /**
1144
     * Tolerance for idle timeouts, to cope with timer undershoots
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     */
1146
    private static final long TIMEOUT_SLOP = 2000000L;
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    /**
1149 1150 1151 1152 1153 1154
     * The maximum stolen->joining link depth allowed in method
     * tryHelpStealer.  Must be a power of two.  Depths for legitimate
     * chains are unbounded, but we use a fixed constant to avoid
     * (otherwise unchecked) cycles and to bound staleness of
     * traversal parameters at the expense of sometimes blocking when
     * we could be helping.
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     */
1156
    private static final int MAX_HELP = 64;
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    /**
1159 1160
     * Increment for seed generators. See class ThreadLocal for
     * explanation.
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     */
1162
    private static final int SEED_INCREMENT = 0x61c88647;
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    /**
1165 1166 1167
     * Bits and masks for control variables
     *
     * Field ctl is a long packed with:
1168
     * AC: Number of active running workers minus target parallelism (16 bits)
1169
     * TC: Number of total workers minus target parallelism (16 bits)
1170 1171
     * ST: true if pool is terminating (1 bit)
     * EC: the wait count of top waiting thread (15 bits)
1172
     * ID: poolIndex of top of Treiber stack of waiters (16 bits)
1173 1174 1175 1176 1177 1178 1179 1180
     *
     * When convenient, we can extract the upper 32 bits of counts and
     * the lower 32 bits of queue state, u = (int)(ctl >>> 32) and e =
     * (int)ctl.  The ec field is never accessed alone, but always
     * together with id and st. The offsets of counts by the target
     * parallelism and the positionings of fields makes it possible to
     * perform the most common checks via sign tests of fields: When
     * ac is negative, there are not enough active workers, when tc is
1181
     * negative, there are not enough total workers, and when e is
1182 1183 1184
     * negative, the pool is terminating.  To deal with these possibly
     * negative fields, we use casts in and out of "short" and/or
     * signed shifts to maintain signedness.
1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199
     *
     * When a thread is queued (inactivated), its eventCount field is
     * set negative, which is the only way to tell if a worker is
     * prevented from executing tasks, even though it must continue to
     * scan for them to avoid queuing races. Note however that
     * eventCount updates lag releases so usage requires care.
     *
     * Field plock is an int packed with:
     * SHUTDOWN: true if shutdown is enabled (1 bit)
     * SEQ:  a sequence lock, with PL_LOCK bit set if locked (30 bits)
     * SIGNAL: set when threads may be waiting on the lock (1 bit)
     *
     * The sequence number enables simple consistency checks:
     * Staleness of read-only operations on the workQueues array can
     * be checked by comparing plock before vs after the reads.
1200 1201 1202 1203 1204 1205 1206 1207 1208
     */

    // bit positions/shifts for fields
    private static final int  AC_SHIFT   = 48;
    private static final int  TC_SHIFT   = 32;
    private static final int  ST_SHIFT   = 31;
    private static final int  EC_SHIFT   = 16;

    // bounds
1209 1210 1211 1212
    private static final int  SMASK      = 0xffff;  // short bits
    private static final int  MAX_CAP    = 0x7fff;  // max #workers - 1
    private static final int  EVENMASK   = 0xfffe;  // even short bits
    private static final int  SQMASK     = 0x007e;  // max 64 (even) slots
1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233
    private static final int  SHORT_SIGN = 1 << 15;
    private static final int  INT_SIGN   = 1 << 31;

    // masks
    private static final long STOP_BIT   = 0x0001L << ST_SHIFT;
    private static final long AC_MASK    = ((long)SMASK) << AC_SHIFT;
    private static final long TC_MASK    = ((long)SMASK) << TC_SHIFT;

    // units for incrementing and decrementing
    private static final long TC_UNIT    = 1L << TC_SHIFT;
    private static final long AC_UNIT    = 1L << AC_SHIFT;

    // masks and units for dealing with u = (int)(ctl >>> 32)
    private static final int  UAC_SHIFT  = AC_SHIFT - 32;
    private static final int  UTC_SHIFT  = TC_SHIFT - 32;
    private static final int  UAC_MASK   = SMASK << UAC_SHIFT;
    private static final int  UTC_MASK   = SMASK << UTC_SHIFT;
    private static final int  UAC_UNIT   = 1 << UAC_SHIFT;
    private static final int  UTC_UNIT   = 1 << UTC_SHIFT;

    // masks and units for dealing with e = (int)ctl
1234 1235
    private static final int E_MASK      = 0x7fffffff; // no STOP_BIT
    private static final int E_SEQ       = 1 << EC_SHIFT;
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1237 1238 1239 1240 1241
    // plock bits
    private static final int SHUTDOWN    = 1 << 31;
    private static final int PL_LOCK     = 2;
    private static final int PL_SIGNAL   = 1;
    private static final int PL_SPINS    = 1 << 8;
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1243 1244 1245 1246
    // access mode for WorkQueue
    static final int LIFO_QUEUE          =  0;
    static final int FIFO_QUEUE          =  1;
    static final int SHARED_QUEUE        = -1;
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1248 1249 1250
    // bounds for #steps in scan loop -- must be power 2 minus 1
    private static final int MIN_SCAN    = 0x1ff;   // cover estimation slop
    private static final int MAX_SCAN    = 0x1ffff; // 4 * max workers
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1252
    // Instance fields
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1254 1255 1256 1257 1258
    /*
     * Field layout of this class tends to matter more than one would
     * like. Runtime layout order is only loosely related to
     * declaration order and may differ across JVMs, but the following
     * empirically works OK on current JVMs.
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     */
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1261 1262
    // Heuristic padding to ameliorate unfortunate memory placements
    volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06;
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1264 1265 1266 1267 1268 1269 1270 1271 1272
    volatile long stealCount;                  // collects worker counts
    volatile long ctl;                         // main pool control
    volatile int plock;                        // shutdown status and seqLock
    volatile int indexSeed;                    // worker/submitter index seed
    final int config;                          // mode and parallelism level
    WorkQueue[] workQueues;                    // main registry
    final ForkJoinWorkerThreadFactory factory;
    final Thread.UncaughtExceptionHandler ueh; // per-worker UEH
    final String workerNamePrefix;             // to create worker name string
1273

1274 1275
    volatile Object pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17;
    volatile Object pad18, pad19, pad1a, pad1b;
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1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323
    /*
     * Acquires the plock lock to protect worker array and related
     * updates. This method is called only if an initial CAS on plock
     * fails. This acts as a spinLock for normal cases, but falls back
     * to builtin monitor to block when (rarely) needed. This would be
     * a terrible idea for a highly contended lock, but works fine as
     * a more conservative alternative to a pure spinlock.
     */
    private int acquirePlock() {
        int spins = PL_SPINS, r = 0, ps, nps;
        for (;;) {
            if (((ps = plock) & PL_LOCK) == 0 &&
                U.compareAndSwapInt(this, PLOCK, ps, nps = ps + PL_LOCK))
                return nps;
            else if (r == 0) { // randomize spins if possible
                Thread t = Thread.currentThread(); WorkQueue w; Submitter z;
                if ((t instanceof ForkJoinWorkerThread) &&
                    (w = ((ForkJoinWorkerThread)t).workQueue) != null)
                    r = w.seed;
                else if ((z = submitters.get()) != null)
                    r = z.seed;
                else
                    r = 1;
            }
            else if (spins >= 0) {
                r ^= r << 1; r ^= r >>> 3; r ^= r << 10; // xorshift
                if (r >= 0)
                    --spins;
            }
            else if (U.compareAndSwapInt(this, PLOCK, ps, ps | PL_SIGNAL)) {
                synchronized (this) {
                    if ((plock & PL_SIGNAL) != 0) {
                        try {
                            wait();
                        } catch (InterruptedException ie) {
                            try {
                                Thread.currentThread().interrupt();
                            } catch (SecurityException ignore) {
                            }
                        }
                    }
                    else
                        notifyAll();
                }
            }
        }
    }
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    /**
1326 1327
     * Unlocks and signals any thread waiting for plock. Called only
     * when CAS of seq value for unlock fails.
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     */
1329 1330 1331 1332
    private void releasePlock(int ps) {
        plock = ps;
        synchronized (this) { notifyAll(); }
    }
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    /**
1335 1336 1337 1338 1339 1340 1341 1342
     * Performs secondary initialization, called when plock is zero.
     * Creates workQueue array and sets plock to a valid value.  The
     * lock body must be exception-free (so no try/finally) so we
     * optimistically allocate new array outside the lock and throw
     * away if (very rarely) not needed. (A similar tactic is used in
     * fullExternalPush.)  Because the plock seq value can eventually
     * wrap around zero, this method harmlessly fails to reinitialize
     * if workQueues exists, while still advancing plock.
1343
     *
1344
     * Additionally tries to create the first worker.
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     */
1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392
    private void initWorkers() {
        WorkQueue[] ws, nws; int ps;
        int p = config & SMASK;        // find power of two table size
        int n = (p > 1) ? p - 1 : 1;   // ensure at least 2 slots
        n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; n |= n >>> 16;
        n = (n + 1) << 1;
        if ((ws = workQueues) == null || ws.length == 0)
            nws = new WorkQueue[n];
        else
            nws = null;
        if (((ps = plock) & PL_LOCK) != 0 ||
            !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
            ps = acquirePlock();
        if (((ws = workQueues) == null || ws.length == 0) && nws != null)
            workQueues = nws;
        int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
        if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
            releasePlock(nps);
        tryAddWorker();
    }

    /**
     * Tries to create and start one worker if fewer than target
     * parallelism level exist. Adjusts counts etc on failure.
     */
    private void tryAddWorker() {
        long c; int u;
        while ((u = (int)((c = ctl) >>> 32)) < 0 &&
               (u & SHORT_SIGN) != 0 && (int)c == 0) {
            long nc = (long)(((u + UTC_UNIT) & UTC_MASK) |
                             ((u + UAC_UNIT) & UAC_MASK)) << 32;
            if (U.compareAndSwapLong(this, CTL, c, nc)) {
                ForkJoinWorkerThreadFactory fac;
                Throwable ex = null;
                ForkJoinWorkerThread wt = null;
                try {
                    if ((fac = factory) != null &&
                        (wt = fac.newThread(this)) != null) {
                        wt.start();
                        break;
                    }
                } catch (Throwable e) {
                    ex = e;
                }
                deregisterWorker(wt, ex);
                break;
            }
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        }
    }

1396
    //  Registering and deregistering workers
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    /**
1399 1400 1401 1402 1403 1404 1405 1406
     * Callback from ForkJoinWorkerThread to establish and record its
     * WorkQueue. To avoid scanning bias due to packing entries in
     * front of the workQueues array, we treat the array as a simple
     * power-of-two hash table using per-thread seed as hash,
     * expanding as needed.
     *
     * @param wt the worker thread
     * @return the worker's queue
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     */
1408 1409 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434
    final WorkQueue registerWorker(ForkJoinWorkerThread wt) {
        Thread.UncaughtExceptionHandler handler; WorkQueue[] ws; int s, ps;
        wt.setDaemon(true);
        if ((handler = ueh) != null)
            wt.setUncaughtExceptionHandler(handler);
        do {} while (!U.compareAndSwapInt(this, INDEXSEED, s = indexSeed,
                                          s += SEED_INCREMENT) ||
                     s == 0); // skip 0
        WorkQueue w = new WorkQueue(this, wt, config >>> 16, s);
        if (((ps = plock) & PL_LOCK) != 0 ||
            !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
            ps = acquirePlock();
        int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
        try {
            if ((ws = workQueues) != null) {    // skip if shutting down
                int n = ws.length, m = n - 1;
                int r = (s << 1) | 1;           // use odd-numbered indices
                if (ws[r &= m] != null) {       // collision
                    int probes = 0;             // step by approx half size
                    int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2;
                    while (ws[r = (r + step) & m] != null) {
                        if (++probes >= n) {
                            workQueues = ws = Arrays.copyOf(ws, n <<= 1);
                            m = n - 1;
                            probes = 0;
                        }
                    }
1435
                }
1436 1437
                w.eventCount = w.poolIndex = r; // volatile write orders
                ws[r] = w;
1438
            }
1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475
        } finally {
            if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
                releasePlock(nps);
        }
        wt.setName(workerNamePrefix.concat(Integer.toString(w.poolIndex)));
        return w;
    }

    /**
     * Final callback from terminating worker, as well as upon failure
     * to construct or start a worker.  Removes record of worker from
     * array, and adjusts counts. If pool is shutting down, tries to
     * complete termination.
     *
     * @param wt the worker thread or null if construction failed
     * @param ex the exception causing failure, or null if none
     */
    final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
        WorkQueue w = null;
        if (wt != null && (w = wt.workQueue) != null) {
            int ps;
            w.qlock = -1;                // ensure set
            long ns = w.nsteals, sc;     // collect steal count
            do {} while (!U.compareAndSwapLong(this, STEALCOUNT,
                                               sc = stealCount, sc + ns));
            if (((ps = plock) & PL_LOCK) != 0 ||
                !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
                ps = acquirePlock();
            int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
            try {
                int idx = w.poolIndex;
                WorkQueue[] ws = workQueues;
                if (ws != null && idx >= 0 && idx < ws.length && ws[idx] == w)
                    ws[idx] = null;
            } finally {
                if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
                    releasePlock(nps);
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            }
        }
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1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510
        long c;                          // adjust ctl counts
        do {} while (!U.compareAndSwapLong
                     (this, CTL, c = ctl, (((c - AC_UNIT) & AC_MASK) |
                                           ((c - TC_UNIT) & TC_MASK) |
                                           (c & ~(AC_MASK|TC_MASK)))));

        if (!tryTerminate(false, false) && w != null && w.array != null) {
            w.cancelAll();               // cancel remaining tasks
            WorkQueue[] ws; WorkQueue v; Thread p; int u, i, e;
            while ((u = (int)((c = ctl) >>> 32)) < 0 && (e = (int)c) >= 0) {
                if (e > 0) {             // activate or create replacement
                    if ((ws = workQueues) == null ||
                        (i = e & SMASK) >= ws.length ||
                        (v = ws[i]) != null)
                        break;
                    long nc = (((long)(v.nextWait & E_MASK)) |
                               ((long)(u + UAC_UNIT) << 32));
                    if (v.eventCount != (e | INT_SIGN))
                        break;
                    if (U.compareAndSwapLong(this, CTL, c, nc)) {
                        v.eventCount = (e + E_SEQ) & E_MASK;
                        if ((p = v.parker) != null)
                            U.unpark(p);
                        break;
                    }
                }
                else {
                    if ((short)u < 0)
                        tryAddWorker();
                    break;
                }
            }
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        }
1512 1513 1514 1515
        if (ex == null)                     // help clean refs on way out
            ForkJoinTask.helpExpungeStaleExceptions();
        else                                // rethrow
            ForkJoinTask.rethrow(ex);
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    }

1518
    // Submissions
1519

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    /**
1521 1522 1523 1524 1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535 1536 1537 1538 1539 1540 1541 1542 1543 1544 1545 1546 1547 1548 1549 1550 1551 1552 1553 1554 1555 1556 1557 1558 1559 1560 1561 1562 1563 1564 1565 1566 1567 1568 1569 1570 1571 1572 1573 1574 1575 1576 1577 1578 1579 1580 1581 1582 1583 1584 1585 1586 1587 1588 1589 1590 1591 1592 1593 1594 1595 1596 1597 1598 1599
     * Unless shutting down, adds the given task to a submission queue
     * at submitter's current queue index (modulo submission
     * range). Only the most common path is directly handled in this
     * method. All others are relayed to fullExternalPush.
     *
     * @param task the task. Caller must ensure non-null.
     */
    final void externalPush(ForkJoinTask<?> task) {
        WorkQueue[] ws; WorkQueue q; Submitter z; int m; ForkJoinTask<?>[] a;
        if ((z = submitters.get()) != null && plock > 0 &&
            (ws = workQueues) != null && (m = (ws.length - 1)) >= 0 &&
            (q = ws[m & z.seed & SQMASK]) != null &&
            U.compareAndSwapInt(q, QLOCK, 0, 1)) { // lock
            int b = q.base, s = q.top, n, an;
            if ((a = q.array) != null && (an = a.length) > (n = s + 1 - b)) {
                int j = (((an - 1) & s) << ASHIFT) + ABASE;
                U.putOrderedObject(a, j, task);
                q.top = s + 1;                     // push on to deque
                q.qlock = 0;
                if (n <= 2)
                    signalWork(q);
                return;
            }
            q.qlock = 0;
        }
        fullExternalPush(task);
    }

    /**
     * Full version of externalPush. This method is called, among
     * other times, upon the first submission of the first task to the
     * pool, so must perform secondary initialization (via
     * initWorkers). It also detects first submission by an external
     * thread by looking up its ThreadLocal, and creates a new shared
     * queue if the one at index if empty or contended. The plock lock
     * body must be exception-free (so no try/finally) so we
     * optimistically allocate new queues outside the lock and throw
     * them away if (very rarely) not needed.
     */
    private void fullExternalPush(ForkJoinTask<?> task) {
        int r = 0; // random index seed
        for (Submitter z = submitters.get();;) {
            WorkQueue[] ws; WorkQueue q; int ps, m, k;
            if (z == null) {
                if (U.compareAndSwapInt(this, INDEXSEED, r = indexSeed,
                                        r += SEED_INCREMENT) && r != 0)
                    submitters.set(z = new Submitter(r));
            }
            else if (r == 0) {               // move to a different index
                r = z.seed;
                r ^= r << 13;                // same xorshift as WorkQueues
                r ^= r >>> 17;
                z.seed = r ^ (r << 5);
            }
            else if ((ps = plock) < 0)
                throw new RejectedExecutionException();
            else if (ps == 0 || (ws = workQueues) == null ||
                     (m = ws.length - 1) < 0)
                initWorkers();
            else if ((q = ws[k = r & m & SQMASK]) != null) {
                if (q.qlock == 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) {
                    ForkJoinTask<?>[] a = q.array;
                    int s = q.top;
                    boolean submitted = false;
                    try {                      // locked version of push
                        if ((a != null && a.length > s + 1 - q.base) ||
                            (a = q.growArray()) != null) {   // must presize
                            int j = (((a.length - 1) & s) << ASHIFT) + ABASE;
                            U.putOrderedObject(a, j, task);
                            q.top = s + 1;
                            submitted = true;
                        }
                    } finally {
                        q.qlock = 0;  // unlock
                    }
                    if (submitted) {
                        signalWork(q);
                        return;
                    }
1600
                }
1601
                r = 0; // move on failure
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            }
1603 1604 1605 1606 1607 1608 1609 1610 1611 1612
            else if (((ps = plock) & PL_LOCK) == 0) { // create new queue
                q = new WorkQueue(this, null, SHARED_QUEUE, r);
                if (((ps = plock) & PL_LOCK) != 0 ||
                    !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
                    ps = acquirePlock();
                if ((ws = workQueues) != null && k < ws.length && ws[k] == null)
                    ws[k] = q;
                int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
                if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
                    releasePlock(nps);
1613 1614
            }
            else
1615
                r = 0; // try elsewhere while lock held
1616
        }
1617 1618 1619 1620 1621 1622 1623 1624 1625 1626 1627 1628 1629 1630 1631 1632 1633 1634 1635 1636 1637 1638 1639 1640 1641 1642 1643 1644 1645 1646 1647 1648 1649 1650 1651
    }

    // Maintaining ctl counts

    /**
     * Increments active count; mainly called upon return from blocking.
     */
    final void incrementActiveCount() {
        long c;
        do {} while (!U.compareAndSwapLong(this, CTL, c = ctl, c + AC_UNIT));
    }

    /**
     * Tries to create or activate a worker if too few are active.
     *
     * @param q the (non-null) queue holding tasks to be signalled
     */
    final void signalWork(WorkQueue q) {
        int hint = q.poolIndex;
        long c; int e, u, i, n; WorkQueue[] ws; WorkQueue w; Thread p;
        while ((u = (int)((c = ctl) >>> 32)) < 0) {
            if ((e = (int)c) > 0) {
                if ((ws = workQueues) != null && ws.length > (i = e & SMASK) &&
                    (w = ws[i]) != null && w.eventCount == (e | INT_SIGN)) {
                    long nc = (((long)(w.nextWait & E_MASK)) |
                               ((long)(u + UAC_UNIT) << 32));
                    if (U.compareAndSwapLong(this, CTL, c, nc)) {
                        w.hint = hint;
                        w.eventCount = (e + E_SEQ) & E_MASK;
                        if ((p = w.parker) != null)
                            U.unpark(p);
                        break;
                    }
                    if (q.top - q.base <= 0)
                        break;
1652
                }
1653 1654 1655 1656 1657 1658 1659
                else
                    break;
            }
            else {
                if ((short)u < 0)
                    tryAddWorker();
                break;
1660
            }
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        }
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    }

1664 1665 1666 1667 1668 1669 1670 1671 1672 1673
    // Scanning for tasks

    /**
     * Top-level runloop for workers, called by ForkJoinWorkerThread.run.
     */
    final void runWorker(WorkQueue w) {
        w.growArray(); // allocate queue
        do { w.runTask(scan(w)); } while (w.qlock >= 0);
    }

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    /**
1675 1676 1677 1678 1679 1680 1681 1682 1683 1684 1685 1686 1687 1688 1689 1690 1691 1692 1693 1694 1695 1696 1697 1698 1699 1700
     * Scans for and, if found, returns one task, else possibly
     * inactivates the worker. This method operates on single reads of
     * volatile state and is designed to be re-invoked continuously,
     * in part because it returns upon detecting inconsistencies,
     * contention, or state changes that indicate possible success on
     * re-invocation.
     *
     * The scan searches for tasks across queues (starting at a random
     * index, and relying on registerWorker to irregularly scatter
     * them within array to avoid bias), checking each at least twice.
     * The scan terminates upon either finding a non-empty queue, or
     * completing the sweep. If the worker is not inactivated, it
     * takes and returns a task from this queue. Otherwise, if not
     * activated, it signals workers (that may include itself) and
     * returns so caller can retry. Also returns for true if the
     * worker array may have changed during an empty scan.  On failure
     * to find a task, we take one of the following actions, after
     * which the caller will retry calling this method unless
     * terminated.
     *
     * * If pool is terminating, terminate the worker.
     *
     * * If not already enqueued, try to inactivate and enqueue the
     * worker on wait queue. Or, if inactivating has caused the pool
     * to be quiescent, relay to idleAwaitWork to possibly shrink
     * pool.
1701
     *
1702 1703 1704 1705 1706 1707 1708 1709 1710 1711 1712 1713 1714 1715 1716 1717 1718 1719 1720 1721 1722 1723 1724 1725 1726 1727 1728 1729 1730 1731 1732 1733 1734
     * * If already enqueued and none of the above apply, possibly
     * park awaiting signal, else lingering to help scan and signal.
     *
     * * If a non-empty queue discovered or left as a hint,
     * help wake up other workers before return
     *
     * @param w the worker (via its WorkQueue)
     * @return a task or null if none found
     */
    private final ForkJoinTask<?> scan(WorkQueue w) {
        WorkQueue[] ws; int m;
        int ps = plock;                          // read plock before ws
        if (w != null && (ws = workQueues) != null && (m = ws.length - 1) >= 0) {
            int ec = w.eventCount;               // ec is negative if inactive
            int r = w.seed; r ^= r << 13; r ^= r >>> 17; w.seed = r ^= r << 5;
            w.hint = -1;                         // update seed and clear hint
            int j = ((m + m + 1) | MIN_SCAN) & MAX_SCAN;
            do {
                WorkQueue q; ForkJoinTask<?>[] a; int b;
                if ((q = ws[(r + j) & m]) != null && (b = q.base) - q.top < 0 &&
                    (a = q.array) != null) {     // probably nonempty
                    int i = (((a.length - 1) & b) << ASHIFT) + ABASE;
                    ForkJoinTask<?> t = (ForkJoinTask<?>)
                        U.getObjectVolatile(a, i);
                    if (q.base == b && ec >= 0 && t != null &&
                        U.compareAndSwapObject(a, i, t, null)) {
                        if ((q.base = b + 1) - q.top < 0)
                            signalWork(q);
                        return t;                // taken
                    }
                    else if ((ec < 0 || j < m) && (int)(ctl >> AC_SHIFT) <= 0) {
                        w.hint = (r + j) & m;    // help signal below
                        break;                   // cannot take
1735 1736
                    }
                }
1737 1738 1739 1740 1741 1742 1743
            } while (--j >= 0);

            int h, e, ns; long c, sc; WorkQueue q;
            if ((ns = w.nsteals) != 0) {
                if (U.compareAndSwapLong(this, STEALCOUNT,
                                         sc = stealCount, sc + ns))
                    w.nsteals = 0;               // collect steals and rescan
1744
            }
1745 1746 1747 1748
            else if (plock != ps)                // consistency check
                ;                                // skip
            else if ((e = (int)(c = ctl)) < 0)
                w.qlock = -1;                    // pool is terminating
1749
            else {
1750 1751 1752 1753 1754 1755 1756 1757 1758 1759 1760 1761 1762 1763 1764 1765 1766 1767 1768 1769 1770 1771 1772 1773 1774 1775 1776 1777 1778 1779 1780 1781 1782 1783 1784 1785 1786 1787 1788 1789 1790 1791 1792 1793 1794 1795 1796
                if ((h = w.hint) < 0) {
                    if (ec >= 0) {               // try to enqueue/inactivate
                        long nc = (((long)ec |
                                    ((c - AC_UNIT) & (AC_MASK|TC_MASK))));
                        w.nextWait = e;          // link and mark inactive
                        w.eventCount = ec | INT_SIGN;
                        if (ctl != c || !U.compareAndSwapLong(this, CTL, c, nc))
                            w.eventCount = ec;   // unmark on CAS failure
                        else if ((int)(c >> AC_SHIFT) == 1 - (config & SMASK))
                            idleAwaitWork(w, nc, c);
                    }
                    else if (w.eventCount < 0 && !tryTerminate(false, false) &&
                             ctl == c) {         // block
                        Thread wt = Thread.currentThread();
                        Thread.interrupted();    // clear status
                        U.putObject(wt, PARKBLOCKER, this);
                        w.parker = wt;           // emulate LockSupport.park
                        if (w.eventCount < 0)    // recheck
                            U.park(false, 0L);
                        w.parker = null;
                        U.putObject(wt, PARKBLOCKER, null);
                    }
                }
                if ((h >= 0 || (h = w.hint) >= 0) &&
                    (ws = workQueues) != null && h < ws.length &&
                    (q = ws[h]) != null) {      // signal others before retry
                    WorkQueue v; Thread p; int u, i, s;
                    for (int n = (config & SMASK) >>> 1;;) {
                        int idleCount = (w.eventCount < 0) ? 0 : -1;
                        if (((s = idleCount - q.base + q.top) <= n &&
                             (n = s) <= 0) ||
                            (u = (int)((c = ctl) >>> 32)) >= 0 ||
                            (e = (int)c) <= 0 || m < (i = e & SMASK) ||
                            (v = ws[i]) == null)
                            break;
                        long nc = (((long)(v.nextWait & E_MASK)) |
                                   ((long)(u + UAC_UNIT) << 32));
                        if (v.eventCount != (e | INT_SIGN) ||
                            !U.compareAndSwapLong(this, CTL, c, nc))
                            break;
                        v.hint = h;
                        v.eventCount = (e + E_SEQ) & E_MASK;
                        if ((p = v.parker) != null)
                            U.unpark(p);
                        if (--n <= 0)
                            break;
                    }
1797
                }
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            }
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        }
1800
        return null;
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    }

    /**
1804 1805 1806 1807 1808 1809
     * If inactivating worker w has caused the pool to become
     * quiescent, checks for pool termination, and, so long as this is
     * not the only worker, waits for event for up to a given
     * duration.  On timeout, if ctl has not changed, terminates the
     * worker, which will in turn wake up another worker to possibly
     * repeat this process.
1810 1811
     *
     * @param w the calling worker
1812 1813 1814 1815 1816 1817 1818 1819 1820 1821
     * @param currentCtl the ctl value triggering possible quiescence
     * @param prevCtl the ctl value to restore if thread is terminated
     */
    private void idleAwaitWork(WorkQueue w, long currentCtl, long prevCtl) {
        if (w != null && w.eventCount < 0 &&
            !tryTerminate(false, false) && (int)prevCtl != 0) {
            int dc = -(short)(currentCtl >>> TC_SHIFT);
            long parkTime = dc < 0 ? FAST_IDLE_TIMEOUT: (dc + 1) * IDLE_TIMEOUT;
            long deadline = System.nanoTime() + parkTime - TIMEOUT_SLOP;
            Thread wt = Thread.currentThread();
1822
            while (ctl == currentCtl) {
1823 1824 1825 1826 1827 1828 1829 1830
                Thread.interrupted();  // timed variant of version in scan()
                U.putObject(wt, PARKBLOCKER, this);
                w.parker = wt;
                if (ctl == currentCtl)
                    U.park(false, parkTime);
                w.parker = null;
                U.putObject(wt, PARKBLOCKER, null);
                if (ctl != currentCtl)
1831
                    break;
1832 1833 1834 1835
                if (deadline - System.nanoTime() <= 0L &&
                    U.compareAndSwapLong(this, CTL, currentCtl, prevCtl)) {
                    w.eventCount = (w.eventCount + E_SEQ) | E_MASK;
                    w.qlock = -1;   // shrink
1836
                    break;
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                }
            }
        }
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    }

    /**
1843 1844 1845
     * Scans through queues looking for work while joining a task; if
     * any present, signals. May return early if more signalling is
     * detectably unneeded.
1846
     *
1847 1848
     * @param task return early if done
     * @param origin an index to start scan
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     */
1850 1851 1852 1853 1854 1855 1856 1857 1858 1859 1860 1861 1862 1863 1864 1865 1866 1867 1868 1869 1870 1871 1872 1873 1874 1875 1876 1877 1878
    private void helpSignal(ForkJoinTask<?> task, int origin) {
        WorkQueue[] ws; WorkQueue w; Thread p; long c; int m, u, e, i, s;
        if (task != null && task.status >= 0 &&
            (u = (int)(ctl >>> 32)) < 0 && (u >> UAC_SHIFT) < 0 &&
            (ws = workQueues) != null && (m = ws.length - 1) >= 0) {
            outer: for (int k = origin, j = m; j >= 0; --j) {
                WorkQueue q = ws[k++ & m];
                for (int n = m;;) { // limit to at most m signals
                    if (task.status < 0)
                        break outer;
                    if (q == null ||
                        ((s = -q.base + q.top) <= n && (n = s) <= 0))
                        break;
                    if ((u = (int)((c = ctl) >>> 32)) >= 0 ||
                        (e = (int)c) <= 0 || m < (i = e & SMASK) ||
                        (w = ws[i]) == null)
                        break outer;
                    long nc = (((long)(w.nextWait & E_MASK)) |
                               ((long)(u + UAC_UNIT) << 32));
                    if (w.eventCount != (e | INT_SIGN))
                        break outer;
                    if (U.compareAndSwapLong(this, CTL, c, nc)) {
                        w.eventCount = (e + E_SEQ) & E_MASK;
                        if ((p = w.parker) != null)
                            U.unpark(p);
                        if (--n <= 0)
                            break;
                    }
                }
1879 1880
            }
        }
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    }

    /**
1884 1885 1886 1887 1888 1889 1890 1891 1892 1893 1894 1895 1896 1897 1898 1899 1900 1901 1902 1903 1904 1905 1906 1907 1908 1909 1910
     * Tries to locate and execute tasks for a stealer of the given
     * task, or in turn one of its stealers, Traces currentSteal ->
     * currentJoin links looking for a thread working on a descendant
     * of the given task and with a non-empty queue to steal back and
     * execute tasks from. The first call to this method upon a
     * waiting join will often entail scanning/search, (which is OK
     * because the joiner has nothing better to do), but this method
     * leaves hints in workers to speed up subsequent calls. The
     * implementation is very branchy to cope with potential
     * inconsistencies or loops encountering chains that are stale,
     * unknown, or so long that they are likely cyclic.
     *
     * @param joiner the joining worker
     * @param task the task to join
     * @return 0 if no progress can be made, negative if task
     * known complete, else positive
     */
    private int tryHelpStealer(WorkQueue joiner, ForkJoinTask<?> task) {
        int stat = 0, steps = 0;                    // bound to avoid cycles
        if (joiner != null && task != null) {       // hoist null checks
            restart: for (;;) {
                ForkJoinTask<?> subtask = task;     // current target
                for (WorkQueue j = joiner, v;;) {   // v is stealer of subtask
                    WorkQueue[] ws; int m, s, h;
                    if ((s = task.status) < 0) {
                        stat = s;
                        break restart;
1911
                    }
1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961
                    if ((ws = workQueues) == null || (m = ws.length - 1) <= 0)
                        break restart;              // shutting down
                    if ((v = ws[h = (j.hint | 1) & m]) == null ||
                        v.currentSteal != subtask) {
                        for (int origin = h;;) {    // find stealer
                            if (((h = (h + 2) & m) & 15) == 1 &&
                                (subtask.status < 0 || j.currentJoin != subtask))
                                continue restart;   // occasional staleness check
                            if ((v = ws[h]) != null &&
                                v.currentSteal == subtask) {
                                j.hint = h;        // save hint
                                break;
                            }
                            if (h == origin)
                                break restart;      // cannot find stealer
                        }
                    }
                    for (;;) { // help stealer or descend to its stealer
                        ForkJoinTask[] a;  int b;
                        if (subtask.status < 0)     // surround probes with
                            continue restart;       //   consistency checks
                        if ((b = v.base) - v.top < 0 && (a = v.array) != null) {
                            int i = (((a.length - 1) & b) << ASHIFT) + ABASE;
                            ForkJoinTask<?> t =
                                (ForkJoinTask<?>)U.getObjectVolatile(a, i);
                            if (subtask.status < 0 || j.currentJoin != subtask ||
                                v.currentSteal != subtask)
                                continue restart;   // stale
                            stat = 1;               // apparent progress
                            if (t != null && v.base == b &&
                                U.compareAndSwapObject(a, i, t, null)) {
                                v.base = b + 1;     // help stealer
                                joiner.runSubtask(t);
                            }
                            else if (v.base == b && ++steps == MAX_HELP)
                                break restart;      // v apparently stalled
                        }
                        else {                      // empty -- try to descend
                            ForkJoinTask<?> next = v.currentJoin;
                            if (subtask.status < 0 || j.currentJoin != subtask ||
                                v.currentSteal != subtask)
                                continue restart;   // stale
                            else if (next == null || ++steps == MAX_HELP)
                                break restart;      // dead-end or maybe cyclic
                            else {
                                subtask = next;
                                j = v;
                                break;
                            }
                        }
1962 1963
                    }
                }
1964
            }
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        }
1966
        return stat;
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    }

    /**
1970 1971 1972 1973 1974 1975
     * Analog of tryHelpStealer for CountedCompleters. Tries to steal
     * and run tasks within the target's computation.
     *
     * @param task the task to join
     * @param mode if shared, exit upon completing any task
     * if all workers are active
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     *
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     */
1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 2039 2040 2041 2042
    private int helpComplete(ForkJoinTask<?> task, int mode) {
        WorkQueue[] ws; WorkQueue q; int m, n, s, u;
        if (task != null && (ws = workQueues) != null &&
            (m = ws.length - 1) >= 0) {
            for (int j = 1, origin = j;;) {
                if ((s = task.status) < 0)
                    return s;
                if ((q = ws[j & m]) != null && q.pollAndExecCC(task)) {
                    origin = j;
                    if (mode == SHARED_QUEUE &&
                        ((u = (int)(ctl >>> 32)) >= 0 || (u >> UAC_SHIFT) >= 0))
                        break;
                }
                else if ((j = (j + 2) & m) == origin)
                    break;
            }
        }
        return 0;
    }

    /**
     * Tries to decrement active count (sometimes implicitly) and
     * possibly release or create a compensating worker in preparation
     * for blocking. Fails on contention or termination. Otherwise,
     * adds a new thread if no idle workers are available and pool
     * may become starved.
     */
    final boolean tryCompensate() {
        int pc = config & SMASK, e, i, tc; long c;
        WorkQueue[] ws; WorkQueue w; Thread p;
        if ((ws = workQueues) != null && (e = (int)(c = ctl)) >= 0) {
            if (e != 0 && (i = e & SMASK) < ws.length &&
                (w = ws[i]) != null && w.eventCount == (e | INT_SIGN)) {
                long nc = ((long)(w.nextWait & E_MASK) |
                           (c & (AC_MASK|TC_MASK)));
                if (U.compareAndSwapLong(this, CTL, c, nc)) {
                    w.eventCount = (e + E_SEQ) & E_MASK;
                    if ((p = w.parker) != null)
                        U.unpark(p);
                    return true;   // replace with idle worker
                }
            }
            else if ((tc = (short)(c >>> TC_SHIFT)) >= 0 &&
                     (int)(c >> AC_SHIFT) + pc > 1) {
                long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK);
                if (U.compareAndSwapLong(this, CTL, c, nc))
                    return true;   // no compensation
            }
            else if (tc + pc < MAX_CAP) {
                long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
                if (U.compareAndSwapLong(this, CTL, c, nc)) {
                    ForkJoinWorkerThreadFactory fac;
                    Throwable ex = null;
                    ForkJoinWorkerThread wt = null;
                    try {
                        if ((fac = factory) != null &&
                            (wt = fac.newThread(this)) != null) {
                            wt.start();
                            return true;
                        }
                    } catch (Throwable rex) {
                        ex = rex;
                    }
                    deregisterWorker(wt, ex); // clean up and return false
                }
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            }
        }
2045
        return false;
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    }

    /**
2049
     * Helps and/or blocks until the given task is done.
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     *
2051 2052 2053 2054 2055 2056 2057 2058 2059 2060 2061 2062 2063 2064 2065 2066
     * @param joiner the joining worker
     * @param task the task
     * @return task status on exit
     */
    final int awaitJoin(WorkQueue joiner, ForkJoinTask<?> task) {
        int s = 0;
        if (joiner != null && task != null && (s = task.status) >= 0) {
            ForkJoinTask<?> prevJoin = joiner.currentJoin;
            joiner.currentJoin = task;
            do {} while ((s = task.status) >= 0 && !joiner.isEmpty() &&
                         joiner.tryRemoveAndExec(task)); // process local tasks
            if (s >= 0 && (s = task.status) >= 0) {
                helpSignal(task, joiner.poolIndex);
                if ((s = task.status) >= 0 &&
                    (task instanceof CountedCompleter))
                    s = helpComplete(task, LIFO_QUEUE);
2067
            }
2068 2069 2070 2071 2072 2073 2074 2075 2076 2077 2078 2079 2080 2081 2082 2083 2084 2085 2086 2087 2088
            while (s >= 0 && (s = task.status) >= 0) {
                if ((!joiner.isEmpty() ||           // try helping
                     (s = tryHelpStealer(joiner, task)) == 0) &&
                    (s = task.status) >= 0) {
                    helpSignal(task, joiner.poolIndex);
                    if ((s = task.status) >= 0 && tryCompensate()) {
                        if (task.trySetSignal() && (s = task.status) >= 0) {
                            synchronized (task) {
                                if (task.status >= 0) {
                                    try {                // see ForkJoinTask
                                        task.wait();     //  for explanation
                                    } catch (InterruptedException ie) {
                                    }
                                }
                                else
                                    task.notifyAll();
                            }
                        }
                        long c;                          // re-activate
                        do {} while (!U.compareAndSwapLong
                                     (this, CTL, c = ctl, c + AC_UNIT));
2089
                    }
2090
                }
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            }
2092
            joiner.currentJoin = prevJoin;
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        }
2094
        return s;
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    }

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    /**
2098 2099 2100 2101 2102 2103
     * Stripped-down variant of awaitJoin used by timed joins. Tries
     * to help join only while there is continuous progress. (Caller
     * will then enter a timed wait.)
     *
     * @param joiner the joining worker
     * @param task the task
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     */
2105 2106 2107 2108 2109 2110 2111 2112 2113 2114 2115 2116
    final void helpJoinOnce(WorkQueue joiner, ForkJoinTask<?> task) {
        int s;
        if (joiner != null && task != null && (s = task.status) >= 0) {
            ForkJoinTask<?> prevJoin = joiner.currentJoin;
            joiner.currentJoin = task;
            do {} while ((s = task.status) >= 0 && !joiner.isEmpty() &&
                         joiner.tryRemoveAndExec(task));
            if (s >= 0 && (s = task.status) >= 0) {
                helpSignal(task, joiner.poolIndex);
                if ((s = task.status) >= 0 &&
                    (task instanceof CountedCompleter))
                    s = helpComplete(task, LIFO_QUEUE);
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            }
2118 2119 2120 2121 2122
            if (s >= 0 && joiner.isEmpty()) {
                do {} while (task.status >= 0 &&
                             tryHelpStealer(joiner, task) > 0);
            }
            joiner.currentJoin = prevJoin;
2123
        }
2124 2125 2126
    }

    /**
2127 2128 2129 2130 2131 2132 2133 2134 2135 2136 2137 2138 2139 2140 2141 2142 2143 2144 2145 2146 2147 2148 2149 2150
     * Returns a (probably) non-empty steal queue, if one is found
     * during a random, then cyclic scan, else null.  This method must
     * be retried by caller if, by the time it tries to use the queue,
     * it is empty.
     * @param r a (random) seed for scanning
     */
    private WorkQueue findNonEmptyStealQueue(int r) {
        for (WorkQueue[] ws;;) {
            int ps = plock, m, n;
            if ((ws = workQueues) == null || (m = ws.length - 1) < 1)
                return null;
            for (int j = (m + 1) << 2; ;) {
                WorkQueue q = ws[(((r + j) << 1) | 1) & m];
                if (q != null && (n = q.base - q.top) < 0) {
                    if (n < -1)
                        signalWork(q);
                    return q;
                }
                else if (--j < 0) {
                    if (plock == ps)
                        return null;
                    break;
                }
            }
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        }
    }
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    /**
2155 2156 2157 2158 2159 2160 2161 2162 2163 2164 2165 2166 2167 2168 2169 2170 2171 2172 2173 2174 2175 2176 2177 2178 2179 2180 2181 2182 2183 2184 2185 2186 2187 2188 2189 2190 2191 2192
     * Runs tasks until {@code isQuiescent()}. We piggyback on
     * active count ctl maintenance, but rather than blocking
     * when tasks cannot be found, we rescan until all others cannot
     * find tasks either.
     */
    final void helpQuiescePool(WorkQueue w) {
        for (boolean active = true;;) {
            ForkJoinTask<?> localTask; // exhaust local queue
            while ((localTask = w.nextLocalTask()) != null)
                localTask.doExec();
            // Similar to loop in scan(), but ignoring submissions
            WorkQueue q = findNonEmptyStealQueue(w.nextSeed());
            if (q != null) {
                ForkJoinTask<?> t; int b;
                if (!active) {      // re-establish active count
                    long c;
                    active = true;
                    do {} while (!U.compareAndSwapLong
                                 (this, CTL, c = ctl, c + AC_UNIT));
                }
                if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null)
                    w.runSubtask(t);
            }
            else {
                long c;
                if (active) {       // decrement active count without queuing
                    active = false;
                    do {} while (!U.compareAndSwapLong
                                 (this, CTL, c = ctl, c -= AC_UNIT));
                }
                else
                    c = ctl;        // re-increment on exit
                if ((int)(c >> AC_SHIFT) + (config & SMASK) == 0) {
                    do {} while (!U.compareAndSwapLong
                                 (this, CTL, c = ctl, c + AC_UNIT));
                    break;
                }
            }
2193 2194 2195 2196
        }
    }

    /**
2197
     * Gets and removes a local or stolen task for the given worker.
2198
     *
2199
     * @return a task, if available
2200
     */
2201 2202 2203 2204 2205 2206 2207 2208 2209
    final ForkJoinTask<?> nextTaskFor(WorkQueue w) {
        for (ForkJoinTask<?> t;;) {
            WorkQueue q; int b;
            if ((t = w.nextLocalTask()) != null)
                return t;
            if ((q = findNonEmptyStealQueue(w.nextSeed())) == null)
                return null;
            if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null)
                return t;
2210 2211 2212 2213
        }
    }

    /**
2214 2215 2216 2217 2218 2219 2220 2221 2222 2223 2224 2225 2226 2227 2228 2229 2230 2231 2232 2233 2234 2235 2236 2237 2238 2239 2240 2241 2242 2243 2244 2245 2246 2247 2248 2249 2250 2251 2252 2253 2254 2255 2256 2257 2258 2259 2260 2261 2262 2263 2264 2265 2266 2267 2268 2269
     * Returns a cheap heuristic guide for task partitioning when
     * programmers, frameworks, tools, or languages have little or no
     * idea about task granularity.  In essence by offering this
     * method, we ask users only about tradeoffs in overhead vs
     * expected throughput and its variance, rather than how finely to
     * partition tasks.
     *
     * In a steady state strict (tree-structured) computation, each
     * thread makes available for stealing enough tasks for other
     * threads to remain active. Inductively, if all threads play by
     * the same rules, each thread should make available only a
     * constant number of tasks.
     *
     * The minimum useful constant is just 1. But using a value of 1
     * would require immediate replenishment upon each steal to
     * maintain enough tasks, which is infeasible.  Further,
     * partitionings/granularities of offered tasks should minimize
     * steal rates, which in general means that threads nearer the top
     * of computation tree should generate more than those nearer the
     * bottom. In perfect steady state, each thread is at
     * approximately the same level of computation tree. However,
     * producing extra tasks amortizes the uncertainty of progress and
     * diffusion assumptions.
     *
     * So, users will want to use values larger, but not much larger
     * than 1 to both smooth over transient shortages and hedge
     * against uneven progress; as traded off against the cost of
     * extra task overhead. We leave the user to pick a threshold
     * value to compare with the results of this call to guide
     * decisions, but recommend values such as 3.
     *
     * When all threads are active, it is on average OK to estimate
     * surplus strictly locally. In steady-state, if one thread is
     * maintaining say 2 surplus tasks, then so are others. So we can
     * just use estimated queue length.  However, this strategy alone
     * leads to serious mis-estimates in some non-steady-state
     * conditions (ramp-up, ramp-down, other stalls). We can detect
     * many of these by further considering the number of "idle"
     * threads, that are known to have zero queued tasks, so
     * compensate by a factor of (#idle/#active) threads.
     *
     * Note: The approximation of #busy workers as #active workers is
     * not very good under current signalling scheme, and should be
     * improved.
     */
    static int getSurplusQueuedTaskCount() {
        Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q;
        if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) {
            int p = (pool = (wt = (ForkJoinWorkerThread)t).pool).config & SMASK;
            int n = (q = wt.workQueue).top - q.base;
            int a = (int)(pool.ctl >> AC_SHIFT) + p;
            return n - (a > (p >>>= 1) ? 0 :
                        a > (p >>>= 1) ? 1 :
                        a > (p >>>= 1) ? 2 :
                        a > (p >>>= 1) ? 4 :
                        8);
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        }
2271
        return 0;
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    }

2274
    //  Termination
2275

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    /**
2277 2278 2279 2280 2281 2282 2283
     * Possibly initiates and/or completes termination.  The caller
     * triggering termination runs three passes through workQueues:
     * (0) Setting termination status, followed by wakeups of queued
     * workers; (1) cancelling all tasks; (2) interrupting lagging
     * threads (likely in external tasks, but possibly also blocked in
     * joins).  Each pass repeats previous steps because of potential
     * lagging thread creation.
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     *
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     * @param now if true, unconditionally terminate, else only
2286 2287
     * if no work and no active workers
     * @param enable if true, enable shutdown when next possible
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     * @return true if now terminating or terminated
     */
2290 2291 2292 2293 2294 2295 2296 2297 2298
    private boolean tryTerminate(boolean now, boolean enable) {
        if (this == commonPool)                     // cannot shut down
            return false;
        for (long c;;) {
            if (((c = ctl) & STOP_BIT) != 0) {      // already terminating
                if ((short)(c >>> TC_SHIFT) == -(config & SMASK)) {
                    synchronized (this) {
                        notifyAll();                // signal when 0 workers
                    }
2299
                }
2300
                return true;
2301
            }
2302 2303 2304 2305 2306 2307 2308 2309 2310
            if (plock >= 0) {                       // not yet enabled
                int ps;
                if (!enable)
                    return false;
                if (((ps = plock) & PL_LOCK) != 0 ||
                    !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
                    ps = acquirePlock();
                if (!U.compareAndSwapInt(this, PLOCK, ps, SHUTDOWN))
                    releasePlock(SHUTDOWN);
2311
            }
2312 2313 2314 2315 2316 2317 2318 2319 2320 2321 2322 2323 2324 2325 2326 2327 2328 2329 2330 2331 2332 2333 2334 2335 2336 2337 2338 2339 2340 2341 2342 2343 2344
            if (!now) {                             // check if idle & no tasks
                if ((int)(c >> AC_SHIFT) != -(config & SMASK) ||
                    hasQueuedSubmissions())
                    return false;
                // Check for unqueued inactive workers. One pass suffices.
                WorkQueue[] ws = workQueues; WorkQueue w;
                if (ws != null) {
                    for (int i = 1; i < ws.length; i += 2) {
                        if ((w = ws[i]) != null && w.eventCount >= 0)
                            return false;
                    }
                }
            }
            if (U.compareAndSwapLong(this, CTL, c, c | STOP_BIT)) {
                for (int pass = 0; pass < 3; ++pass) {
                    WorkQueue[] ws = workQueues;
                    if (ws != null) {
                        WorkQueue w; Thread wt;
                        int n = ws.length;
                        for (int i = 0; i < n; ++i) {
                            if ((w = ws[i]) != null) {
                                w.qlock = -1;
                                if (pass > 0) {
                                    w.cancelAll();
                                    if (pass > 1 && (wt = w.owner) != null) {
                                        if (!wt.isInterrupted()) {
                                            try {
                                                wt.interrupt();
                                            } catch (SecurityException ignore) {
                                            }
                                        }
                                        U.unpark(wt);
                                    }
2345
                                }
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                            }
                        }
2348 2349 2350 2351 2352 2353 2354 2355 2356 2357 2358 2359 2360 2361 2362 2363
                        // Wake up workers parked on event queue
                        int i, e; long cc; Thread p;
                        while ((e = (int)(cc = ctl) & E_MASK) != 0 &&
                               (i = e & SMASK) < n &&
                               (w = ws[i]) != null) {
                            long nc = ((long)(w.nextWait & E_MASK) |
                                       ((cc + AC_UNIT) & AC_MASK) |
                                       (cc & (TC_MASK|STOP_BIT)));
                            if (w.eventCount == (e | INT_SIGN) &&
                                U.compareAndSwapLong(this, CTL, cc, nc)) {
                                w.eventCount = (e + E_SEQ) & E_MASK;
                                w.qlock = -1;
                                if ((p = w.parker) != null)
                                    U.unpark(p);
                            }
                        }
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                    }
                }
            }
        }
    }

2370 2371
    // external operations on common pool

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    /**
2373 2374
     * Returns common pool queue for a thread that has submitted at
     * least one task.
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     */
2376 2377 2378 2379 2380 2381 2382
    static WorkQueue commonSubmitterQueue() {
        ForkJoinPool p; WorkQueue[] ws; int m; Submitter z;
        return ((z = submitters.get()) != null &&
                (p = commonPool) != null &&
                (ws = p.workQueues) != null &&
                (m = ws.length - 1) >= 0) ?
            ws[m & z.seed & SQMASK] : null;
2383 2384 2385
    }

    /**
2386 2387 2388 2389 2390 2391 2392 2393 2394 2395 2396 2397 2398 2399 2400 2401 2402 2403 2404 2405 2406
     * Tries to pop the given task from submitter's queue in common pool.
     */
    static boolean tryExternalUnpush(ForkJoinTask<?> t) {
        ForkJoinPool p; WorkQueue[] ws; WorkQueue q; Submitter z;
        ForkJoinTask<?>[] a;  int m, s;
        if (t != null &&
            (z = submitters.get()) != null &&
            (p = commonPool) != null &&
            (ws = p.workQueues) != null &&
            (m = ws.length - 1) >= 0 &&
            (q = ws[m & z.seed & SQMASK]) != null &&
            (s = q.top) != q.base &&
            (a = q.array) != null) {
            long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE;
            if (U.getObject(a, j) == t &&
                U.compareAndSwapInt(q, QLOCK, 0, 1)) {
                if (q.array == a && q.top == s && // recheck
                    U.compareAndSwapObject(a, j, t, null)) {
                    q.top = s - 1;
                    q.qlock = 0;
                    return true;
2407
                }
2408
                q.qlock = 0;
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            }
        }
2411
        return false;
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    }

    /**
2415 2416 2417
     * Tries to pop and run local tasks within the same computation
     * as the given root. On failure, tries to help complete from
     * other queues via helpComplete.
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     */
2419 2420 2421 2422 2423 2424 2425 2426 2427 2428 2429 2430 2431 2432 2433 2434 2435 2436 2437 2438 2439 2440 2441 2442 2443 2444 2445 2446 2447 2448 2449 2450 2451 2452 2453 2454 2455 2456 2457
    private void externalHelpComplete(WorkQueue q, ForkJoinTask<?> root) {
        ForkJoinTask<?>[] a; int m;
        if (q != null && (a = q.array) != null && (m = (a.length - 1)) >= 0 &&
            root != null && root.status >= 0) {
            for (;;) {
                int s, u; Object o; CountedCompleter<?> task = null;
                if ((s = q.top) - q.base > 0) {
                    long j = ((m & (s - 1)) << ASHIFT) + ABASE;
                    if ((o = U.getObject(a, j)) != null &&
                        (o instanceof CountedCompleter)) {
                        CountedCompleter<?> t = (CountedCompleter<?>)o, r = t;
                        do {
                            if (r == root) {
                                if (U.compareAndSwapInt(q, QLOCK, 0, 1)) {
                                    if (q.array == a && q.top == s &&
                                        U.compareAndSwapObject(a, j, t, null)) {
                                        q.top = s - 1;
                                        task = t;
                                    }
                                    q.qlock = 0;
                                }
                                break;
                            }
                        } while ((r = r.completer) != null);
                    }
                }
                if (task != null)
                    task.doExec();
                if (root.status < 0 ||
                    (u = (int)(ctl >>> 32)) >= 0 || (u >> UAC_SHIFT) >= 0)
                    break;
                if (task == null) {
                    helpSignal(root, q.poolIndex);
                    if (root.status >= 0)
                        helpComplete(root, SHARED_QUEUE);
                    break;
                }
            }
        }
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    }

    /**
2461 2462 2463 2464 2465 2466 2467 2468 2469 2470 2471 2472 2473 2474 2475 2476 2477 2478 2479 2480 2481 2482 2483 2484 2485 2486 2487 2488 2489 2490 2491 2492 2493 2494 2495 2496
     * Tries to help execute or signal availability of the given task
     * from submitter's queue in common pool.
     */
    static void externalHelpJoin(ForkJoinTask<?> t) {
        // Some hard-to-avoid overlap with tryExternalUnpush
        ForkJoinPool p; WorkQueue[] ws; WorkQueue q, w; Submitter z;
        ForkJoinTask<?>[] a;  int m, s, n;
        if (t != null &&
            (z = submitters.get()) != null &&
            (p = commonPool) != null &&
            (ws = p.workQueues) != null &&
            (m = ws.length - 1) >= 0 &&
            (q = ws[m & z.seed & SQMASK]) != null &&
            (a = q.array) != null) {
            int am = a.length - 1;
            if ((s = q.top) != q.base) {
                long j = ((am & (s - 1)) << ASHIFT) + ABASE;
                if (U.getObject(a, j) == t &&
                    U.compareAndSwapInt(q, QLOCK, 0, 1)) {
                    if (q.array == a && q.top == s &&
                        U.compareAndSwapObject(a, j, t, null)) {
                        q.top = s - 1;
                        q.qlock = 0;
                        t.doExec();
                    }
                    else
                        q.qlock = 0;
                }
            }
            if (t.status >= 0) {
                if (t instanceof CountedCompleter)
                    p.externalHelpComplete(q, t);
                else
                    p.helpSignal(t, q.poolIndex);
            }
        }
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    }

    /**
2500
     * Restricted version of helpQuiescePool for external callers
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     */
2502 2503 2504 2505 2506 2507 2508
    static void externalHelpQuiescePool() {
        ForkJoinPool p; ForkJoinTask<?> t; WorkQueue q; int b;
        if ((p = commonPool) != null &&
            (q = p.findNonEmptyStealQueue(1)) != null &&
            (b = q.base) - q.top < 0 &&
            (t = q.pollAt(b)) != null)
            t.doExec();
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    }

2511
    // Exported methods
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    // Constructors

    /**
     * Creates a {@code ForkJoinPool} with parallelism equal to {@link
     * java.lang.Runtime#availableProcessors}, using the {@linkplain
     * #defaultForkJoinWorkerThreadFactory default thread factory},
     * no UncaughtExceptionHandler, and non-async LIFO processing mode.
     *
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public ForkJoinPool() {
        this(Runtime.getRuntime().availableProcessors(),
             defaultForkJoinWorkerThreadFactory, null, false);
    }

    /**
     * Creates a {@code ForkJoinPool} with the indicated parallelism
     * level, the {@linkplain
     * #defaultForkJoinWorkerThreadFactory default thread factory},
     * no UncaughtExceptionHandler, and non-async LIFO processing mode.
     *
     * @param parallelism the parallelism level
     * @throws IllegalArgumentException if parallelism less than or
     *         equal to zero, or greater than implementation limit
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public ForkJoinPool(int parallelism) {
        this(parallelism, defaultForkJoinWorkerThreadFactory, null, false);
    }

    /**
     * Creates a {@code ForkJoinPool} with the given parameters.
     *
     * @param parallelism the parallelism level. For default value,
     * use {@link java.lang.Runtime#availableProcessors}.
     * @param factory the factory for creating new threads. For default value,
     * use {@link #defaultForkJoinWorkerThreadFactory}.
     * @param handler the handler for internal worker threads that
     * terminate due to unrecoverable errors encountered while executing
     * tasks. For default value, use {@code null}.
     * @param asyncMode if true,
     * establishes local first-in-first-out scheduling mode for forked
     * tasks that are never joined. This mode may be more appropriate
     * than default locally stack-based mode in applications in which
     * worker threads only process event-style asynchronous tasks.
     * For default value, use {@code false}.
     * @throws IllegalArgumentException if parallelism less than or
     *         equal to zero, or greater than implementation limit
     * @throws NullPointerException if the factory is null
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public ForkJoinPool(int parallelism,
                        ForkJoinWorkerThreadFactory factory,
                        Thread.UncaughtExceptionHandler handler,
                        boolean asyncMode) {
        checkPermission();
        if (factory == null)
            throw new NullPointerException();
2580
        if (parallelism <= 0 || parallelism > MAX_CAP)
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            throw new IllegalArgumentException();
        this.factory = factory;
        this.ueh = handler;
2584
        this.config = parallelism | (asyncMode ? (FIFO_QUEUE << 16) : 0);
2585 2586
        long np = (long)(-parallelism); // offset ctl counts
        this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
2587
        int pn = nextPoolId();
2588
        StringBuilder sb = new StringBuilder("ForkJoinPool-");
2589
        sb.append(Integer.toString(pn));
2590 2591
        sb.append("-worker-");
        this.workerNamePrefix = sb.toString();
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    }

2594 2595 2596 2597 2598 2599 2600 2601 2602 2603 2604 2605 2606 2607 2608 2609 2610 2611 2612 2613 2614 2615 2616 2617 2618 2619
    /**
     * Constructor for common pool, suitable only for static initialization.
     * Basically the same as above, but uses smallest possible initial footprint.
     */
    ForkJoinPool(int parallelism, long ctl,
                 ForkJoinWorkerThreadFactory factory,
                 Thread.UncaughtExceptionHandler handler) {
        this.config = parallelism;
        this.ctl = ctl;
        this.factory = factory;
        this.ueh = handler;
        this.workerNamePrefix = "ForkJoinPool.commonPool-worker-";
    }

    /**
     * Returns the common pool instance. This pool is statically
     * constructed; its run state is unaffected by attempts to
     * {@link #shutdown} or {@link #shutdownNow}.
     *
     * @return the common pool instance
     */
    public static ForkJoinPool commonPool() {
        // assert commonPool != null : "static init error";
        return commonPool;
    }

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    // Execution methods

    /**
     * Performs the given task, returning its result upon completion.
2624 2625 2626 2627 2628 2629 2630
     * If the computation encounters an unchecked Exception or Error,
     * it is rethrown as the outcome of this invocation.  Rethrown
     * exceptions behave in the same way as regular exceptions, but,
     * when possible, contain stack traces (as displayed for example
     * using {@code ex.printStackTrace()}) of both the current thread
     * as well as the thread actually encountering the exception;
     * minimally only the latter.
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     *
     * @param task the task
     * @return the task's result
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     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
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    public <T> T invoke(ForkJoinTask<T> task) {
2639 2640
        if (task == null)
            throw new NullPointerException();
2641 2642
        externalPush(task);
        return task.join();
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    }

    /**
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     * Arranges for (asynchronous) execution of the given task.
     *
     * @param task the task
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     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
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    public void execute(ForkJoinTask<?> task) {
2654 2655
        if (task == null)
            throw new NullPointerException();
2656
        externalPush(task);
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    }

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    // AbstractExecutorService methods

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    /**
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
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    public void execute(Runnable task) {
2667 2668
        if (task == null)
            throw new NullPointerException();
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        ForkJoinTask<?> job;
        if (task instanceof ForkJoinTask<?>) // avoid re-wrap
            job = (ForkJoinTask<?>) task;
        else
2673 2674
            job = new ForkJoinTask.AdaptedRunnableAction(task);
        externalPush(job);
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    }

    /**
     * Submits a ForkJoinTask for execution.
     *
     * @param task the task to submit
     * @return the task
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
2687 2688
        if (task == null)
            throw new NullPointerException();
2689
        externalPush(task);
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        return task;
    }

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    /**
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public <T> ForkJoinTask<T> submit(Callable<T> task) {
2699 2700
        ForkJoinTask<T> job = new ForkJoinTask.AdaptedCallable<T>(task);
        externalPush(job);
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        return job;
    }

    /**
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public <T> ForkJoinTask<T> submit(Runnable task, T result) {
2710 2711
        ForkJoinTask<T> job = new ForkJoinTask.AdaptedRunnable<T>(task, result);
        externalPush(job);
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        return job;
    }

    /**
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public ForkJoinTask<?> submit(Runnable task) {
2721 2722
        if (task == null)
            throw new NullPointerException();
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        ForkJoinTask<?> job;
        if (task instanceof ForkJoinTask<?>) // avoid re-wrap
            job = (ForkJoinTask<?>) task;
        else
2727 2728
            job = new ForkJoinTask.AdaptedRunnableAction(task);
        externalPush(job);
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        return job;
    }
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    /**
     * @throws NullPointerException       {@inheritDoc}
     * @throws RejectedExecutionException {@inheritDoc}
     */
    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
2737 2738 2739 2740 2741 2742
        // In previous versions of this class, this method constructed
        // a task to run ForkJoinTask.invokeAll, but now external
        // invocation of multiple tasks is at least as efficient.
        List<ForkJoinTask<T>> fs = new ArrayList<ForkJoinTask<T>>(tasks.size());
        // Workaround needed because method wasn't declared with
        // wildcards in return type but should have been.
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        @SuppressWarnings({"unchecked", "rawtypes"})
2744
            List<Future<T>> futures = (List<Future<T>>) (List) fs;
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2746 2747 2748 2749 2750 2751 2752 2753 2754 2755 2756 2757 2758 2759 2760
        boolean done = false;
        try {
            for (Callable<T> t : tasks) {
                ForkJoinTask<T> f = new ForkJoinTask.AdaptedCallable<T>(t);
                externalPush(f);
                fs.add(f);
            }
            for (ForkJoinTask<T> f : fs)
                f.quietlyJoin();
            done = true;
            return futures;
        } finally {
            if (!done)
                for (ForkJoinTask<T> f : fs)
                    f.cancel(false);
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        }
    }

    /**
     * Returns the factory used for constructing new workers.
     *
     * @return the factory used for constructing new workers
     */
    public ForkJoinWorkerThreadFactory getFactory() {
        return factory;
    }

    /**
     * Returns the handler for internal worker threads that terminate
     * due to unrecoverable errors encountered while executing tasks.
     *
     * @return the handler, or {@code null} if none
     */
    public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler() {
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        return ueh;
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    }

    /**
     * Returns the targeted parallelism level of this pool.
     *
     * @return the targeted parallelism level of this pool
     */
    public int getParallelism() {
2789 2790 2791 2792 2793 2794 2795 2796 2797 2798
        return config & SMASK;
    }

    /**
     * Returns the targeted parallelism level of the common pool.
     *
     * @return the targeted parallelism level of the common pool
     */
    public static int getCommonPoolParallelism() {
        return commonPoolParallelism;
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    }

    /**
     * Returns the number of worker threads that have started but not
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     * yet terminated.  The result returned by this method may differ
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     * from {@link #getParallelism} when threads are created to
     * maintain parallelism when others are cooperatively blocked.
     *
     * @return the number of worker threads
     */
    public int getPoolSize() {
2810
        return (config & SMASK) + (short)(ctl >>> TC_SHIFT);
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    }

    /**
     * Returns {@code true} if this pool uses local first-in-first-out
     * scheduling mode for forked tasks that are never joined.
     *
     * @return {@code true} if this pool uses async mode
     */
    public boolean getAsyncMode() {
2820
        return (config >>> 16) == FIFO_QUEUE;
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    }

    /**
     * Returns an estimate of the number of worker threads that are
     * not blocked waiting to join tasks or for other managed
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     * synchronization. This method may overestimate the
     * number of running threads.
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     *
     * @return the number of worker threads
     */
    public int getRunningThreadCount() {
2832 2833 2834 2835 2836 2837 2838 2839 2840
        int rc = 0;
        WorkQueue[] ws; WorkQueue w;
        if ((ws = workQueues) != null) {
            for (int i = 1; i < ws.length; i += 2) {
                if ((w = ws[i]) != null && w.isApparentlyUnblocked())
                    ++rc;
            }
        }
        return rc;
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    }

    /**
     * Returns an estimate of the number of threads that are currently
     * stealing or executing tasks. This method may overestimate the
     * number of active threads.
     *
     * @return the number of active threads
     */
    public int getActiveThreadCount() {
2851
        int r = (config & SMASK) + (int)(ctl >> AC_SHIFT);
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        return (r <= 0) ? 0 : r; // suppress momentarily negative values
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2853 2854 2855 2856 2857 2858 2859 2860 2861 2862 2863 2864 2865 2866
    }

    /**
     * Returns {@code true} if all worker threads are currently idle.
     * An idle worker is one that cannot obtain a task to execute
     * because none are available to steal from other threads, and
     * there are no pending submissions to the pool. This method is
     * conservative; it might not return {@code true} immediately upon
     * idleness of all threads, but will eventually become true if
     * threads remain inactive.
     *
     * @return {@code true} if all threads are currently idle
     */
    public boolean isQuiescent() {
2867
        return (int)(ctl >> AC_SHIFT) + (config & SMASK) == 0;
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    }

    /**
     * Returns an estimate of the total number of tasks stolen from
     * one thread's work queue by another. The reported value
     * underestimates the actual total number of steals when the pool
     * is not quiescent. This value may be useful for monitoring and
     * tuning fork/join programs: in general, steal counts should be
     * high enough to keep threads busy, but low enough to avoid
     * overhead and contention across threads.
     *
     * @return the number of steals
     */
    public long getStealCount() {
2882 2883 2884 2885 2886 2887 2888 2889 2890
        long count = stealCount;
        WorkQueue[] ws; WorkQueue w;
        if ((ws = workQueues) != null) {
            for (int i = 1; i < ws.length; i += 2) {
                if ((w = ws[i]) != null)
                    count += w.nsteals;
            }
        }
        return count;
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    }

    /**
     * Returns an estimate of the total number of tasks currently held
     * in queues by worker threads (but not including tasks submitted
     * to the pool that have not begun executing). This value is only
     * an approximation, obtained by iterating across all threads in
     * the pool. This method may be useful for tuning task
     * granularities.
     *
     * @return the number of queued tasks
     */
    public long getQueuedTaskCount() {
        long count = 0;
2905 2906 2907 2908 2909 2910
        WorkQueue[] ws; WorkQueue w;
        if ((ws = workQueues) != null) {
            for (int i = 1; i < ws.length; i += 2) {
                if ((w = ws[i]) != null)
                    count += w.queueSize();
            }
2911
        }
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        return count;
    }

    /**
     * Returns an estimate of the number of tasks submitted to this
2917 2918
     * pool that have not yet begun executing.  This method may take
     * time proportional to the number of submissions.
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     *
     * @return the number of queued submissions
     */
    public int getQueuedSubmissionCount() {
2923 2924 2925 2926 2927 2928 2929 2930 2931
        int count = 0;
        WorkQueue[] ws; WorkQueue w;
        if ((ws = workQueues) != null) {
            for (int i = 0; i < ws.length; i += 2) {
                if ((w = ws[i]) != null)
                    count += w.queueSize();
            }
        }
        return count;
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2932 2933 2934 2935 2936 2937 2938 2939 2940
    }

    /**
     * Returns {@code true} if there are any tasks submitted to this
     * pool that have not yet begun executing.
     *
     * @return {@code true} if there are any queued submissions
     */
    public boolean hasQueuedSubmissions() {
2941 2942 2943 2944 2945 2946 2947 2948
        WorkQueue[] ws; WorkQueue w;
        if ((ws = workQueues) != null) {
            for (int i = 0; i < ws.length; i += 2) {
                if ((w = ws[i]) != null && !w.isEmpty())
                    return true;
            }
        }
        return false;
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2949 2950 2951 2952 2953 2954 2955 2956 2957 2958
    }

    /**
     * Removes and returns the next unexecuted submission if one is
     * available.  This method may be useful in extensions to this
     * class that re-assign work in systems with multiple pools.
     *
     * @return the next submission, or {@code null} if none
     */
    protected ForkJoinTask<?> pollSubmission() {
2959 2960 2961 2962 2963
        WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t;
        if ((ws = workQueues) != null) {
            for (int i = 0; i < ws.length; i += 2) {
                if ((w = ws[i]) != null && (t = w.poll()) != null)
                    return t;
2964 2965 2966
            }
        }
        return null;
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2967 2968 2969 2970 2971 2972 2973 2974 2975 2976 2977 2978 2979 2980 2981 2982 2983 2984 2985 2986
    }

    /**
     * Removes all available unexecuted submitted and forked tasks
     * from scheduling queues and adds them to the given collection,
     * without altering their execution status. These may include
     * artificially generated or wrapped tasks. This method is
     * designed to be invoked only when the pool is known to be
     * quiescent. Invocations at other times may not remove all
     * tasks. A failure encountered while attempting to add elements
     * to collection {@code c} may result in elements being in
     * neither, either or both collections when the associated
     * exception is thrown.  The behavior of this operation is
     * undefined if the specified collection is modified while the
     * operation is in progress.
     *
     * @param c the collection to transfer elements into
     * @return the number of elements transferred
     */
    protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
2987
        int count = 0;
2988 2989 2990 2991 2992 2993 2994 2995 2996
        WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t;
        if ((ws = workQueues) != null) {
            for (int i = 0; i < ws.length; ++i) {
                if ((w = ws[i]) != null) {
                    while ((t = w.poll()) != null) {
                        c.add(t);
                        ++count;
                    }
                }
2997 2998
            }
        }
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        return count;
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    }

    /**
     * Returns a string identifying this pool, as well as its state,
     * including indications of run state, parallelism level, and
     * worker and task counts.
     *
     * @return a string identifying this pool, as well as its state
     */
    public String toString() {
3010 3011 3012
        // Use a single pass through workQueues to collect counts
        long qt = 0L, qs = 0L; int rc = 0;
        long st = stealCount;
3013
        long c = ctl;
3014 3015 3016 3017 3018 3019 3020 3021 3022 3023 3024 3025 3026 3027 3028 3029 3030
        WorkQueue[] ws; WorkQueue w;
        if ((ws = workQueues) != null) {
            for (int i = 0; i < ws.length; ++i) {
                if ((w = ws[i]) != null) {
                    int size = w.queueSize();
                    if ((i & 1) == 0)
                        qs += size;
                    else {
                        qt += size;
                        st += w.nsteals;
                        if (w.isApparentlyUnblocked())
                            ++rc;
                    }
                }
            }
        }
        int pc = (config & SMASK);
3031
        int tc = pc + (short)(c >>> TC_SHIFT);
3032 3033 3034
        int ac = pc + (int)(c >> AC_SHIFT);
        if (ac < 0) // ignore transient negative
            ac = 0;
3035 3036
        String level;
        if ((c & STOP_BIT) != 0)
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            level = (tc == 0) ? "Terminated" : "Terminating";
3038
        else
3039
            level = plock < 0 ? "Shutting down" : "Running";
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3040
        return super.toString() +
3041
            "[" + level +
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3042 3043 3044 3045
            ", parallelism = " + pc +
            ", size = " + tc +
            ", active = " + ac +
            ", running = " + rc +
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3046 3047 3048 3049 3050 3051 3052
            ", steals = " + st +
            ", tasks = " + qt +
            ", submissions = " + qs +
            "]";
    }

    /**
3053 3054 3055 3056 3057 3058 3059
     * Possibly initiates an orderly shutdown in which previously
     * submitted tasks are executed, but no new tasks will be
     * accepted. Invocation has no effect on execution state if this
     * is the {@link #commonPool}, and no additional effect if
     * already shut down.  Tasks that are in the process of being
     * submitted concurrently during the course of this method may or
     * may not be rejected.
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     *
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public void shutdown() {
        checkPermission();
3068
        tryTerminate(false, true);
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    }

    /**
3072 3073 3074 3075 3076 3077 3078 3079 3080 3081
     * Possibly attempts to cancel and/or stop all tasks, and reject
     * all subsequently submitted tasks.  Invocation has no effect on
     * execution state if this is the {@link #commonPool}, and no
     * additional effect if already shut down. Otherwise, tasks that
     * are in the process of being submitted or executed concurrently
     * during the course of this method may or may not be
     * rejected. This method cancels both existing and unexecuted
     * tasks, in order to permit termination in the presence of task
     * dependencies. So the method always returns an empty list
     * (unlike the case for some other Executors).
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3082 3083 3084 3085 3086 3087 3088 3089 3090
     *
     * @return an empty list
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public List<Runnable> shutdownNow() {
        checkPermission();
3091
        tryTerminate(true, true);
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        return Collections.emptyList();
    }

    /**
     * Returns {@code true} if all tasks have completed following shut down.
     *
     * @return {@code true} if all tasks have completed following shut down
     */
    public boolean isTerminated() {
3101 3102
        long c = ctl;
        return ((c & STOP_BIT) != 0L &&
3103
                (short)(c >>> TC_SHIFT) == -(config & SMASK));
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3104 3105 3106 3107 3108 3109 3110
    }

    /**
     * Returns {@code true} if the process of termination has
     * commenced but not yet completed.  This method may be useful for
     * debugging. A return of {@code true} reported a sufficient
     * period after shutdown may indicate that submitted tasks have
3111
     * ignored or suppressed interruption, or are waiting for I/O,
3112 3113 3114 3115
     * causing this executor not to properly terminate. (See the
     * advisory notes for class {@link ForkJoinTask} stating that
     * tasks should not normally entail blocking operations.  But if
     * they do, they must abort them on interrupt.)
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3116 3117 3118 3119
     *
     * @return {@code true} if terminating but not yet terminated
     */
    public boolean isTerminating() {
3120 3121
        long c = ctl;
        return ((c & STOP_BIT) != 0L &&
3122
                (short)(c >>> TC_SHIFT) != -(config & SMASK));
3123 3124
    }

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3125 3126 3127 3128 3129 3130
    /**
     * Returns {@code true} if this pool has been shut down.
     *
     * @return {@code true} if this pool has been shut down
     */
    public boolean isShutdown() {
3131
        return plock < 0;
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3132 3133 3134
    }

    /**
3135 3136 3137 3138 3139
     * Blocks until all tasks have completed execution after a
     * shutdown request, or the timeout occurs, or the current thread
     * is interrupted, whichever happens first. Note that the {@link
     * #commonPool()} never terminates until program shutdown so
     * this method will always time out.
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3140 3141 3142 3143 3144 3145 3146 3147 3148
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return {@code true} if this executor terminated and
     *         {@code false} if the timeout elapsed before termination
     * @throws InterruptedException if interrupted while waiting
     */
    public boolean awaitTermination(long timeout, TimeUnit unit)
        throws InterruptedException {
3149
        long nanos = unit.toNanos(timeout);
3150 3151 3152 3153 3154 3155 3156 3157 3158 3159 3160 3161
        if (isTerminated())
            return true;
        long startTime = System.nanoTime();
        boolean terminated = false;
        synchronized (this) {
            for (long waitTime = nanos, millis = 0L;;) {
                if (terminated = isTerminated() ||
                    waitTime <= 0L ||
                    (millis = unit.toMillis(waitTime)) <= 0L)
                    break;
                wait(millis);
                waitTime = nanos - (System.nanoTime() - startTime);
3162
            }
D
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3163
        }
3164
        return terminated;
D
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3165 3166 3167 3168 3169 3170
    }

    /**
     * Interface for extending managed parallelism for tasks running
     * in {@link ForkJoinPool}s.
     *
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3171 3172 3173 3174
     * <p>A {@code ManagedBlocker} provides two methods.  Method
     * {@code isReleasable} must return {@code true} if blocking is
     * not necessary. Method {@code block} blocks the current thread
     * if necessary (perhaps internally invoking {@code isReleasable}
3175 3176 3177 3178 3179 3180 3181 3182 3183
     * before actually blocking). These actions are performed by any
     * thread invoking {@link ForkJoinPool#managedBlock}.  The
     * unusual methods in this API accommodate synchronizers that may,
     * but don't usually, block for long periods. Similarly, they
     * allow more efficient internal handling of cases in which
     * additional workers may be, but usually are not, needed to
     * ensure sufficient parallelism.  Toward this end,
     * implementations of method {@code isReleasable} must be amenable
     * to repeated invocation.
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3184 3185 3186 3187 3188 3189 3190 3191 3192 3193 3194 3195 3196 3197 3198 3199 3200
     *
     * <p>For example, here is a ManagedBlocker based on a
     * ReentrantLock:
     *  <pre> {@code
     * class ManagedLocker implements ManagedBlocker {
     *   final ReentrantLock lock;
     *   boolean hasLock = false;
     *   ManagedLocker(ReentrantLock lock) { this.lock = lock; }
     *   public boolean block() {
     *     if (!hasLock)
     *       lock.lock();
     *     return true;
     *   }
     *   public boolean isReleasable() {
     *     return hasLock || (hasLock = lock.tryLock());
     *   }
     * }}</pre>
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3201 3202 3203 3204 3205 3206 3207 3208 3209 3210 3211 3212 3213 3214 3215 3216 3217 3218 3219 3220
     *
     * <p>Here is a class that possibly blocks waiting for an
     * item on a given queue:
     *  <pre> {@code
     * class QueueTaker<E> implements ManagedBlocker {
     *   final BlockingQueue<E> queue;
     *   volatile E item = null;
     *   QueueTaker(BlockingQueue<E> q) { this.queue = q; }
     *   public boolean block() throws InterruptedException {
     *     if (item == null)
     *       item = queue.take();
     *     return true;
     *   }
     *   public boolean isReleasable() {
     *     return item != null || (item = queue.poll()) != null;
     *   }
     *   public E getItem() { // call after pool.managedBlock completes
     *     return item;
     *   }
     * }}</pre>
D
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3221 3222 3223 3224 3225 3226 3227 3228 3229 3230 3231 3232 3233 3234 3235 3236 3237 3238 3239 3240 3241 3242 3243
     */
    public static interface ManagedBlocker {
        /**
         * Possibly blocks the current thread, for example waiting for
         * a lock or condition.
         *
         * @return {@code true} if no additional blocking is necessary
         * (i.e., if isReleasable would return true)
         * @throws InterruptedException if interrupted while waiting
         * (the method is not required to do so, but is allowed to)
         */
        boolean block() throws InterruptedException;

        /**
         * Returns {@code true} if blocking is unnecessary.
         */
        boolean isReleasable();
    }

    /**
     * Blocks in accord with the given blocker.  If the current thread
     * is a {@link ForkJoinWorkerThread}, this method possibly
     * arranges for a spare thread to be activated if necessary to
D
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3244
     * ensure sufficient parallelism while the current thread is blocked.
D
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3245 3246 3247 3248 3249 3250 3251 3252 3253 3254 3255 3256 3257 3258 3259
     *
     * <p>If the caller is not a {@link ForkJoinTask}, this method is
     * behaviorally equivalent to
     *  <pre> {@code
     * while (!blocker.isReleasable())
     *   if (blocker.block())
     *     return;
     * }</pre>
     *
     * If the caller is a {@code ForkJoinTask}, then the pool may
     * first be expanded to ensure parallelism, and later adjusted.
     *
     * @param blocker the blocker
     * @throws InterruptedException if blocker.block did so
     */
D
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3260
    public static void managedBlock(ManagedBlocker blocker)
D
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3261 3262
        throws InterruptedException {
        Thread t = Thread.currentThread();
D
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3263
        if (t instanceof ForkJoinWorkerThread) {
3264 3265 3266 3267 3268 3269 3270 3271 3272 3273 3274 3275 3276 3277 3278 3279 3280 3281 3282 3283 3284 3285 3286 3287 3288
            ForkJoinPool p = ((ForkJoinWorkerThread)t).pool;
            while (!blocker.isReleasable()) { // variant of helpSignal
                WorkQueue[] ws; WorkQueue q; int m, u;
                if ((ws = p.workQueues) != null && (m = ws.length - 1) >= 0) {
                    for (int i = 0; i <= m; ++i) {
                        if (blocker.isReleasable())
                            return;
                        if ((q = ws[i]) != null && q.base - q.top < 0) {
                            p.signalWork(q);
                            if ((u = (int)(p.ctl >>> 32)) >= 0 ||
                                (u >> UAC_SHIFT) >= 0)
                                break;
                        }
                    }
                }
                if (p.tryCompensate()) {
                    try {
                        do {} while (!blocker.isReleasable() &&
                                     !blocker.block());
                    } finally {
                        p.incrementActiveCount();
                    }
                    break;
                }
            }
D
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3289 3290
        }
        else {
3291 3292
            do {} while (!blocker.isReleasable() &&
                         !blocker.block());
D
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3293 3294 3295 3296 3297 3298 3299 3300
        }
    }

    // AbstractExecutorService overrides.  These rely on undocumented
    // fact that ForkJoinTask.adapt returns ForkJoinTasks that also
    // implement RunnableFuture.

    protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
3301
        return new ForkJoinTask.AdaptedRunnable<T>(runnable, value);
D
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3302 3303 3304
    }

    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
3305
        return new ForkJoinTask.AdaptedCallable<T>(callable);
D
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3306 3307 3308
    }

    // Unsafe mechanics
3309 3310 3311 3312
    private static final sun.misc.Unsafe U;
    private static final long CTL;
    private static final long PARKBLOCKER;
    private static final int ABASE;
3313
    private static final int ASHIFT;
3314 3315 3316 3317
    private static final long STEALCOUNT;
    private static final long PLOCK;
    private static final long INDEXSEED;
    private static final long QLOCK;
3318 3319

    static {
3320
        int s; // initialize field offsets for CAS etc
D
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3321
        try {
3322
            U = sun.misc.Unsafe.getUnsafe();
3323
            Class<?> k = ForkJoinPool.class;
3324
            CTL = U.objectFieldOffset
3325
                (k.getDeclaredField("ctl"));
3326
            STEALCOUNT = U.objectFieldOffset
3327
                (k.getDeclaredField("stealCount"));
3328 3329 3330 3331 3332 3333 3334 3335 3336 3337 3338 3339 3340 3341
            PLOCK = U.objectFieldOffset
                (k.getDeclaredField("plock"));
            INDEXSEED = U.objectFieldOffset
                (k.getDeclaredField("indexSeed"));
            Class<?> tk = Thread.class;
            PARKBLOCKER = U.objectFieldOffset
                (tk.getDeclaredField("parkBlocker"));
            Class<?> wk = WorkQueue.class;
            QLOCK = U.objectFieldOffset
                (wk.getDeclaredField("qlock"));
            Class<?> ak = ForkJoinTask[].class;
            ABASE = U.arrayBaseOffset(ak);
            s = U.arrayIndexScale(ak);
            ASHIFT = 31 - Integer.numberOfLeadingZeros(s);
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        } catch (Exception e) {
            throw new Error(e);
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        }
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        if ((s & (s-1)) != 0)
            throw new Error("data type scale not a power of two");
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        submitters = new ThreadLocal<Submitter>();
        ForkJoinWorkerThreadFactory fac = defaultForkJoinWorkerThreadFactory =
            new DefaultForkJoinWorkerThreadFactory();
        modifyThreadPermission = new RuntimePermission("modifyThread");

        /*
         * Establish common pool parameters.  For extra caution,
         * computations to set up common pool state are here; the
         * constructor just assigns these values to fields.
         */

        int par = 0;
        Thread.UncaughtExceptionHandler handler = null;
        try {  // TBD: limit or report ignored exceptions?
            String pp = System.getProperty
                ("java.util.concurrent.ForkJoinPool.common.parallelism");
            String hp = System.getProperty
                ("java.util.concurrent.ForkJoinPool.common.exceptionHandler");
            String fp = System.getProperty
                ("java.util.concurrent.ForkJoinPool.common.threadFactory");
            if (fp != null)
                fac = ((ForkJoinWorkerThreadFactory)ClassLoader.
                       getSystemClassLoader().loadClass(fp).newInstance());
            if (hp != null)
                handler = ((Thread.UncaughtExceptionHandler)ClassLoader.
                           getSystemClassLoader().loadClass(hp).newInstance());
            if (pp != null)
                par = Integer.parseInt(pp);
        } catch (Exception ignore) {
        }

        if (par <= 0)
            par = Runtime.getRuntime().availableProcessors();
        if (par > MAX_CAP)
            par = MAX_CAP;
        commonPoolParallelism = par;
        long np = (long)(-par); // precompute initial ctl value
        long ct = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);

        commonPool = new ForkJoinPool(par, ct, fac, handler);
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    }
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}