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7023006: Reduce unnecessary thread activity in ForkJoinPool 

Reviewed-by: chegar, dholmes
上级 41df01e6
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src/share/classes/java/util/concurrent/ForkJoinPool.java
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......@@ -40,6 +40,7 @@ import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.Random;
import java.util.concurrent.AbstractExecutorService;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
......@@ -51,6 +52,7 @@ import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.Condition;
/**
* An {@link ExecutorService} for running {@link ForkJoinTask}s.
......@@ -158,239 +160,208 @@ public class ForkJoinPool extends AbstractExecutorService {
* set of worker threads: Submissions from non-FJ threads enter
* into a submission queue. Workers take these tasks and typically
* split them into subtasks that may be stolen by other workers.
* The main work-stealing mechanics implemented in class
* ForkJoinWorkerThread 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 worker queues, and
* lastly to new submissions. These mechanics do not consider
* 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.)
*
* Beyond work-stealing support and essential bookkeeping, the
* main responsibility of this framework is to take actions 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. Given that the creation costs of most threads on most
* systems mainly surrounds setting up runtime stacks, thread
* creation and switching is usually not much more expensive than
* stack creation and switching, and is more flexible). Instead we
* 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 worker queues, and
* lastly to new submissions.
*
* 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
* essentially atomic control state into a single 64bit volatile
* variable ("ctl"). This variable is read on the order of 10-100
* times as often as it is modified (always via CAS). (There is
* some additional control state, for example variable "shutdown"
* for which we can cope with uncoordinated updates.) This
* streamlines synchronization and control at the expense of messy
* constructions needed to repack status bits upon updates.
* Updates tend not to contend with each other except during
* bursts while submitted tasks begin or end. In some cases when
* they do contend, threads can instead do something else
* (usually, scan for tasks) until contention subsides.
*
* To enable 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.
*
* Recording Workers. Workers are recorded in the "workers" array
* that is created upon pool construction and expanded if (rarely)
* necessary. This is an array as opposed to some other data
* structure to support index-based random steals by workers.
* Updates to the array recording new workers and unrecording
* terminated ones are protected from each other by a seqLock
* (scanGuard) but the array is otherwise concurrently readable,
* and accessed directly by workers. To simplify index-based
* operations, the array size is always a power of two, and all
* readers must tolerate null slots. To avoid flailing during
* start-up, the array is presized to hold twice #parallelism
* workers (which is unlikely to need further resizing during
* execution). But to avoid dealing with so many null slots,
* variable scanGuard includes a mask for the nearest power of two
* that contains all current workers. 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 workers are
* via indices into the workers array (which is one source of some
* of the messy code constructions here). In essence, the workers
* array serves as a weak reference mechanism. Thus for example
* the wait queue field of ctl stores worker indices, not worker
* references. Access to the workers 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 workers array, as well as queue arrays, check
* that the array 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.
*
* Wait Queuing. Unlike HPC work-stealing frameworks, we cannot
* 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
* generated. 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 to both wake up waiters (by advancing their
* count) and avoid ABA effects. Successors are held in worker
* field "nextWait". Queuing deals with several intrinsic races,
* mainly that a task-producing thread can miss seeing (and
* signalling) another thread that gave up looking for work but
* has not yet entered the wait queue. We solve this by requiring
* a full sweep of all workers both before (in scan()) and after
* (in tryAwaitWork()) 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 "parked" field of
* ForkJoinWorkerThread to reduce unnecessary calls to unpark.
* (Use of the parked field requires a secondary recheck to avoid
* missed signals.)
*
* Signalling. We create or wake up workers only when there
* appears to be at least one task they might be able to find and
* execute. When a submission is added or another worker adds a
* task to a queue that previously had two or fewer tasks, they
* signal waiting workers (or trigger creation of new ones if
* fewer than the given parallelism level -- see signalWork).
* These primary signals are buttressed by signals during rescans
* as well as those performed when a worker steals a task and
* notices that there are more tasks too; together these cover the
* signals needed in cases when more than two tasks are pushed
* but untaken.
*
* Trimming workers. To release resources after periods of lack of
* use, a worker starting to wait when the pool is quiescent will
* time out and terminate if the pool has remained quiescent for
* SHRINK_RATE nanosecs. This will slowly propagate, eventually
* terminating all workers after long periods of non-use.
*
* Submissions. External submissions are maintained in an
* array-based queue that is structured identically to
* ForkJoinWorkerThread queues except for the use of
* submissionLock in method addSubmission. Unlike the case for
* worker queues, multiple external threads can add new
* submissions, so adding requires a lock.
*
* Compensation. Beyond work-stealing support and lifecycle
* control, the main responsibility of this framework is to take
* actions 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:
*
* Helping: Arranging for the joiner to execute some task that it
* would be running if the steal had not occurred. Method
* ForkJoinWorkerThread.helpJoinTask tracks joining->stealing
* ForkJoinWorkerThread.joinTask tracks joining->stealing
* links to try to find such a task.
*
* Compensating: Unless there are already enough live threads,
* method helpMaintainParallelism() may create or
* re-activate a spare thread to compensate for blocked
* joiners until they unblock.
*
* It is impossible to keep exactly the target (parallelism)
* number of threads running at any given time. Determining
* existence of conservatively safe helping targets, the
* availability of already-created spares, and the apparent need
* to create new spares are all racy and require heuristic
* guidance, so we rely on multiple retries of each. Compensation
* occurs in slow-motion. It is triggered only upon timeouts of
* Object.wait used for joins. This reduces poor decisions that
* would otherwise be made when threads are waiting for others
* that are stalled because of unrelated activities such as
* garbage collection.
* method tryPreBlock() may create or re-activate a spare
* thread to compensate for blocked joiners until they
* unblock.
*
* The ManagedBlocker extension API can't use helping so relies
* only on compensation in method awaitBlocker.
*
* The main throughput advantages of work-stealing stem from
* decentralized control -- workers mostly steal tasks from each
* other. We do not want to negate this by creating bottlenecks
* implementing other management responsibilities. So we use a
* collection of techniques that avoid, reduce, or cope well with
* contention. These entail several instances of bit-packing into
* CASable fields to maintain only the minimally required
* atomicity. To enable such packing, we restrict maximum
* parallelism to (1<<15)-1 (enabling twice this (to accommodate
* unbalanced increments and decrements) to fit into a 16 bit
* field, which is far in excess of normal operating range. Even
* though updates to some of these bookkeeping fields do sometimes
* contend with each other, they don't normally cache-contend with
* updates to others enough to warrant memory padding or
* isolation. So they are all held as fields of ForkJoinPool
* objects. The main capabilities are as follows:
*
* 1. Creating and removing workers. Workers are recorded in the
* "workers" array. This is an array as opposed to some other data
* structure to support index-based random steals by workers.
* Updates to the array recording new workers and unrecording
* terminated ones are protected from each other by a lock
* (workerLock) but the array is otherwise concurrently readable,
* and accessed directly by workers. To simplify index-based
* operations, the array size is always a power of two, and all
* readers must tolerate null slots. Currently, 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 workers are via indices into
* the workers array (which is one source of some of the unusual
* code constructions here). In essence, the workers array serves
* as a WeakReference mechanism. Thus for example the event queue
* stores worker indices, not worker references. Access to the
* workers in associated methods (for example releaseEventWaiters)
* 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 shutdown, in which case
* it is OK to give up. On termination, we just clobber these
* data structures without trying to use them.
*
* 2. Bookkeeping for dynamically adding and removing workers. We
* aim to approximately maintain the given level of parallelism.
* When some workers are known to be blocked (on joins or via
* ManagedBlocker), we may create or resume others to take their
* place until they unblock (see below). Implementing this
* requires counts of the number of "running" threads (i.e., those
* that are neither blocked nor artificially suspended) as well as
* the total number. These two values are packed into one field,
* "workerCounts" because we need accurate snapshots when deciding
* to create, resume or suspend. Note however that the
* correspondence of these counts to reality is not guaranteed. In
* particular updates for unblocked threads may lag until they
* actually wake up.
*
* 3. Maintaining global run state. The run state of the pool
* consists of a runLevel (SHUTDOWN, TERMINATING, etc) similar to
* those in other Executor implementations, as well as a count of
* "active" workers -- those that are, or soon will be, or
* recently were executing tasks. The runLevel and active count
* are packed together in order to correctly trigger shutdown and
* termination. Without care, active counts can be subject to very
* high contention. We substantially reduce this contention by
* relaxing update rules. A worker must claim active status
* prospectively, by activating if it sees that a submitted or
* stealable task exists (it may find after activating that the
* task no longer exists). It stays active while processing this
* task (if it exists) and any other local subtasks it produces,
* until it cannot find any other tasks. It then tries
* inactivating (see method preStep), but upon update contention
* instead scans for more tasks, later retrying inactivation if it
* doesn't find any.
*
* 4. Managing idle workers waiting for tasks. We cannot let
* workers spin indefinitely scanning for tasks when none are
* available. On the other hand, we must quickly prod them into
* action when new tasks are submitted or generated. We
* park/unpark these idle workers using an event-count scheme.
* Field eventCount is incremented upon events that may enable
* workers that previously could not find a task to now find one:
* Submission of a new task to the pool, or another worker pushing
* a task onto a previously empty queue. (We also use this
* mechanism for configuration and termination actions that
* require wakeups of idle workers). Each worker maintains its
* last known event count, and blocks when a scan for work did not
* find a task AND its lastEventCount matches the current
* eventCount. Waiting idle workers are recorded in a variant of
* Treiber stack headed by field eventWaiters which, when nonzero,
* encodes the thread index and count awaited for by the worker
* thread most recently calling eventSync. This thread in turn has
* a record (field nextEventWaiter) for the next waiting worker.
* In addition to allowing simpler decisions about need for
* wakeup, the event count bits in eventWaiters serve the role of
* tags to avoid ABA errors in Treiber stacks. Upon any wakeup,
* released threads also try to release at most two others. The
* net effect is a tree-like diffusion of signals, where released
* threads (and possibly others) help with unparks. To further
* reduce contention effects a bit, failed CASes to increment
* field eventCount are tolerated without retries in signalWork.
* Conceptually they are merged into the same event, which is OK
* when their only purpose is to enable workers to scan for work.
*
* 5. Managing suspension of extra workers. When a worker notices
* (usually upon timeout of a wait()) that there are too few
* running threads, we may create a new thread to maintain
* parallelism level, or at least avoid starvation. Usually, extra
* threads are needed for only very short periods, yet join
* dependencies are such that we sometimes need them in
* bursts. Rather than create new threads each time this happens,
* we suspend no-longer-needed extra ones as "spares". For most
* purposes, we don't distinguish "extra" spare threads from
* normal "core" threads: On each call to preStep (the only point
* at which we can do this) a worker checks to see if there are
* now too many running workers, and if so, suspends itself.
* Method helpMaintainParallelism looks for suspended threads to
* resume before considering creating a new replacement. The
* spares themselves are encoded on another variant of a Treiber
* Stack, headed at field "spareWaiters". Note that the use of
* spares is intrinsically racy. One thread may become a spare at
* about the same time as another is needlessly being created. We
* counteract this and related slop in part by requiring resumed
* spares to immediately recheck (in preStep) to see whether they
* should re-suspend.
*
* 6. Killing off unneeded workers. A timeout mechanism is used to
* shed unused workers: The oldest (first) event queue waiter uses
* a timed rather than hard wait. When this wait times out without
* a normal wakeup, it tries to shutdown any one (for convenience
* the newest) other spare or event waiter via
* tryShutdownUnusedWorker. This eventually reduces the number of
* worker threads to a minimum of one after a long enough period
* without use.
*
* 7. Deciding when to create new workers. The main dynamic
* control in this class is deciding when to create extra threads
* in method helpMaintainParallelism. We would like to keep
* exactly #parallelism threads running, which is an impossible
* task. We always need to create one when the number of running
* threads would become zero and all workers are busy. Beyond
* this, we must rely on heuristics that work well in the
* presence of transient phenomena such as GC stalls, dynamic
* compilation, and wake-up lags. These transients are extremely
* common -- we are normally trying to fully saturate the CPUs on
* a machine, so almost any activity other than running tasks
* impedes accuracy. Our main defense is to allow parallelism to
* lapse for a while during joins, and use a timeout to see if,
* after the resulting settling, there is still a need for
* additional workers. This also better copes with the fact that
* some of the methods in this class tend to never become compiled
* (but are interpreted), so some components of the entire set of
* controls might execute 100 times faster than others. And
* similarly for cases where the apparent lack of work is just due
* to GC stalls and other transient system activity.
*
* Beware that there is a lot of representation-level coupling
* 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
* to create new spares are all racy and require heuristic
* guidance, so we rely on multiple retries of each. Currently,
* in keeping with on-demand signalling policy, we compensate only
* if blocking would leave less than one active (non-waiting,
* non-blocked) worker. Additionally, to avoid some false alarms
* due to GC, lagging counters, system activity, etc, compensated
* blocking for joins is only attempted after rechecks stabilize
* (retries are interspersed with Thread.yield, for good
* citizenship). The variable blockedCount, incremented before
* blocking and decremented after, is sometimes needed to
* distinguish cases of waiting for work vs blocking on joins or
* other managed sync. Both cases are equivalent for most pool
* control, so we can update non-atomically. (Additionally,
* contention on blockedCount alleviates some contention on ctl).
*
* Shutdown and Termination. A call to shutdownNow atomically sets
* the ctl stop bit and then (non-atomically) sets each workers
* "terminate" 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 quiesence (i.e., that
* there is no more work) which is reflected in active counts so
* long as there are no current blockers, as well as possible
* re-evaluations during independent changes in blocking or
* quiescing workers.
*
* Style notes: There is a lot of representation-level coupling
* among classes ForkJoinPool, ForkJoinWorkerThread, and
* ForkJoinTask. For example, direct access to "workers" array by
* ForkJoinTask. Most fields of ForkJoinWorkerThread maintain
* data structures managed by ForkJoinPool, so are directly
* accessed. Conversely we allow access to "workers" array by
* workers, and direct access to ForkJoinTask.status by both
* ForkJoinPool and ForkJoinWorkerThread. There is little point
* trying to reduce this, since any associated future changes in
* representations will need to be accompanied by algorithmic
* changes anyway.
*
* Style notes: 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). Also 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 that
* help some methods perform reasonably even when interpreted (not
* compiled), at the expense of some messy constructions that
* reduce byte code counts.
*
* The order of declarations in this file is: (1) statics (2)
* fields (along with constants used when unpacking some of them)
* (3) internal control methods (4) callbacks and other support
* for ForkJoinTask and ForkJoinWorkerThread classes, (5) exported
* methods (plus a few little helpers).
* changes anyway. All together, these low-level implementation
* choices produce as much as a factor of 4 performance
* improvement compared to naive implementations, and enable the
* processing of billions of tasks per second, at the expense of
* some ugliness.
*
* 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 that help some methods perform reasonably even
* when interpreted (not compiled).
*
* The order of declarations in this file is: (1) declarations of
* statics (2) fields (along with constants used when unpacking
* some of them), listed in an order that tends to reduce
* contention among them a bit under most JVMs. (3) internal
* control methods (4) callbacks and other support for
* ForkJoinTask and ForkJoinWorkerThread classes, (5) exported
* methods (plus a few little helpers). (6) static block
* initializing all statics in a minimally dependent order.
*/
/**
......@@ -425,15 +396,13 @@ public class ForkJoinPool extends AbstractExecutorService {
* overridden in ForkJoinPool constructors.
*/
public static final ForkJoinWorkerThreadFactory
defaultForkJoinWorkerThreadFactory =
new DefaultForkJoinWorkerThreadFactory();
defaultForkJoinWorkerThreadFactory;
/**
* Permission required for callers of methods that may start or
* kill threads.
*/
private static final RuntimePermission modifyThreadPermission =
new RuntimePermission("modifyThread");
private static final RuntimePermission modifyThreadPermission;
/**
* If there is a security manager, makes sure caller has
......@@ -448,63 +417,59 @@ public class ForkJoinPool extends AbstractExecutorService {
/**
* Generator for assigning sequence numbers as pool names.
*/
private static final AtomicInteger poolNumberGenerator =
new AtomicInteger();
private static final AtomicInteger poolNumberGenerator;
/**
* The time to block in a join (see awaitJoin) before checking if
* a new worker should be (re)started to maintain parallelism
* level. The value should be short enough to maintain global
* responsiveness and progress but long enough to avoid
* counterproductive firings during GC stalls or unrelated system
* activity, and to not bog down systems with continual re-firings
* on GCs or legitimately long waits.
* Generator for initial random seeds for worker victim
* selection. This is used only to create initial seeds. Random
* steals use a cheaper xorshift generator per steal attempt. We
* don't expect much contention on seedGenerator, so just use a
* plain Random.
*/
private static final long JOIN_TIMEOUT_MILLIS = 250L; // 4 per second
static final Random workerSeedGenerator;
/**
* The wakeup interval (in nanoseconds) for the oldest worker
* waiting for an event to invoke tryShutdownUnusedWorker to
* shrink the number of workers. The exact value does not matter
* too much. It must be short enough to release resources during
* sustained periods of idleness, but not so short that threads
* are continually re-created.
* Array holding all worker threads in the pool. Initialized upon
* construction. Array size must be a power of two. Updates and
* replacements are protected by scanGuard, but the array is
* always kept in a consistent enough state to be randomly
* accessed without locking by workers performing work-stealing,
* as well as other traversal-based methods in this class, so long
* as reads memory-acquire by first reading ctl. All readers must
* tolerate that some array slots may be null.
*/
private static final long SHRINK_RATE_NANOS =
30L * 1000L * 1000L * 1000L; // 2 per minute
ForkJoinWorkerThread[] workers;
/**
* Absolute bound for parallelism level. Twice this number plus
* one (i.e., 0xfff) must fit into a 16bit field to enable
* word-packing for some counts and indices.
* Initial size for submission queue array. Must be a power of
* two. In many applications, these always stay small so we use a
* small initial cap.
*/
private static final int MAX_WORKERS = 0x7fff;
private static final int INITIAL_QUEUE_CAPACITY = 8;
/**
* Array holding all worker threads in the pool. Array size must
* be a power of two. Updates and replacements are protected by
* workerLock, but the array is always kept in a consistent enough
* state to be randomly accessed without locking by workers
* performing work-stealing, as well as other traversal-based
* methods in this class. All readers must tolerate that some
* array slots may be null.
* Maximum size for submission queue array. Must be a power of two
* less than or equal to 1 << (31 - width of array entry) to
* ensure lack of index wraparound, but is capped at a lower
* value to help users trap runaway computations.
*/
volatile ForkJoinWorkerThread[] workers;
private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M
/**
* Queue for external submissions.
* Array serving as submission queue. Initialized upon construction.
*/
private final LinkedTransferQueue<ForkJoinTask<?>> submissionQueue;
private ForkJoinTask<?>[] submissionQueue;
/**
* Lock protecting updates to workers array.
* Lock protecting submissions array for addSubmission
*/
private final ReentrantLock workerLock;
private final ReentrantLock submissionLock;
/**
* Latch released upon termination.
* Condition for awaitTermination, using submissionLock for
* convenience.
*/
private final Phaser termination;
private final Condition termination;
/**
* Creation factory for worker threads.
......@@ -512,87 +477,103 @@ public class ForkJoinPool extends AbstractExecutorService {
private final ForkJoinWorkerThreadFactory factory;
/**
* Sum of per-thread steal counts, updated only when threads are
* idle or terminating.
* The uncaught exception handler used when any worker abruptly
* terminates.
*/
private volatile long stealCount;
final Thread.UncaughtExceptionHandler ueh;
/**
* Encoded record of top of Treiber stack of threads waiting for
* events. The top 32 bits contain the count being waited for. The
* bottom 16 bits contains one plus the pool index of waiting
* worker thread. (Bits 16-31 are unused.)
* Prefix for assigning names to worker threads
*/
private volatile long eventWaiters;
private static final int EVENT_COUNT_SHIFT = 32;
private static final int WAITER_ID_MASK = (1 << 16) - 1;
private final String workerNamePrefix;
/**
* A counter for events that may wake up worker threads:
* - Submission of a new task to the pool
* - A worker pushing a task on an empty queue
* - termination
* Sum of per-thread steal counts, updated only when threads are
* idle or terminating.
*/
private volatile int eventCount;
private volatile long stealCount;
/**
* Encoded record of top of Treiber stack of spare threads waiting
* for resumption. The top 16 bits contain an arbitrary count to
* avoid ABA effects. The bottom 16bits contains one plus the pool
* index of waiting worker thread.
*/
private volatile int spareWaiters;
private static final int SPARE_COUNT_SHIFT = 16;
private static final int SPARE_ID_MASK = (1 << 16) - 1;
* Main pool control -- a long packed with:
* AC: Number of active running workers minus target parallelism (16 bits)
* TC: Number of total workers minus target parallelism (16bits)
* ST: true if pool is terminating (1 bit)
* EC: the wait count of top waiting thread (15 bits)
* ID: ~poolIndex of top of Treiber stack of waiting threads (16 bits)
*
* 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
* negative, there are not enough total workers, when id is
* negative, there is at least one waiting worker, and when e is
* 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.
*/
volatile long ctl;
// 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
private static final int MAX_ID = 0x7fff; // max poolIndex
private static final int SMASK = 0xffff; // mask short bits
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
private static final int E_MASK = 0x7fffffff; // no STOP_BIT
private static final int EC_UNIT = 1 << EC_SHIFT;
/**
* Lifecycle control. The low word contains the number of workers
* that are (probably) executing tasks. This value is atomically
* incremented before a worker gets a task to run, and decremented
* when a worker has no tasks and cannot find any. Bits 16-18
* contain runLevel value. When all are zero, the pool is
* running. Level transitions are monotonic (running -> shutdown
* -> terminating -> terminated) so each transition adds a bit.
* These are bundled together to ensure consistent read for
* termination checks (i.e., that runLevel is at least SHUTDOWN
* and active threads is zero).
*
* Notes: Most direct CASes are dependent on these bitfield
* positions. Also, this field is non-private to enable direct
* performance-sensitive CASes in ForkJoinWorkerThread.
* The target parallelism level.
*/
volatile int runState;
// Note: The order among run level values matters.
private static final int RUNLEVEL_SHIFT = 16;
private static final int SHUTDOWN = 1 << RUNLEVEL_SHIFT;
private static final int TERMINATING = 1 << (RUNLEVEL_SHIFT + 1);
private static final int TERMINATED = 1 << (RUNLEVEL_SHIFT + 2);
private static final int ACTIVE_COUNT_MASK = (1 << RUNLEVEL_SHIFT) - 1;
final int parallelism;
/**
* Holds number of total (i.e., created and not yet terminated)
* and running (i.e., not blocked on joins or other managed sync)
* threads, packed together to ensure consistent snapshot when
* making decisions about creating and suspending spare
* threads. Updated only by CAS. Note that adding a new worker
* requires incrementing both counts, since workers start off in
* running state.
* Index (mod submission queue length) of next element to take
* from submission queue. Usage is identical to that for
* per-worker queues -- see ForkJoinWorkerThread internal
* documentation.
*/
private volatile int workerCounts;
volatile int queueBase;
private static final int TOTAL_COUNT_SHIFT = 16;
private static final int RUNNING_COUNT_MASK = (1 << TOTAL_COUNT_SHIFT) - 1;
private static final int ONE_RUNNING = 1;
private static final int ONE_TOTAL = 1 << TOTAL_COUNT_SHIFT;
/**
* Index (mod submission queue length) of next element to add
* in submission queue. Usage is identical to that for
* per-worker queues -- see ForkJoinWorkerThread internal
* documentation.
*/
int queueTop;
/**
* The target parallelism level.
* Accessed directly by ForkJoinWorkerThreads.
* True when shutdown() has been called.
*/
final int parallelism;
volatile boolean shutdown;
/**
* True if use local fifo, not default lifo, for local polling
......@@ -601,557 +582,664 @@ public class ForkJoinPool extends AbstractExecutorService {
final boolean locallyFifo;
/**
* The uncaught exception handler used when any worker abruptly
* terminates.
* The number of threads in ForkJoinWorkerThreads.helpQuiescePool.
* When non-zero, suppresses automatic shutdown when active
* counts become zero.
*/
private final Thread.UncaughtExceptionHandler ueh;
volatile int quiescerCount;
/**
* Pool number, just for assigning useful names to worker threads
* The number of threads blocked in join.
*/
private final int poolNumber;
volatile int blockedCount;
// Utilities for CASing fields. Note that most of these
// are usually manually inlined by callers
/**
* Counter for worker Thread names (unrelated to their poolIndex)
*/
private volatile int nextWorkerNumber;
/**
* Increments running count part of workerCounts.
* The index for the next created worker. Accessed under scanGuard.
*/
final void incrementRunningCount() {
int c;
do {} while (!UNSAFE.compareAndSwapInt(this, workerCountsOffset,
c = workerCounts,
c + ONE_RUNNING));
}
private int nextWorkerIndex;
/**
* Tries to increment running count part of workerCounts.
* SeqLock and index masking for updates to workers array. Locked
* when SG_UNIT is set. Unlocking clears bit by adding
* SG_UNIT. Staleness of read-only operations can be checked by
* comparing scanGuard to value before the reads. The low 16 bits
* (i.e, anding with SMASK) hold (the smallest power of two
* covering all worker indices, minus one, and is used to avoid
* dealing with large numbers of null slots when the workers array
* is overallocated.
*/
final boolean tryIncrementRunningCount() {
int c;
return UNSAFE.compareAndSwapInt(this, workerCountsOffset,
c = workerCounts,
c + ONE_RUNNING);
}
volatile int scanGuard;
private static final int SG_UNIT = 1 << 16;
/**
* Tries to decrement running count unless already zero.
* The wakeup interval (in nanoseconds) for a worker waiting for a
* task when the pool is quiescent to instead try to shrink the
* number of workers. The exact value does not matter too
* much. It must be short enough to release resources during
* sustained periods of idleness, but not so short that threads
* are continually re-created.
*/
final boolean tryDecrementRunningCount() {
int wc = workerCounts;
if ((wc & RUNNING_COUNT_MASK) == 0)
return false;
return UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING);
}
private static final long SHRINK_RATE =
4L * 1000L * 1000L * 1000L; // 4 seconds
/**
* Forces decrement of encoded workerCounts, awaiting nonzero if
* (rarely) necessary when other count updates lag.
* Top-level loop for worker threads: On each step: if the
* previous step swept through all queues and found no tasks, or
* there are excess threads, then possibly blocks. Otherwise,
* scans for and, if found, executes a task. Returns when pool
* and/or worker terminate.
*
* @param dr -- either zero or ONE_RUNNING
* @param dt -- either zero or ONE_TOTAL
* @param w the worker
*/
private void decrementWorkerCounts(int dr, int dt) {
for (;;) {
int wc = workerCounts;
if ((wc & RUNNING_COUNT_MASK) - dr < 0 ||
(wc >>> TOTAL_COUNT_SHIFT) - dt < 0) {
if ((runState & TERMINATED) != 0)
return; // lagging termination on a backout
Thread.yield();
}
if (UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - (dr + dt)))
return;
final void work(ForkJoinWorkerThread w) {
boolean swept = false; // true on empty scans
long c;
while (!w.terminate && (int)(c = ctl) >= 0) {
int a; // active count
if (!swept && (a = (int)(c >> AC_SHIFT)) <= 0)
swept = scan(w, a);
else if (tryAwaitWork(w, c))
swept = false;
}
}
// Signalling
/**
* Tries decrementing active count; fails on contention.
* Called when workers cannot find tasks to run.
* Wakes up or creates a worker.
*/
final boolean tryDecrementActiveCount() {
int c;
return UNSAFE.compareAndSwapInt(this, runStateOffset,
c = runState, c - 1);
final void signalWork() {
/*
* The while condition is true if: (there is are too few total
* workers OR there is at least one waiter) AND (there are too
* few active workers OR the pool is terminating). The value
* of e distinguishes the remaining cases: zero (no waiters)
* for create, negative if terminating (in which case do
* nothing), else release a waiter. The secondary checks for
* release (non-null array etc) can fail if the pool begins
* terminating after the test, and don't impose any added cost
* because JVMs must perform null and bounds checks anyway.
*/
long c; int e, u;
while ((((e = (int)(c = ctl)) | (u = (int)(c >>> 32))) &
(INT_SIGN|SHORT_SIGN)) == (INT_SIGN|SHORT_SIGN) && e >= 0) {
if (e > 0) { // release a waiting worker
int i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws;
if ((ws = workers) == null ||
(i = ~e & SMASK) >= ws.length ||
(w = ws[i]) == null)
break;
long nc = (((long)(w.nextWait & E_MASK)) |
((long)(u + UAC_UNIT) << 32));
if (w.eventCount == e &&
UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
w.eventCount = (e + EC_UNIT) & E_MASK;
if (w.parked)
UNSAFE.unpark(w);
break;
}
}
else if (UNSAFE.compareAndSwapLong
(this, ctlOffset, c,
(long)(((u + UTC_UNIT) & UTC_MASK) |
((u + UAC_UNIT) & UAC_MASK)) << 32)) {
addWorker();
break;
}
}
}
/**
* Advances to at least the given level. Returns true if not
* already in at least the given level.
* Variant of signalWork to help release waiters on rescans.
* Tries once to release a waiter if active count < 0.
*
* @return false if failed due to contention, else true
*/
private boolean advanceRunLevel(int level) {
for (;;) {
int s = runState;
if ((s & level) != 0)
private boolean tryReleaseWaiter() {
long c; int e, i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws;
if ((e = (int)(c = ctl)) > 0 &&
(int)(c >> AC_SHIFT) < 0 &&
(ws = workers) != null &&
(i = ~e & SMASK) < ws.length &&
(w = ws[i]) != null) {
long nc = ((long)(w.nextWait & E_MASK) |
((c + AC_UNIT) & (AC_MASK|TC_MASK)));
if (w.eventCount != e ||
!UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc))
return false;
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, s | level))
return true;
w.eventCount = (e + EC_UNIT) & E_MASK;
if (w.parked)
UNSAFE.unpark(w);
}
return true;
}
// workers array maintenance
// Scanning for tasks
/**
* Records and returns a workers array index for new worker.
* Scans for and, if found, executes one task. Scans start at a
* random index of workers array, and randomly select the first
* (2*#workers)-1 probes, and then, if all empty, resort to 2
* circular sweeps, which is necessary to check quiescence. and
* taking a submission only if no stealable tasks were found. The
* steal code inside the loop is a specialized form of
* ForkJoinWorkerThread.deqTask, followed bookkeeping to support
* helpJoinTask and signal propagation. The code for submission
* queues is almost identical. On each steal, the worker completes
* not only the task, but also all local tasks that this task may
* have generated. On detecting staleness or contention when
* trying to take a task, this method returns without finishing
* sweep, which allows global state rechecks before retry.
*
* @param w the worker
* @param a the number of active workers
* @return true if swept all queues without finding a task
*/
private int recordWorker(ForkJoinWorkerThread w) {
// Try using slot totalCount-1. If not available, scan and/or resize
int k = (workerCounts >>> TOTAL_COUNT_SHIFT) - 1;
final ReentrantLock lock = this.workerLock;
lock.lock();
try {
private boolean scan(ForkJoinWorkerThread w, int a) {
int g = scanGuard; // mask 0 avoids useless scans if only one active
int m = (parallelism == 1 - a && blockedCount == 0) ? 0 : g & SMASK;
ForkJoinWorkerThread[] ws = workers;
int n = ws.length;
if (k < 0 || k >= n || ws[k] != null) {
for (k = 0; k < n && ws[k] != null; ++k)
;
if (k == n)
ws = workers = Arrays.copyOf(ws, n << 1);
if (ws == null || ws.length <= m) // staleness check
return false;
for (int r = w.seed, k = r, j = -(m + m); j <= m + m; ++j) {
ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
ForkJoinWorkerThread v = ws[k & m];
if (v != null && (b = v.queueBase) != v.queueTop &&
(q = v.queue) != null && (i = (q.length - 1) & b) >= 0) {
long u = (i << ASHIFT) + ABASE;
if ((t = q[i]) != null && v.queueBase == b &&
UNSAFE.compareAndSwapObject(q, u, t, null)) {
int d = (v.queueBase = b + 1) - v.queueTop;
v.stealHint = w.poolIndex;
if (d != 0)
signalWork(); // propagate if nonempty
w.execTask(t);
}
r ^= r << 13; r ^= r >>> 17; w.seed = r ^ (r << 5);
return false; // store next seed
}
else if (j < 0) { // xorshift
r ^= r << 13; r ^= r >>> 17; k = r ^= r << 5;
}
ws[k] = w;
int c = eventCount; // advance event count to ensure visibility
UNSAFE.compareAndSwapInt(this, eventCountOffset, c, c+1);
} finally {
lock.unlock();
else
++k;
}
return k;
if (scanGuard != g) // staleness check
return false;
else { // try to take submission
ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
if ((b = queueBase) != queueTop &&
(q = submissionQueue) != null &&
(i = (q.length - 1) & b) >= 0) {
long u = (i << ASHIFT) + ABASE;
if ((t = q[i]) != null && queueBase == b &&
UNSAFE.compareAndSwapObject(q, u, t, null)) {
queueBase = b + 1;
w.execTask(t);
}
/**
* Nulls out record of worker in workers array.
*/
private void forgetWorker(ForkJoinWorkerThread w) {
int idx = w.poolIndex;
// Locking helps method recordWorker avoid unnecessary expansion
final ReentrantLock lock = this.workerLock;
lock.lock();
try {
return false;
}
return true; // all queues empty
}
}
/**
* Tries to enqueue worker w in wait queue and await change in
* worker's eventCount. If the pool is quiescent, possibly
* terminates worker upon exit. Otherwise, before blocking,
* rescans queues to avoid missed signals. Upon finding work,
* releases at least one worker (which may be the current
* worker). Rescans restart upon detected staleness or failure to
* release due to contention. Note the unusual conventions about
* Thread.interrupt here and elsewhere: Because interrupts are
* used solely to alert threads to check termination, which is
* checked here anyway, 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.
*
* @param w the calling worker
* @param c the ctl value on entry
* @return true if waited or another thread was released upon enq
*/
private boolean tryAwaitWork(ForkJoinWorkerThread w, long c) {
int v = w.eventCount;
w.nextWait = (int)c; // w's successor record
long nc = (long)(v & E_MASK) | ((c - AC_UNIT) & (AC_MASK|TC_MASK));
if (ctl != c || !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
long d = ctl; // return true if lost to a deq, to force scan
return (int)d != (int)c && ((d - c) & AC_MASK) >= 0L;
}
for (int sc = w.stealCount; sc != 0;) { // accumulate stealCount
long s = stealCount;
if (UNSAFE.compareAndSwapLong(this, stealCountOffset, s, s + sc))
sc = w.stealCount = 0;
else if (w.eventCount != v)
return true; // update next time
}
if (parallelism + (int)(nc >> AC_SHIFT) == 0 &&
blockedCount == 0 && quiescerCount == 0)
idleAwaitWork(w, nc, c, v); // quiescent
for (boolean rescanned = false;;) {
if (w.eventCount != v)
return true;
if (!rescanned) {
int g = scanGuard, m = g & SMASK;
ForkJoinWorkerThread[] ws = workers;
if (idx >= 0 && idx < ws.length && ws[idx] == w) // verify
ws[idx] = null;
} finally {
lock.unlock();
if (ws != null && m < ws.length) {
rescanned = true;
for (int i = 0; i <= m; ++i) {
ForkJoinWorkerThread u = ws[i];
if (u != null) {
if (u.queueBase != u.queueTop &&
!tryReleaseWaiter())
rescanned = false; // contended
if (w.eventCount != v)
return true;
}
}
/**
* Final callback from terminating worker. Removes record of
* worker from array, and adjusts counts. If pool is shutting
* down, tries to complete termination.
*
* @param w the worker
*/
final void workerTerminated(ForkJoinWorkerThread w) {
forgetWorker(w);
decrementWorkerCounts(w.isTrimmed() ? 0 : ONE_RUNNING, ONE_TOTAL);
while (w.stealCount != 0) // collect final count
tryAccumulateStealCount(w);
tryTerminate(false);
}
// Waiting for and signalling events
/**
* Releases workers blocked on a count not equal to current count.
* Normally called after precheck that eventWaiters isn't zero to
* avoid wasted array checks. Gives up upon a change in count or
* upon releasing four workers, letting others take over.
*/
private void releaseEventWaiters() {
ForkJoinWorkerThread[] ws = workers;
int n = ws.length;
long h = eventWaiters;
int ec = eventCount;
int releases = 4;
ForkJoinWorkerThread w; int id;
while ((id = (((int)h) & WAITER_ID_MASK) - 1) >= 0 &&
(int)(h >>> EVENT_COUNT_SHIFT) != ec &&
id < n && (w = ws[id]) != null) {
if (UNSAFE.compareAndSwapLong(this, eventWaitersOffset,
h, w.nextWaiter)) {
LockSupport.unpark(w);
if (--releases == 0)
break;
if (scanGuard != g || // stale
(queueBase != queueTop && !tryReleaseWaiter()))
rescanned = false;
if (!rescanned)
Thread.yield(); // reduce contention
else
Thread.interrupted(); // clear before park
}
if (eventCount != ec)
break;
h = eventWaiters;
else {
w.parked = true; // must recheck
if (w.eventCount != v) {
w.parked = false;
return true;
}
LockSupport.park(this);
rescanned = w.parked = false;
}
}
/**
* Tries to advance eventCount and releases waiters. Called only
* from workers.
*/
final void signalWork() {
int c; // try to increment event count -- CAS failure OK
UNSAFE.compareAndSwapInt(this, eventCountOffset, c = eventCount, c+1);
if (eventWaiters != 0L)
releaseEventWaiters();
}
/**
* Adds the given worker to event queue and blocks until
* terminating or event count advances from the given value
* If inactivating worker w has caused pool to become
* quiescent, check for pool termination, and wait for event
* for up to SHRINK_RATE nanosecs (rescans are unnecessary in
* this case because quiescence reflects consensus about lack
* of work). On timeout, if ctl has not changed, terminate the
* worker. Upon its termination (see deregisterWorker), it may
* wake up another worker to possibly repeat this process.
*
* @param w the calling worker thread
* @param ec the count
* @param w the calling worker
* @param currentCtl the ctl value after enqueuing w
* @param prevCtl the ctl value if w terminated
* @param v the eventCount w awaits change
*/
private void eventSync(ForkJoinWorkerThread w, int ec) {
long nh = (((long)ec) << EVENT_COUNT_SHIFT) | ((long)(w.poolIndex+1));
long h;
while ((runState < SHUTDOWN || !tryTerminate(false)) &&
(((int)(h = eventWaiters) & WAITER_ID_MASK) == 0 ||
(int)(h >>> EVENT_COUNT_SHIFT) == ec) &&
eventCount == ec) {
if (UNSAFE.compareAndSwapLong(this, eventWaitersOffset,
w.nextWaiter = h, nh)) {
awaitEvent(w, ec);
private void idleAwaitWork(ForkJoinWorkerThread w, long currentCtl,
long prevCtl, int v) {
if (w.eventCount == v) {
if (shutdown)
tryTerminate(false);
ForkJoinTask.helpExpungeStaleExceptions(); // help clean weak refs
while (ctl == currentCtl) {
long startTime = System.nanoTime();
w.parked = true;
if (w.eventCount == v) // must recheck
LockSupport.parkNanos(this, SHRINK_RATE);
w.parked = false;
if (w.eventCount != v)
break;
else if (System.nanoTime() - startTime < SHRINK_RATE)
Thread.interrupted(); // spurious wakeup
else if (UNSAFE.compareAndSwapLong(this, ctlOffset,
currentCtl, prevCtl)) {
w.terminate = true; // restore previous
w.eventCount = ((int)currentCtl + EC_UNIT) & E_MASK;
break;
}
}
}
}
// Submissions
/**
* Blocks the given worker (that has already been entered as an
* event waiter) until terminating or event count advances from
* the given value. The oldest (first) waiter uses a timed wait to
* occasionally one-by-one shrink the number of workers (to a
* minimum of one) if the pool has not been used for extended
* periods.
* Enqueues the given task in the submissionQueue. Same idea as
* ForkJoinWorkerThread.pushTask except for use of submissionLock.
*
* @param w the calling worker thread
* @param ec the count
* @param t the task
*/
private void awaitEvent(ForkJoinWorkerThread w, int ec) {
while (eventCount == ec) {
if (tryAccumulateStealCount(w)) { // transfer while idle
boolean untimed = (w.nextWaiter != 0L ||
(workerCounts & RUNNING_COUNT_MASK) <= 1);
long startTime = untimed ? 0 : System.nanoTime();
Thread.interrupted(); // clear/ignore interrupt
if (w.isTerminating() || eventCount != ec)
break; // recheck after clear
if (untimed)
LockSupport.park(w);
else {
LockSupport.parkNanos(w, SHRINK_RATE_NANOS);
if (eventCount != ec || w.isTerminating())
break;
if (System.nanoTime() - startTime >= SHRINK_RATE_NANOS)
tryShutdownUnusedWorker(ec);
}
private void addSubmission(ForkJoinTask<?> t) {
final ReentrantLock lock = this.submissionLock;
lock.lock();
try {
ForkJoinTask<?>[] q; int s, m;
if ((q = submissionQueue) != null) { // ignore if queue removed
long u = (((s = queueTop) & (m = q.length-1)) << ASHIFT)+ABASE;
UNSAFE.putOrderedObject(q, u, t);
queueTop = s + 1;
if (s - queueBase == m)
growSubmissionQueue();
}
} finally {
lock.unlock();
}
signalWork();
}
// Maintaining parallelism
// (pollSubmission is defined below with exported methods)
/**
* Pushes worker onto the spare stack.
* Creates or doubles submissionQueue array.
* Basically identical to ForkJoinWorkerThread version.
*/
final void pushSpare(ForkJoinWorkerThread w) {
int ns = (++w.spareCount << SPARE_COUNT_SHIFT) | (w.poolIndex + 1);
do {} while (!UNSAFE.compareAndSwapInt(this, spareWaitersOffset,
w.nextSpare = spareWaiters,ns));
private void growSubmissionQueue() {
ForkJoinTask<?>[] oldQ = submissionQueue;
int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY;
if (size > MAXIMUM_QUEUE_CAPACITY)
throw new RejectedExecutionException("Queue capacity exceeded");
if (size < INITIAL_QUEUE_CAPACITY)
size = INITIAL_QUEUE_CAPACITY;
ForkJoinTask<?>[] q = submissionQueue = new ForkJoinTask<?>[size];
int mask = size - 1;
int top = queueTop;
int oldMask;
if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) {
for (int b = queueBase; b != top; ++b) {
long u = ((b & oldMask) << ASHIFT) + ABASE;
Object x = UNSAFE.getObjectVolatile(oldQ, u);
if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null))
UNSAFE.putObjectVolatile
(q, ((b & mask) << ASHIFT) + ABASE, x);
}
}
}
// Blocking support
/**
* Tries (once) to resume a spare if the number of running
* threads is less than target.
* Tries to increment blockedCount, decrement active count
* (sometimes implicitly) and possibly release or create a
* compensating worker in preparation for blocking. Fails
* on contention or termination.
*
* @return true if the caller can block, else should recheck and retry
*/
private void tryResumeSpare() {
int sw, id;
ForkJoinWorkerThread[] ws = workers;
int n = ws.length;
ForkJoinWorkerThread w;
if ((sw = spareWaiters) != 0 &&
(id = (sw & SPARE_ID_MASK) - 1) >= 0 &&
id < n && (w = ws[id]) != null &&
(runState >= TERMINATING ||
(workerCounts & RUNNING_COUNT_MASK) < parallelism) &&
spareWaiters == sw &&
UNSAFE.compareAndSwapInt(this, spareWaitersOffset,
sw, w.nextSpare)) {
int c; // increment running count before resume
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
if (w.tryUnsuspend())
LockSupport.unpark(w);
else // back out if w was shutdown
decrementWorkerCounts(ONE_RUNNING, 0);
}
}
/**
* Tries to increase the number of running workers if below target
* parallelism: If a spare exists tries to resume it via
* tryResumeSpare. Otherwise, if not enough total workers or all
* existing workers are busy, adds a new worker. In all cases also
* helps wake up releasable workers waiting for work.
*/
private void helpMaintainParallelism() {
private boolean tryPreBlock() {
int b = blockedCount;
if (UNSAFE.compareAndSwapInt(this, blockedCountOffset, b, b + 1)) {
int pc = parallelism;
int wc, rs, tc;
while (((wc = workerCounts) & RUNNING_COUNT_MASK) < pc &&
(rs = runState) < TERMINATING) {
if (spareWaiters != 0)
tryResumeSpare();
else if ((tc = wc >>> TOTAL_COUNT_SHIFT) >= MAX_WORKERS ||
(tc >= pc && (rs & ACTIVE_COUNT_MASK) != tc))
break; // enough total
else if (runState == rs && workerCounts == wc &&
UNSAFE.compareAndSwapInt(this, workerCountsOffset, wc,
wc + (ONE_RUNNING|ONE_TOTAL))) {
ForkJoinWorkerThread w = null;
Throwable fail = null;
try {
w = factory.newThread(this);
} catch (Throwable ex) {
fail = ex;
}
if (w == null) { // null or exceptional factory return
decrementWorkerCounts(ONE_RUNNING, ONE_TOTAL);
tryTerminate(false); // handle failure during shutdown
// If originating from an external caller,
// propagate exception, else ignore
if (fail != null && runState < TERMINATING &&
!(Thread.currentThread() instanceof
ForkJoinWorkerThread))
UNSAFE.throwException(fail);
break;
}
w.start(recordWorker(w), ueh);
if ((workerCounts >>> TOTAL_COUNT_SHIFT) >= pc)
break; // add at most one unless total below target
do {
ForkJoinWorkerThread[] ws; ForkJoinWorkerThread w;
int e, ac, tc, rc, i;
long c = ctl;
int u = (int)(c >>> 32);
if ((e = (int)c) < 0) {
// skip -- terminating
}
else if ((ac = (u >> UAC_SHIFT)) <= 0 && e != 0 &&
(ws = workers) != null &&
(i = ~e & SMASK) < ws.length &&
(w = ws[i]) != null) {
long nc = ((long)(w.nextWait & E_MASK) |
(c & (AC_MASK|TC_MASK)));
if (w.eventCount == e &&
UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
w.eventCount = (e + EC_UNIT) & E_MASK;
if (w.parked)
UNSAFE.unpark(w);
return true; // release an idle worker
}
}
else if ((tc = (short)(u >>> UTC_SHIFT)) >= 0 && ac + pc > 1) {
long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK);
if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc))
return true; // no compensation needed
}
else if (tc + pc < MAX_ID) {
long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
addWorker();
return true; // create a replacement
}
}
// try to back out on any failure and let caller retry
} while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset,
b = blockedCount, b - 1));
}
return false;
}
if (eventWaiters != 0L)
releaseEventWaiters();
/**
* Decrements blockedCount and increments active count
*/
private void postBlock() {
long c;
do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, // no mask
c = ctl, c + AC_UNIT));
int b;
do {} while(!UNSAFE.compareAndSwapInt(this, blockedCountOffset,
b = blockedCount, b - 1));
}
/**
* Callback from the oldest waiter in awaitEvent waking up after a
* period of non-use. If all workers are idle, tries (once) to
* shutdown an event waiter or a spare, if one exists. Note that
* we don't need CAS or locks here because the method is called
* only from one thread occasionally waking (and even misfires are
* OK). Note that until the shutdown worker fully terminates,
* workerCounts will overestimate total count, which is tolerable.
* Possibly blocks waiting for the given task to complete, or
* cancels the task if terminating. Fails to wait if contended.
*
* @param ec the event count waited on by caller (to abort
* attempt if count has since changed).
* @param joinMe the task
*/
private void tryShutdownUnusedWorker(int ec) {
if (runState == 0 && eventCount == ec) { // only trigger if all idle
ForkJoinWorkerThread[] ws = workers;
int n = ws.length;
ForkJoinWorkerThread w = null;
boolean shutdown = false;
int sw;
long h;
if ((sw = spareWaiters) != 0) { // prefer killing spares
int id = (sw & SPARE_ID_MASK) - 1;
if (id >= 0 && id < n && (w = ws[id]) != null &&
UNSAFE.compareAndSwapInt(this, spareWaitersOffset,
sw, w.nextSpare))
shutdown = true;
}
else if ((h = eventWaiters) != 0L) {
long nh;
int id = (((int)h) & WAITER_ID_MASK) - 1;
if (id >= 0 && id < n && (w = ws[id]) != null &&
(nh = w.nextWaiter) != 0L && // keep at least one worker
UNSAFE.compareAndSwapLong(this, eventWaitersOffset, h, nh))
shutdown = true;
}
if (w != null && shutdown) {
w.shutdown();
LockSupport.unpark(w);
final void tryAwaitJoin(ForkJoinTask<?> joinMe) {
int s;
Thread.interrupted(); // clear interrupts before checking termination
if (joinMe.status >= 0) {
if (tryPreBlock()) {
joinMe.tryAwaitDone(0L);
postBlock();
}
else if ((ctl & STOP_BIT) != 0L)
joinMe.cancelIgnoringExceptions();
}
releaseEventWaiters(); // in case of interference
}
/**
* Callback from workers invoked upon each top-level action (i.e.,
* stealing a task or taking a submission and running it).
* Performs one or more of the following:
*
* 1. If the worker is active and either did not run a task
* or there are too many workers, try to set its active status
* to inactive and update activeCount. On contention, we may
* try again in this or a subsequent call.
*
* 2. If not enough total workers, help create some.
*
* 3. If there are too many running workers, suspend this worker
* (first forcing inactive if necessary). If it is not needed,
* it may be shutdown while suspended (via
* tryShutdownUnusedWorker). Otherwise, upon resume it
* rechecks running thread count and need for event sync.
*
* 4. If worker did not run a task, await the next task event via
* eventSync if necessary (first forcing inactivation), upon
* which the worker may be shutdown via
* tryShutdownUnusedWorker. Otherwise, help release any
* existing event waiters that are now releasable,
* Possibly blocks the given worker waiting for joinMe to
* complete or timeout
*
* @param w the worker
* @param ran true if worker ran a task since last call to this method
* @param joinMe the task
* @param millis the wait time for underlying Object.wait
*/
final void preStep(ForkJoinWorkerThread w, boolean ran) {
int wec = w.lastEventCount;
boolean active = w.active;
boolean inactivate = false;
int pc = parallelism;
while (w.runState == 0) {
int rs = runState;
if (rs >= TERMINATING) { // propagate shutdown
w.shutdown();
final void timedAwaitJoin(ForkJoinTask<?> joinMe, long nanos) {
while (joinMe.status >= 0) {
Thread.interrupted();
if ((ctl & STOP_BIT) != 0L) {
joinMe.cancelIgnoringExceptions();
break;
}
if ((inactivate || (active && (rs & ACTIVE_COUNT_MASK) >= pc)) &&
UNSAFE.compareAndSwapInt(this, runStateOffset, rs, --rs)) {
inactivate = active = w.active = false;
if (rs == SHUTDOWN) { // all inactive and shut down
tryTerminate(false);
continue;
}
}
int wc = workerCounts; // try to suspend as spare
if ((wc & RUNNING_COUNT_MASK) > pc) {
if (!(inactivate |= active) && // must inactivate to suspend
workerCounts == wc &&
UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING))
w.suspendAsSpare();
}
else if ((wc >>> TOTAL_COUNT_SHIFT) < pc)
helpMaintainParallelism(); // not enough workers
else if (ran)
if (tryPreBlock()) {
long last = System.nanoTime();
while (joinMe.status >= 0) {
long millis = TimeUnit.NANOSECONDS.toMillis(nanos);
if (millis <= 0)
break;
else {
long h = eventWaiters;
int ec = eventCount;
if (h != 0L && (int)(h >>> EVENT_COUNT_SHIFT) != ec)
releaseEventWaiters(); // release others before waiting
else if (ec != wec) {
w.lastEventCount = ec; // no need to wait
joinMe.tryAwaitDone(millis);
if (joinMe.status < 0)
break;
if ((ctl & STOP_BIT) != 0L) {
joinMe.cancelIgnoringExceptions();
break;
}
long now = System.nanoTime();
nanos -= now - last;
last = now;
}
else if (!(inactivate |= active))
eventSync(w, wec); // must inactivate before sync
postBlock();
break;
}
}
}
/**
* Helps and/or blocks awaiting join of the given task.
* See above for explanation.
*
* @param joinMe the task to join
* @param worker the current worker thread
* @param timed true if wait should time out
* @param nanos timeout value if timed
* If necessary, compensates for blocker, and blocks
*/
final void awaitJoin(ForkJoinTask<?> joinMe, ForkJoinWorkerThread worker,
boolean timed, long nanos) {
long startTime = timed ? System.nanoTime() : 0L;
int retries = 2 + (parallelism >> 2); // #helpJoins before blocking
boolean running = true; // false when count decremented
while (joinMe.status >= 0) {
if (runState >= TERMINATING) {
joinMe.cancelIgnoringExceptions();
break;
private void awaitBlocker(ManagedBlocker blocker)
throws InterruptedException {
while (!blocker.isReleasable()) {
if (tryPreBlock()) {
try {
do {} while (!blocker.isReleasable() && !blocker.block());
} finally {
postBlock();
}
running = worker.helpJoinTask(joinMe, running);
if (joinMe.status < 0)
break;
if (retries > 0) {
--retries;
continue;
}
int wc = workerCounts;
if ((wc & RUNNING_COUNT_MASK) != 0) {
if (running) {
if (!UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING))
continue;
running = false;
}
long h = eventWaiters;
if (h != 0L && (int)(h >>> EVENT_COUNT_SHIFT) != eventCount)
releaseEventWaiters();
if ((workerCounts & RUNNING_COUNT_MASK) != 0) {
long ms; int ns;
if (!timed) {
ms = JOIN_TIMEOUT_MILLIS;
ns = 0;
}
else { // at most JOIN_TIMEOUT_MILLIS per wait
long nt = nanos - (System.nanoTime() - startTime);
if (nt <= 0L)
break;
ms = nt / 1000000;
if (ms > JOIN_TIMEOUT_MILLIS) {
ms = JOIN_TIMEOUT_MILLIS;
ns = 0;
}
else
ns = (int) (nt % 1000000);
}
joinMe.internalAwaitDone(ms, ns);
}
if (joinMe.status < 0)
break;
// Creating, registering and deregistring workers
/**
* Tries to create and start a worker; minimally rolls back counts
* on failure.
*/
private void addWorker() {
Throwable ex = null;
ForkJoinWorkerThread t = null;
try {
t = factory.newThread(this);
} catch (Throwable e) {
ex = e;
}
if (t == null) { // null or exceptional factory return
long c; // adjust counts
do {} while (!UNSAFE.compareAndSwapLong
(this, ctlOffset, c = ctl,
(((c - AC_UNIT) & AC_MASK) |
((c - TC_UNIT) & TC_MASK) |
(c & ~(AC_MASK|TC_MASK)))));
// Propagate exception if originating from an external caller
if (!tryTerminate(false) && ex != null &&
!(Thread.currentThread() instanceof ForkJoinWorkerThread))
UNSAFE.throwException(ex);
}
helpMaintainParallelism();
else
t.start();
}
if (!running) {
int c;
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
/**
* Callback from ForkJoinWorkerThread constructor to assign a
* public name
*/
final String nextWorkerName() {
for (int n;;) {
if (UNSAFE.compareAndSwapInt(this, nextWorkerNumberOffset,
n = nextWorkerNumber, ++n))
return workerNamePrefix + n;
}
}
/**
* Same idea as awaitJoin, but no helping, retries, or timeouts.
* Callback from ForkJoinWorkerThread constructor to
* determine its poolIndex and record in workers array.
*
* @param w the worker
* @return the worker's pool index
*/
final void awaitBlocker(ManagedBlocker blocker)
throws InterruptedException {
while (!blocker.isReleasable()) {
int wc = workerCounts;
if ((wc & RUNNING_COUNT_MASK) == 0)
helpMaintainParallelism();
else if (UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING)) {
final int registerWorker(ForkJoinWorkerThread w) {
/*
* In the typical case, a new worker acquires the lock, uses
* next available index and returns quickly. Since we should
* not block callers (ultimately from signalWork or
* tryPreBlock) waiting for the lock needed to do this, we
* instead help release other workers while waiting for the
* lock.
*/
for (int g;;) {
ForkJoinWorkerThread[] ws;
if (((g = scanGuard) & SG_UNIT) == 0 &&
UNSAFE.compareAndSwapInt(this, scanGuardOffset,
g, g | SG_UNIT)) {
int k = nextWorkerIndex;
try {
while (!blocker.isReleasable()) {
long h = eventWaiters;
if (h != 0L &&
(int)(h >>> EVENT_COUNT_SHIFT) != eventCount)
releaseEventWaiters();
else if ((workerCounts & RUNNING_COUNT_MASK) == 0 &&
runState < TERMINATING)
helpMaintainParallelism();
else if (blocker.block())
break;
if ((ws = workers) != null) { // ignore on shutdown
int n = ws.length;
if (k < 0 || k >= n || ws[k] != null) {
for (k = 0; k < n && ws[k] != null; ++k)
;
if (k == n)
ws = workers = Arrays.copyOf(ws, n << 1);
}
ws[k] = w;
nextWorkerIndex = k + 1;
int m = g & SMASK;
g = k >= m? ((m << 1) + 1) & SMASK : g + (SG_UNIT<<1);
}
} finally {
int c;
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
scanGuard = g;
}
return k;
}
else if ((ws = workers) != null) { // help release others
for (ForkJoinWorkerThread u : ws) {
if (u != null && u.queueBase != u.queueTop) {
if (tryReleaseWaiter())
break;
}
}
}
}
}
/**
* Final callback from terminating worker. Removes record of
* worker from array, and adjusts counts. If pool is shutting
* down, tries to complete termination.
*
* @param w the worker
*/
final void deregisterWorker(ForkJoinWorkerThread w, Throwable ex) {
int idx = w.poolIndex;
int sc = w.stealCount;
int steps = 0;
// Remove from array, adjust worker counts and collect steal count.
// We can intermix failed removes or adjusts with steal updates
do {
long s, c;
int g;
if (steps == 0 && ((g = scanGuard) & SG_UNIT) == 0 &&
UNSAFE.compareAndSwapInt(this, scanGuardOffset,
g, g |= SG_UNIT)) {
ForkJoinWorkerThread[] ws = workers;
if (ws != null && idx >= 0 &&
idx < ws.length && ws[idx] == w)
ws[idx] = null; // verify
nextWorkerIndex = idx;
scanGuard = g + SG_UNIT;
steps = 1;
}
if (steps == 1 &&
UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl,
(((c - AC_UNIT) & AC_MASK) |
((c - TC_UNIT) & TC_MASK) |
(c & ~(AC_MASK|TC_MASK)))))
steps = 2;
if (sc != 0 &&
UNSAFE.compareAndSwapLong(this, stealCountOffset,
s = stealCount, s + sc))
sc = 0;
} while (steps != 2 || sc != 0);
if (!tryTerminate(false)) {
if (ex != null) // possibly replace if died abnormally
signalWork();
else
tryReleaseWaiter();
}
}
// Shutdown and termination
/**
* Possibly initiates and/or completes termination.
......@@ -1161,48 +1249,52 @@ public class ForkJoinPool extends AbstractExecutorService {
* @return true if now terminating or terminated
*/
private boolean tryTerminate(boolean now) {
if (now)
advanceRunLevel(SHUTDOWN); // ensure at least SHUTDOWN
else if (runState < SHUTDOWN ||
!submissionQueue.isEmpty() ||
(runState & ACTIVE_COUNT_MASK) != 0)
long c;
while (((c = ctl) & STOP_BIT) == 0) {
if (!now) {
if ((int)(c >> AC_SHIFT) != -parallelism)
return false;
if (advanceRunLevel(TERMINATING))
if (!shutdown || blockedCount != 0 || quiescerCount != 0 ||
queueBase != queueTop) {
if (ctl == c) // staleness check
return false;
continue;
}
}
if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, c | STOP_BIT))
startTerminating();
// Finish now if all threads terminated; else in some subsequent call
if ((workerCounts >>> TOTAL_COUNT_SHIFT) == 0) {
advanceRunLevel(TERMINATED);
termination.forceTermination();
}
if ((short)(c >>> TC_SHIFT) == -parallelism) { // signal when 0 workers
final ReentrantLock lock = this.submissionLock;
lock.lock();
try {
termination.signalAll();
} finally {
lock.unlock();
}
}
return true;
}
/**
* Actions on transition to TERMINATING
*
* Runs up to four passes through workers: (0) shutting down each
* (without waking up if parked) to quickly spread notifications
* without unnecessary bouncing around event queues etc (1) wake
* up and help cancel tasks (2) interrupt (3) mop up races with
* interrupted workers
* Runs up to three passes through workers: (0) Setting
* termination status for each worker, followed by wakeups up to
* queued workers; (1) helping cancel 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.
*/
private void startTerminating() {
cancelSubmissions();
for (int passes = 0; passes < 4 && workerCounts != 0; ++passes) {
int c; // advance event count
UNSAFE.compareAndSwapInt(this, eventCountOffset,
c = eventCount, c+1);
eventWaiters = 0L; // clobber lists
spareWaiters = 0;
for (ForkJoinWorkerThread w : workers) {
for (int pass = 0; pass < 3; ++pass) {
ForkJoinWorkerThread[] ws = workers;
if (ws != null) {
for (ForkJoinWorkerThread w : ws) {
if (w != null) {
w.shutdown();
if (passes > 0 && !w.isTerminated()) {
w.terminate = true;
if (pass > 0) {
w.cancelTasks();
LockSupport.unpark(w);
if (passes > 1 && !w.isInterrupted()) {
if (pass > 1 && !w.isInterrupted()) {
try {
w.interrupt();
} catch (SecurityException ignore) {
......@@ -1211,47 +1303,78 @@ public class ForkJoinPool extends AbstractExecutorService {
}
}
}
terminateWaiters();
}
}
}
/**
* Clears out and cancels submissions, ignoring exceptions.
* Polls and cancels all submissions. Called only during termination.
*/
private void cancelSubmissions() {
ForkJoinTask<?> task;
while ((task = submissionQueue.poll()) != null) {
while (queueBase != queueTop) {
ForkJoinTask<?> task = pollSubmission();
if (task != null) {
try {
task.cancel(false);
} catch (Throwable ignore) {
}
}
}
// misc support for ForkJoinWorkerThread
}
/**
* Returns pool number.
* Tries to set the termination status of waiting workers, and
* then wakes them up (after which they will terminate).
*/
final int getPoolNumber() {
return poolNumber;
private void terminateWaiters() {
ForkJoinWorkerThread[] ws = workers;
if (ws != null) {
ForkJoinWorkerThread w; long c; int i, e;
int n = ws.length;
while ((i = ~(e = (int)(c = ctl)) & SMASK) < n &&
(w = ws[i]) != null && w.eventCount == (e & E_MASK)) {
if (UNSAFE.compareAndSwapLong(this, ctlOffset, c,
(long)(w.nextWait & E_MASK) |
((c + AC_UNIT) & AC_MASK) |
(c & (TC_MASK|STOP_BIT)))) {
w.terminate = true;
w.eventCount = e + EC_UNIT;
if (w.parked)
UNSAFE.unpark(w);
}
}
}
}
// misc ForkJoinWorkerThread support
/**
* Tries to accumulate steal count from a worker, clearing
* the worker's value if successful.
* Increment or decrement quiescerCount. Needed only to prevent
* triggering shutdown if a worker is transiently inactive while
* checking quiescence.
*
* @return true if worker steal count now zero
* @param delta 1 for increment, -1 for decrement
*/
final boolean tryAccumulateStealCount(ForkJoinWorkerThread w) {
int sc = w.stealCount;
long c = stealCount;
// CAS even if zero, for fence effects
if (UNSAFE.compareAndSwapLong(this, stealCountOffset, c, c + sc)) {
if (sc != 0)
w.stealCount = 0;
return true;
final void addQuiescerCount(int delta) {
int c;
do {} while(!UNSAFE.compareAndSwapInt(this, quiescerCountOffset,
c = quiescerCount, c + delta));
}
return sc == 0;
/**
* Directly increment or decrement active count without
* queuing. This method is used to transiently assert inactivation
* while checking quiescence.
*
* @param delta 1 for increment, -1 for decrement
*/
final void addActiveCount(int delta) {
long d = delta < 0 ? -AC_UNIT : AC_UNIT;
long c;
do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl,
((c + d) & AC_MASK) |
(c & ~AC_MASK)));
}
/**
......@@ -1259,16 +1382,17 @@ public class ForkJoinPool extends AbstractExecutorService {
* active thread.
*/
final int idlePerActive() {
int pc = parallelism; // use parallelism, not rc
int ac = runState; // no mask -- artificially boosts during shutdown
// Use exact results for small values, saturate past 4
return ((pc <= ac) ? 0 :
(pc >>> 1 <= ac) ? 1 :
(pc >>> 2 <= ac) ? 3 :
pc >>> 3);
// Approximate at powers of two for small values, saturate past 4
int p = parallelism;
int a = p + (int)(ctl >> AC_SHIFT);
return (a > (p >>>= 1) ? 0 :
a > (p >>>= 1) ? 1 :
a > (p >>>= 1) ? 2 :
a > (p >>>= 1) ? 4 :
8);
}
// Public and protected methods
// Exported methods
// Constructors
......@@ -1337,49 +1461,42 @@ public class ForkJoinPool extends AbstractExecutorService {
checkPermission();
if (factory == null)
throw new NullPointerException();
if (parallelism <= 0 || parallelism > MAX_WORKERS)
if (parallelism <= 0 || parallelism > MAX_ID)
throw new IllegalArgumentException();
this.parallelism = parallelism;
this.factory = factory;
this.ueh = handler;
this.locallyFifo = asyncMode;
int arraySize = initialArraySizeFor(parallelism);
this.workers = new ForkJoinWorkerThread[arraySize];
this.submissionQueue = new LinkedTransferQueue<ForkJoinTask<?>>();
this.workerLock = new ReentrantLock();
this.termination = new Phaser(1);
this.poolNumber = poolNumberGenerator.incrementAndGet();
}
/**
* Returns initial power of two size for workers array.
* @param pc the initial parallelism level
*/
private static int initialArraySizeFor(int pc) {
// If possible, initially allocate enough space for one spare
int size = pc < MAX_WORKERS ? pc + 1 : MAX_WORKERS;
// See Hackers Delight, sec 3.2. We know MAX_WORKERS < (1 >>> 16)
size |= size >>> 1;
size |= size >>> 2;
size |= size >>> 4;
size |= size >>> 8;
return size + 1;
long np = (long)(-parallelism); // offset ctl counts
this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
this.submissionQueue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
// initialize workers array with room for 2*parallelism if possible
int n = parallelism << 1;
if (n >= MAX_ID)
n = MAX_ID;
else { // See Hackers Delight, sec 3.2, where n < (1 << 16)
n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8;
}
workers = new ForkJoinWorkerThread[n + 1];
this.submissionLock = new ReentrantLock();
this.termination = submissionLock.newCondition();
StringBuilder sb = new StringBuilder("ForkJoinPool-");
sb.append(poolNumberGenerator.incrementAndGet());
sb.append("-worker-");
this.workerNamePrefix = sb.toString();
}
// Execution methods
/**
* Submits task and creates, starts, or resumes some workers if necessary
*/
private <T> void doSubmit(ForkJoinTask<T> task) {
submissionQueue.offer(task);
int c; // try to increment event count -- CAS failure OK
UNSAFE.compareAndSwapInt(this, eventCountOffset, c = eventCount, c+1);
helpMaintainParallelism();
}
/**
* Performs the given task, returning its result upon completion.
* 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.
*
* @param task the task
* @return the task's result
......@@ -1388,16 +1505,16 @@ public class ForkJoinPool extends AbstractExecutorService {
* scheduled for execution
*/
public <T> T invoke(ForkJoinTask<T> task) {
Thread t = Thread.currentThread();
if (task == null)
throw new NullPointerException();
if (runState >= SHUTDOWN)
if (shutdown)
throw new RejectedExecutionException();
Thread t = Thread.currentThread();
if ((t instanceof ForkJoinWorkerThread) &&
((ForkJoinWorkerThread)t).pool == this)
return task.invoke(); // bypass submit if in same pool
else {
doSubmit(task);
addSubmission(task);
return task.join();
}
}
......@@ -1407,14 +1524,15 @@ public class ForkJoinPool extends AbstractExecutorService {
* computation in the current pool, else submits as external task.
*/
private <T> void forkOrSubmit(ForkJoinTask<T> task) {
if (runState >= SHUTDOWN)
throw new RejectedExecutionException();
ForkJoinWorkerThread w;
Thread t = Thread.currentThread();
if (shutdown)
throw new RejectedExecutionException();
if ((t instanceof ForkJoinWorkerThread) &&
((ForkJoinWorkerThread)t).pool == this)
task.fork();
(w = (ForkJoinWorkerThread)t).pool == this)
w.pushTask(task);
else
doSubmit(task);
addSubmission(task);
}
/**
......@@ -1571,7 +1689,7 @@ public class ForkJoinPool extends AbstractExecutorService {
* @return the number of worker threads
*/
public int getPoolSize() {
return workerCounts >>> TOTAL_COUNT_SHIFT;
return parallelism + (short)(ctl >>> TC_SHIFT);
}
/**
......@@ -1593,7 +1711,8 @@ public class ForkJoinPool extends AbstractExecutorService {
* @return the number of worker threads
*/
public int getRunningThreadCount() {
return workerCounts & RUNNING_COUNT_MASK;
int r = parallelism + (int)(ctl >> AC_SHIFT);
return r <= 0? 0 : r; // suppress momentarily negative values
}
/**
......@@ -1604,7 +1723,8 @@ public class ForkJoinPool extends AbstractExecutorService {
* @return the number of active threads
*/
public int getActiveThreadCount() {
return runState & ACTIVE_COUNT_MASK;
int r = parallelism + (int)(ctl >> AC_SHIFT) + blockedCount;
return r <= 0? 0 : r; // suppress momentarily negative values
}
/**
......@@ -1619,7 +1739,7 @@ public class ForkJoinPool extends AbstractExecutorService {
* @return {@code true} if all threads are currently idle
*/
public boolean isQuiescent() {
return (runState & ACTIVE_COUNT_MASK) == 0;
return parallelism + (int)(ctl >> AC_SHIFT) + blockedCount == 0;
}
/**
......@@ -1649,21 +1769,25 @@ public class ForkJoinPool extends AbstractExecutorService {
*/
public long getQueuedTaskCount() {
long count = 0;
for (ForkJoinWorkerThread w : workers)
ForkJoinWorkerThread[] ws;
if ((short)(ctl >>> TC_SHIFT) > -parallelism &&
(ws = workers) != null) {
for (ForkJoinWorkerThread w : ws)
if (w != null)
count += w.getQueueSize();
count -= w.queueBase - w.queueTop; // must read base first
}
return count;
}
/**
* Returns an estimate of the number of tasks submitted to this
* pool that have not yet begun executing. This method takes time
* proportional to the number of submissions.
* pool that have not yet begun executing. This method may take
* time proportional to the number of submissions.
*
* @return the number of queued submissions
*/
public int getQueuedSubmissionCount() {
return submissionQueue.size();
return -queueBase + queueTop;
}
/**
......@@ -1673,7 +1797,7 @@ public class ForkJoinPool extends AbstractExecutorService {
* @return {@code true} if there are any queued submissions
*/
public boolean hasQueuedSubmissions() {
return !submissionQueue.isEmpty();
return queueBase != queueTop;
}
/**
......@@ -1684,7 +1808,19 @@ public class ForkJoinPool extends AbstractExecutorService {
* @return the next submission, or {@code null} if none
*/
protected ForkJoinTask<?> pollSubmission() {
return submissionQueue.poll();
ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
while ((b = queueBase) != queueTop &&
(q = submissionQueue) != null &&
(i = (q.length - 1) & b) >= 0) {
long u = (i << ASHIFT) + ABASE;
if ((t = q[i]) != null &&
queueBase == b &&
UNSAFE.compareAndSwapObject(q, u, t, null)) {
queueBase = b + 1;
return t;
}
}
return null;
}
/**
......@@ -1705,10 +1841,21 @@ public class ForkJoinPool extends AbstractExecutorService {
* @return the number of elements transferred
*/
protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
int count = submissionQueue.drainTo(c);
for (ForkJoinWorkerThread w : workers)
int count = 0;
while (queueBase != queueTop) {
ForkJoinTask<?> t = pollSubmission();
if (t != null) {
c.add(t);
++count;
}
}
ForkJoinWorkerThread[] ws;
if ((short)(ctl >>> TC_SHIFT) > -parallelism &&
(ws = workers) != null) {
for (ForkJoinWorkerThread w : ws)
if (w != null)
count += w.drainTasksTo(c);
}
return count;
}
......@@ -1723,14 +1870,20 @@ public class ForkJoinPool extends AbstractExecutorService {
long st = getStealCount();
long qt = getQueuedTaskCount();
long qs = getQueuedSubmissionCount();
int wc = workerCounts;
int tc = wc >>> TOTAL_COUNT_SHIFT;
int rc = wc & RUNNING_COUNT_MASK;
int pc = parallelism;
int rs = runState;
int ac = rs & ACTIVE_COUNT_MASK;
long c = ctl;
int tc = pc + (short)(c >>> TC_SHIFT);
int rc = pc + (int)(c >> AC_SHIFT);
if (rc < 0) // ignore transient negative
rc = 0;
int ac = rc + blockedCount;
String level;
if ((c & STOP_BIT) != 0)
level = (tc == 0)? "Terminated" : "Terminating";
else
level = shutdown? "Shutting down" : "Running";
return super.toString() +
"[" + runLevelToString(rs) +
"[" + level +
", parallelism = " + pc +
", size = " + tc +
", active = " + ac +
......@@ -1741,13 +1894,6 @@ public class ForkJoinPool extends AbstractExecutorService {
"]";
}
private static String runLevelToString(int s) {
return ((s & TERMINATED) != 0 ? "Terminated" :
((s & TERMINATING) != 0 ? "Terminating" :
((s & SHUTDOWN) != 0 ? "Shutting down" :
"Running")));
}
/**
* Initiates an orderly shutdown in which previously submitted
* tasks are executed, but no new tasks will be accepted.
......@@ -1762,7 +1908,7 @@ public class ForkJoinPool extends AbstractExecutorService {
*/
public void shutdown() {
checkPermission();
advanceRunLevel(SHUTDOWN);
shutdown = true;
tryTerminate(false);
}
......@@ -1784,6 +1930,7 @@ public class ForkJoinPool extends AbstractExecutorService {
*/
public List<Runnable> shutdownNow() {
checkPermission();
shutdown = true;
tryTerminate(true);
return Collections.emptyList();
}
......@@ -1794,7 +1941,9 @@ public class ForkJoinPool extends AbstractExecutorService {
* @return {@code true} if all tasks have completed following shut down
*/
public boolean isTerminated() {
return runState >= TERMINATED;
long c = ctl;
return ((c & STOP_BIT) != 0L &&
(short)(c >>> TC_SHIFT) == -parallelism);
}
/**
......@@ -1811,14 +1960,16 @@ public class ForkJoinPool extends AbstractExecutorService {
* @return {@code true} if terminating but not yet terminated
*/
public boolean isTerminating() {
return (runState & (TERMINATING|TERMINATED)) == TERMINATING;
long c = ctl;
return ((c & STOP_BIT) != 0L &&
(short)(c >>> TC_SHIFT) != -parallelism);
}
/**
* Returns true if terminating or terminated. Used by ForkJoinWorkerThread.
*/
final boolean isAtLeastTerminating() {
return runState >= TERMINATING;
return (ctl & STOP_BIT) != 0L;
}
/**
......@@ -1827,7 +1978,7 @@ public class ForkJoinPool extends AbstractExecutorService {
* @return {@code true} if this pool has been shut down
*/
public boolean isShutdown() {
return runState >= SHUTDOWN;
return shutdown;
}
/**
......@@ -1843,12 +1994,20 @@ public class ForkJoinPool extends AbstractExecutorService {
*/
public boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException {
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.submissionLock;
lock.lock();
try {
termination.awaitAdvanceInterruptibly(0, timeout, unit);
} catch (TimeoutException ex) {
for (;;) {
if (isTerminated())
return true;
if (nanos <= 0)
return false;
nanos = termination.awaitNanos(nanos);
}
} finally {
lock.unlock();
}
return true;
}
/**
......@@ -1859,13 +2018,15 @@ public class ForkJoinPool extends AbstractExecutorService {
* {@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}
* before actually blocking). 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.
* 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.
*
* <p>For example, here is a ManagedBlocker based on a
* ReentrantLock:
......@@ -1967,29 +2128,47 @@ public class ForkJoinPool extends AbstractExecutorService {
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long workerCountsOffset =
objectFieldOffset("workerCounts", ForkJoinPool.class);
private static final long runStateOffset =
objectFieldOffset("runState", ForkJoinPool.class);
private static final long eventCountOffset =
objectFieldOffset("eventCount", ForkJoinPool.class);
private static final long eventWaitersOffset =
objectFieldOffset("eventWaiters", ForkJoinPool.class);
private static final long stealCountOffset =
objectFieldOffset("stealCount", ForkJoinPool.class);
private static final long spareWaitersOffset =
objectFieldOffset("spareWaiters", ForkJoinPool.class);
private static long objectFieldOffset(String field, Class<?> klazz) {
private static final sun.misc.Unsafe UNSAFE;
private static final long ctlOffset;
private static final long stealCountOffset;
private static final long blockedCountOffset;
private static final long quiescerCountOffset;
private static final long scanGuardOffset;
private static final long nextWorkerNumberOffset;
private static final long ABASE;
private static final int ASHIFT;
static {
poolNumberGenerator = new AtomicInteger();
workerSeedGenerator = new Random();
modifyThreadPermission = new RuntimePermission("modifyThread");
defaultForkJoinWorkerThreadFactory =
new DefaultForkJoinWorkerThreadFactory();
int s;
try {
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
} catch (NoSuchFieldException e) {
// Convert Exception to corresponding Error
NoSuchFieldError error = new NoSuchFieldError(field);
error.initCause(e);
throw error;
}
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class k = ForkJoinPool.class;
ctlOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("ctl"));
stealCountOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("stealCount"));
blockedCountOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("blockedCount"));
quiescerCountOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("quiescerCount"));
scanGuardOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("scanGuard"));
nextWorkerNumberOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("nextWorkerNumber"));
Class a = ForkJoinTask[].class;
ABASE = UNSAFE.arrayBaseOffset(a);
s = UNSAFE.arrayIndexScale(a);
} 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);
}
}
src/share/classes/java/util/concurrent/ForkJoinTask.java
浏览文件 @ 32c540e9
......@@ -41,7 +41,8 @@ import java.util.Collections;
import java.util.List;
import java.util.RandomAccess;
import java.util.Map;
import java.util.WeakHashMap;
import java.lang.ref.WeakReference;
import java.lang.ref.ReferenceQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
......@@ -52,6 +53,8 @@ import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.ReentrantLock;
import java.lang.reflect.Constructor;
/**
* Abstract base class for tasks that run within a {@link ForkJoinPool}.
......@@ -95,7 +98,11 @@ import java.util.concurrent.TimeoutException;
* rethrown to callers attempting to join them. These exceptions may
* additionally include {@link RejectedExecutionException} stemming
* from internal resource exhaustion, such as failure to allocate
* internal task queues.
* internal task queues. 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 thread that initiated the computation as well as the thread
* actually encountering the exception; minimally only the latter.
*
* <p>The primary method for awaiting completion and extracting
* results of a task is {@link #join}, but there are several variants:
......@@ -192,8 +199,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
* status maintenance (2) execution and awaiting completion (3)
* user-level methods that additionally report results. This is
* sometimes hard to see because this file orders exported methods
* in a way that flows well in javadocs. In particular, most
* join mechanics are in method quietlyJoin, below.
* in a way that flows well in javadocs.
*/
/*
......@@ -215,91 +221,67 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
/** The run status of this task */
volatile int status; // accessed directly by pool and workers
private static final int NORMAL = -1;
private static final int CANCELLED = -2;
private static final int EXCEPTIONAL = -3;
private static final int SIGNAL = 1;
/**
* Table of exceptions thrown by tasks, to enable reporting by
* callers. Because exceptions are rare, we don't directly keep
* them with task objects, but instead use a weak ref table. Note
* that cancellation exceptions don't appear in the table, but are
* instead recorded as status values.
* TODO: Use ConcurrentReferenceHashMap
*/
static final Map<ForkJoinTask<?>, Throwable> exceptionMap =
Collections.synchronizedMap
(new WeakHashMap<ForkJoinTask<?>, Throwable>());
// Maintaining completion status
/**
* Marks completion and wakes up threads waiting to join this task,
* also clearing signal request bits.
*
* @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
* @return completion status on exit
*/
private void setCompletion(int completion) {
int s;
while ((s = status) >= 0) {
private int setCompletion(int completion) {
for (int s;;) {
if ((s = status) < 0)
return s;
if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) {
if (s != 0)
synchronized (this) { notifyAll(); }
break;
return completion;
}
}
}
/**
* Records exception and sets exceptional completion.
* Tries to block a worker thread until completed or timed out.
* Uses Object.wait time argument conventions.
* May fail on contention or interrupt.
*
* @return status on exit
* @param millis if > 0, wait time.
*/
private void setExceptionalCompletion(Throwable rex) {
exceptionMap.put(this, rex);
setCompletion(EXCEPTIONAL);
}
/**
* Blocks a worker thread until completed or timed out. Called
* only by pool.
*/
final void internalAwaitDone(long millis, int nanos) {
int s = status;
if ((s == 0 &&
UNSAFE.compareAndSwapInt(this, statusOffset, 0, SIGNAL)) ||
s > 0) {
try { // the odd construction reduces lock bias effects
final void tryAwaitDone(long millis) {
int s;
try {
if (((s = status) > 0 ||
(s == 0 &&
UNSAFE.compareAndSwapInt(this, statusOffset, 0, SIGNAL))) &&
status > 0) {
synchronized (this) {
if (status > 0)
wait(millis, nanos);
else
notifyAll();
wait(millis);
}
} catch (InterruptedException ie) {
cancelIfTerminating();
}
} catch (InterruptedException ie) {
// caller must check termination
}
}
/**
* Blocks a non-worker-thread until completion.
* @return status upon completion
*/
private void externalAwaitDone() {
if (status >= 0) {
private int externalAwaitDone() {
int s;
if ((s = status) >= 0) {
boolean interrupted = false;
synchronized (this) {
for (;;) {
int s = status;
while ((s = status) >= 0) {
if (s == 0)
UNSAFE.compareAndSwapInt(this, statusOffset,
0, SIGNAL);
else if (s < 0) {
notifyAll();
break;
}
else {
try {
wait();
......@@ -312,54 +294,309 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
if (interrupted)
Thread.currentThread().interrupt();
}
return s;
}
/**
* Blocks a non-worker-thread until completion or interruption or timeout.
*/
private void externalInterruptibleAwaitDone(boolean timed, long nanos)
private int externalInterruptibleAwaitDone(long millis)
throws InterruptedException {
int s;
if (Thread.interrupted())
throw new InterruptedException();
if (status >= 0) {
long startTime = timed ? System.nanoTime() : 0L;
if ((s = status) >= 0) {
synchronized (this) {
for (;;) {
long nt;
int s = status;
while ((s = status) >= 0) {
if (s == 0)
UNSAFE.compareAndSwapInt(this, statusOffset,
0, SIGNAL);
else if (s < 0) {
notifyAll();
else {
wait(millis);
if (millis > 0L)
break;
}
else if (!timed)
wait();
else if ((nt = nanos - (System.nanoTime()-startTime)) > 0L)
wait(nt / 1000000, (int)(nt % 1000000));
else
break;
}
}
}
return s;
}
/**
* Unless done, calls exec and records status if completed, but
* doesn't wait for completion otherwise. Primary execution method
* for ForkJoinWorkerThread.
* Primary execution method for stolen tasks. Unless done, calls
* exec and records status if completed, but doesn't wait for
* completion otherwise.
*/
final void quietlyExec() {
final void doExec() {
if (status >= 0) {
boolean completed;
try {
if (status < 0 || !exec())
return;
completed = exec();
} catch (Throwable rex) {
setExceptionalCompletion(rex);
return;
}
if (completed)
setCompletion(NORMAL); // must be outside try block
}
}
/**
* Primary mechanics for join, get, quietlyJoin.
* @return status upon completion
*/
private int doJoin() {
Thread t; ForkJoinWorkerThread w; int s; boolean completed;
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
if ((s = status) < 0)
return s;
if ((w = (ForkJoinWorkerThread)t).unpushTask(this)) {
try {
completed = exec();
} catch (Throwable rex) {
return setExceptionalCompletion(rex);
}
if (completed)
return setCompletion(NORMAL);
}
return w.joinTask(this);
}
else
return externalAwaitDone();
}
/**
* Primary mechanics for invoke, quietlyInvoke.
* @return status upon completion
*/
private int doInvoke() {
int s; boolean completed;
if ((s = status) < 0)
return s;
try {
completed = exec();
} catch (Throwable rex) {
return setExceptionalCompletion(rex);
}
if (completed)
return setCompletion(NORMAL);
else
return doJoin();
}
// Exception table support
/**
* Table of exceptions thrown by tasks, to enable reporting by
* callers. Because exceptions are rare, we don't directly keep
* them with task objects, but instead use a weak ref table. Note
* that cancellation exceptions don't appear in the table, but are
* instead recorded as status values.
*
* Note: These statics are initialized below in static block.
*/
private static final ExceptionNode[] exceptionTable;
private static final ReentrantLock exceptionTableLock;
private static final ReferenceQueue<Object> exceptionTableRefQueue;
/**
* Fixed capacity for exceptionTable.
*/
private static final int EXCEPTION_MAP_CAPACITY = 32;
/**
* Key-value nodes for exception table. The chained hash table
* uses identity comparisons, full locking, and weak references
* for keys. The table has a fixed capacity because it only
* maintains task exceptions long enough for joiners to access
* them, so should never become very large for sustained
* periods. However, since we do not know when the last joiner
* completes, we must use weak references and expunge them. We do
* so on each operation (hence full locking). Also, some thread in
* any ForkJoinPool will call helpExpungeStaleExceptions when its
* pool becomes isQuiescent.
*/
static final class ExceptionNode extends WeakReference<ForkJoinTask<?>>{
final Throwable ex;
ExceptionNode next;
final long thrower; // use id not ref to avoid weak cycles
ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next) {
super(task, exceptionTableRefQueue);
this.ex = ex;
this.next = next;
this.thrower = Thread.currentThread().getId();
}
}
/**
* Records exception and sets exceptional completion.
*
* @return status on exit
*/
private int setExceptionalCompletion(Throwable ex) {
int h = System.identityHashCode(this);
final ReentrantLock lock = exceptionTableLock;
lock.lock();
try {
expungeStaleExceptions();
ExceptionNode[] t = exceptionTable;
int i = h & (t.length - 1);
for (ExceptionNode e = t[i]; ; e = e.next) {
if (e == null) {
t[i] = new ExceptionNode(this, ex, t[i]);
break;
}
if (e.get() == this) // already present
break;
}
} finally {
lock.unlock();
}
return setCompletion(EXCEPTIONAL);
}
/**
* Removes exception node and clears status
*/
private void clearExceptionalCompletion() {
int h = System.identityHashCode(this);
final ReentrantLock lock = exceptionTableLock;
lock.lock();
try {
ExceptionNode[] t = exceptionTable;
int i = h & (t.length - 1);
ExceptionNode e = t[i];
ExceptionNode pred = null;
while (e != null) {
ExceptionNode next = e.next;
if (e.get() == this) {
if (pred == null)
t[i] = next;
else
pred.next = next;
break;
}
pred = e;
e = next;
}
expungeStaleExceptions();
status = 0;
} finally {
lock.unlock();
}
}
/**
* Returns a rethrowable exception for the given task, if
* available. To provide accurate stack traces, if the exception
* was not thrown by the current thread, we try to create a new
* exception of the same type as the one thrown, but with the
* recorded exception as its cause. If there is no such
* constructor, we instead try to use a no-arg constructor,
* followed by initCause, to the same effect. If none of these
* apply, or any fail due to other exceptions, we return the
* recorded exception, which is still correct, although it may
* contain a misleading stack trace.
*
* @return the exception, or null if none
*/
private Throwable getThrowableException() {
if (status != EXCEPTIONAL)
return null;
int h = System.identityHashCode(this);
ExceptionNode e;
final ReentrantLock lock = exceptionTableLock;
lock.lock();
try {
expungeStaleExceptions();
ExceptionNode[] t = exceptionTable;
e = t[h & (t.length - 1)];
while (e != null && e.get() != this)
e = e.next;
} finally {
lock.unlock();
}
Throwable ex;
if (e == null || (ex = e.ex) == null)
return null;
if (e.thrower != Thread.currentThread().getId()) {
Class ec = ex.getClass();
try {
Constructor<?> noArgCtor = null;
Constructor<?>[] cs = ec.getConstructors();// public ctors only
for (int i = 0; i < cs.length; ++i) {
Constructor<?> c = cs[i];
Class<?>[] ps = c.getParameterTypes();
if (ps.length == 0)
noArgCtor = c;
else if (ps.length == 1 && ps[0] == Throwable.class)
return (Throwable)(c.newInstance(ex));
}
if (noArgCtor != null) {
Throwable wx = (Throwable)(noArgCtor.newInstance());
wx.initCause(ex);
return wx;
}
} catch (Exception ignore) {
}
}
return ex;
}
/**
* Poll stale refs and remove them. Call only while holding lock.
*/
private static void expungeStaleExceptions() {
for (Object x; (x = exceptionTableRefQueue.poll()) != null;) {
if (x instanceof ExceptionNode) {
ForkJoinTask<?> key = ((ExceptionNode)x).get();
ExceptionNode[] t = exceptionTable;
int i = System.identityHashCode(key) & (t.length - 1);
ExceptionNode e = t[i];
ExceptionNode pred = null;
while (e != null) {
ExceptionNode next = e.next;
if (e == x) {
if (pred == null)
t[i] = next;
else
pred.next = next;
break;
}
pred = e;
e = next;
}
}
}
}
/**
* If lock is available, poll stale refs and remove them.
* Called from ForkJoinPool when pools become quiescent.
*/
static final void helpExpungeStaleExceptions() {
final ReentrantLock lock = exceptionTableLock;
if (lock.tryLock()) {
try {
expungeStaleExceptions();
} finally {
lock.unlock();
}
}
}
/**
* Report the result of invoke or join; called only upon
* non-normal return of internal versions.
*/
private V reportResult() {
int s; Throwable ex;
if ((s = status) == CANCELLED)
throw new CancellationException();
if (s == EXCEPTIONAL && (ex = getThrowableException()) != null)
UNSAFE.throwException(ex);
return getRawResult();
}
// public methods
......@@ -399,10 +636,9 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
* @return the computed result
*/
public final V join() {
quietlyJoin();
Throwable ex;
if (status < NORMAL && (ex = getException()) != null)
UNSAFE.throwException(ex);
if (doJoin() != NORMAL)
return reportResult();
else
return getRawResult();
}
......@@ -415,10 +651,9 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
* @return the computed result
*/
public final V invoke() {
quietlyInvoke();
Throwable ex;
if (status < NORMAL && (ex = getException()) != null)
UNSAFE.throwException(ex);
if (doInvoke() != NORMAL)
return reportResult();
else
return getRawResult();
}
......@@ -483,24 +718,18 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
}
else if (i != 0)
t.fork();
else {
t.quietlyInvoke();
if (ex == null && t.status < NORMAL)
else if (t.doInvoke() < NORMAL && ex == null)
ex = t.getException();
}
}
for (int i = 1; i <= last; ++i) {
ForkJoinTask<?> t = tasks[i];
if (t != null) {
if (ex != null)
t.cancel(false);
else {
t.quietlyJoin();
if (ex == null && t.status < NORMAL)
else if (t.doJoin() < NORMAL && ex == null)
ex = t.getException();
}
}
}
if (ex != null)
UNSAFE.throwException(ex);
}
......@@ -546,24 +775,18 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
}
else if (i != 0)
t.fork();
else {
t.quietlyInvoke();
if (ex == null && t.status < NORMAL)
else if (t.doInvoke() < NORMAL && ex == null)
ex = t.getException();
}
}
for (int i = 1; i <= last; ++i) {
ForkJoinTask<?> t = ts.get(i);
if (t != null) {
if (ex != null)
t.cancel(false);
else {
t.quietlyJoin();
if (ex == null && t.status < NORMAL)
else if (t.doJoin() < NORMAL && ex == null)
ex = t.getException();
}
}
}
if (ex != null)
UNSAFE.throwException(ex);
return tasks;
......@@ -597,8 +820,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
* @return {@code true} if this task is now cancelled
*/
public boolean cancel(boolean mayInterruptIfRunning) {
setCompletion(CANCELLED);
return status == CANCELLED;
return setCompletion(CANCELLED) == CANCELLED;
}
/**
......@@ -614,21 +836,6 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
}
}
/**
* Cancels if current thread is a terminating worker thread,
* ignoring any exceptions thrown by cancel.
*/
final void cancelIfTerminating() {
Thread t = Thread.currentThread();
if ((t instanceof ForkJoinWorkerThread) &&
((ForkJoinWorkerThread) t).isTerminating()) {
try {
cancel(false);
} catch (Throwable ignore) {
}
}
}
public final boolean isDone() {
return status < 0;
}
......@@ -668,7 +875,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
int s = status;
return ((s >= NORMAL) ? null :
(s == CANCELLED) ? new CancellationException() :
exceptionMap.get(this));
getThrowableException());
}
/**
......@@ -726,19 +933,13 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
* member of a ForkJoinPool and was interrupted while waiting
*/
public final V get() throws InterruptedException, ExecutionException {
Thread t = Thread.currentThread();
if (t instanceof ForkJoinWorkerThread)
quietlyJoin();
else
externalInterruptibleAwaitDone(false, 0L);
int s = status;
if (s != NORMAL) {
int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
doJoin() : externalInterruptibleAwaitDone(0L);
Throwable ex;
if (s == CANCELLED)
throw new CancellationException();
if (s == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null)
if (s == EXCEPTIONAL && (ex = getThrowableException()) != null)
throw new ExecutionException(ex);
}
return getRawResult();
}
......@@ -758,20 +959,39 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
*/
public final V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
long nanos = unit.toNanos(timeout);
Thread t = Thread.currentThread();
if (t instanceof ForkJoinWorkerThread)
((ForkJoinWorkerThread)t).joinTask(this, true, nanos);
else
externalInterruptibleAwaitDone(true, nanos);
if (t instanceof ForkJoinWorkerThread) {
ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
long nanos = unit.toNanos(timeout);
if (status >= 0) {
boolean completed = false;
if (w.unpushTask(this)) {
try {
completed = exec();
} catch (Throwable rex) {
setExceptionalCompletion(rex);
}
}
if (completed)
setCompletion(NORMAL);
else if (status >= 0 && nanos > 0)
w.pool.timedAwaitJoin(this, nanos);
}
}
else {
long millis = unit.toMillis(timeout);
if (millis > 0)
externalInterruptibleAwaitDone(millis);
}
int s = status;
if (s != NORMAL) {
Throwable ex;
if (s == CANCELLED)
throw new CancellationException();
if (s == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null)
throw new ExecutionException(ex);
if (s != EXCEPTIONAL)
throw new TimeoutException();
if ((ex = getThrowableException()) != null)
throw new ExecutionException(ex);
}
return getRawResult();
}
......@@ -783,28 +1003,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
* known to have aborted.
*/
public final void quietlyJoin() {
Thread t;
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
if (status >= 0) {
if (w.unpushTask(this)) {
boolean completed;
try {
completed = exec();
} catch (Throwable rex) {
setExceptionalCompletion(rex);
return;
}
if (completed) {
setCompletion(NORMAL);
return;
}
}
w.joinTask(this, false, 0L);
}
}
else
externalAwaitDone();
doJoin();
}
/**
......@@ -813,19 +1012,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
* exception.
*/
public final void quietlyInvoke() {
if (status >= 0) {
boolean completed;
try {
completed = exec();
} catch (Throwable rex) {
setExceptionalCompletion(rex);
return;
}
if (completed)
setCompletion(NORMAL);
else
quietlyJoin();
}
doInvoke();
}
/**
......@@ -864,7 +1051,8 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
*/
public void reinitialize() {
if (status == EXCEPTIONAL)
exceptionMap.remove(this);
clearExceptionalCompletion();
else
status = 0;
}
......@@ -1176,23 +1364,23 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
s.defaultReadObject();
Object ex = s.readObject();
if (ex != null)
setExceptionalCompletion((Throwable) ex);
setExceptionalCompletion((Throwable)ex);
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long statusOffset =
objectFieldOffset("status", ForkJoinTask.class);
private static long objectFieldOffset(String field, Class<?> klazz) {
private static final sun.misc.Unsafe UNSAFE;
private static final long statusOffset;
static {
exceptionTableLock = new ReentrantLock();
exceptionTableRefQueue = new ReferenceQueue<Object>();
exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY];
try {
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
} catch (NoSuchFieldException e) {
// Convert Exception to corresponding Error
NoSuchFieldError error = new NoSuchFieldError(field);
error.initCause(e);
throw error;
UNSAFE = sun.misc.Unsafe.getUnsafe();
statusOffset = UNSAFE.objectFieldOffset
(ForkJoinTask.class.getDeclaredField("status"));
} catch (Exception e) {
throw new Error(e);
}
}
}
src/share/classes/java/util/concurrent/ForkJoinWorkerThread.java
浏览文件 @ 32c540e9
......@@ -35,9 +35,7 @@
package java.util.concurrent;
import java.util.Random;
import java.util.Collection;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.RejectedExecutionException;
/**
......@@ -84,33 +82,38 @@ public class ForkJoinWorkerThread extends Thread {
* a footprint as possible even in programs generating huge
* numbers of tasks. To accomplish this, we shift the CAS
* arbitrating pop vs deq (steal) from being on the indices
* ("base" and "sp") to the slots themselves (mainly via method
* "casSlotNull()"). So, both a successful pop and deq 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 sp.
* 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 sp == 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
* deq have not fully committed. Note that this means that the deq
* 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
* ("queueBase" and "queueTop") to the slots themselves (mainly
* via method "casSlotNull()"). So, both a successful pop and deq
* 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
* queueBase or queueTop. 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 queueTop == queueBase means the queue is empty,
* but otherwise may err on the side of possibly making the queue
* appear nonempty when a push, pop, or deq have not fully
* committed. Note that this means that the deq 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 deq or new push on
* any empty queue to complete. One reason this works well here is
* that apparently-nonempty often means soon-to-be-stealable,
* which gives threads a chance to set activation status if
* necessary before stealing.
* any empty queue to complete.
*
* This approach also enables support for "async mode" where local
* task processing is in FIFO, not LIFO order; simply by using a
* version of deq rather than pop when locallyFifo is true (as set
* by the ForkJoinPool). This allows use in message-passing
* frameworks in which tasks are never joined.
* 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.)
*
* When a worker would otherwise be blocked waiting to join a
* task, it first tries a form of linear helping: Each worker
......@@ -137,29 +140,26 @@ public class ForkJoinWorkerThread extends Thread {
* 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_DEPTH) and fall back to suspending the worker and if
* necessary replacing it with a spare (see
* ForkJoinPool.awaitJoin).
* MAX_HELP) and fall back to suspending the worker and if
* necessary replacing it with another.
*
* Efficient implementation of these algorithms currently relies
* on an uncomfortable amount of "Unsafe" mechanics. To maintain
* correct orderings, reads and writes of variable base require
* volatile ordering. Variable sp does not require volatile
* writes but still needs store-ordering, which we accomplish by
* pre-incrementing sp before filling the slot with an ordered
* store. (Pre-incrementing also enables backouts used in
* joinTask.) Because they are protected by volatile base reads,
* reads of the queue array and its slots by other threads do not
* need volatile load semantics, but writes (in push) require
* store order and CASes (in pop and deq) require (volatile) CAS
* semantics. (Michael, Saraswat, and Vechev's algorithm has
* similar properties, but without support for nulling slots.)
* Since these combinations aren't supported using ordinary
* volatiles, the only way to accomplish these efficiently is to
* use direct Unsafe calls. (Using external AtomicIntegers and
* AtomicReferenceArrays for the indices and array is
* significantly slower because of memory locality and indirection
* effects.)
* correct orderings, reads and writes of variable queueBase
* require volatile ordering. Variable queueTop need not be
* volatile because non-local reads always follow those of
* queueBase. Similarly, because they are protected by volatile
* queueBase reads, reads of the queue array and its slots by
* other threads do not need volatile load semantics, but writes
* (in push) require store order and CASes (in pop and deq)
* require (volatile) CAS semantics. (Michael, Saraswat, and
* Vechev's algorithm has similar properties, but without support
* for nulling slots.) Since these combinations aren't supported
* using ordinary volatiles, the only way to accomplish these
* efficiently is to use direct Unsafe calls. (Using external
* AtomicIntegers and AtomicReferenceArrays for the indices and
* array is significantly slower because of memory locality and
* indirection effects.)
*
* Further, performance on most platforms is very sensitive to
* placement and sizing of the (resizable) queue array. Even
......@@ -167,30 +167,13 @@ public class ForkJoinWorkerThread extends Thread {
* initial size must be large enough to counteract cache
* contention effects across multiple queues (especially in the
* presence of GC cardmarking). Also, to improve thread-locality,
* queues are initialized after starting. All together, these
* low-level implementation choices produce as much as a factor of
* 4 performance improvement compared to naive implementations,
* and enable the processing of billions of tasks per second,
* sometimes at the expense of ugliness.
* queues are initialized after starting.
*/
/**
* Generator for initial random seeds for random victim
* selection. This is used only to create initial seeds. Random
* steals use a cheaper xorshift generator per steal attempt. We
* expect only rare contention on seedGenerator, so just use a
* plain Random.
* Mask for pool indices encoded as shorts
*/
private static final Random seedGenerator = new Random();
/**
* The maximum stolen->joining link depth allowed in helpJoinTask.
* Depths for legitimate chains are unbounded, but we use a fixed
* constant to avoid (otherwise unchecked) cycles and bound
* staleness of traversal parameters at the expense of sometimes
* blocking when we could be helping.
*/
private static final int MAX_HELP_DEPTH = 8;
private static final int SMASK = 0xffff;
/**
* Capacity of work-stealing queue array upon initialization.
......@@ -200,12 +183,19 @@ public class ForkJoinWorkerThread extends Thread {
private static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
/**
* Maximum work-stealing queue array size. Must be less than or
* equal to 1 << (31 - width of array entry) to ensure lack of
* index wraparound. The value is set in the static block
* at the end of this file after obtaining width.
* Maximum size for queue array. Must be a power of two
* less than or equal to 1 << (31 - width of array entry) to
* ensure lack of index wraparound, but is capped at a lower
* value to help users trap runaway computations.
*/
private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M
/**
* The work-stealing queue array. Size must be a power of two.
* Initialized when started (as oposed to when constructed), to
* improve memory locality.
*/
private static final int MAXIMUM_QUEUE_CAPACITY;
ForkJoinTask<?>[] queue;
/**
* The pool this thread works in. Accessed directly by ForkJoinTask.
......@@ -213,25 +203,19 @@ public class ForkJoinWorkerThread extends Thread {
final ForkJoinPool pool;
/**
* The work-stealing queue array. Size must be a power of two.
* Initialized in onStart, to improve memory locality.
* Index (mod queue.length) of next queue slot to push to or pop
* from. It is written only by owner thread, and accessed by other
* threads only after reading (volatile) queueBase. Both queueTop
* and queueBase are allowed to wrap around on overflow, but
* (queueTop - queueBase) still estimates size.
*/
private ForkJoinTask<?>[] queue;
int queueTop;
/**
* Index (mod queue.length) of least valid queue slot, which is
* always the next position to steal from if nonempty.
*/
private volatile int base;
/**
* Index (mod queue.length) of next queue slot to push to or pop
* from. It is written only by owner thread, and accessed by other
* threads only after reading (volatile) base. Both sp and base
* are allowed to wrap around on overflow, but (sp - base) still
* estimates size.
*/
private int sp;
volatile int queueBase;
/**
* The index of most recent stealer, used as a hint to avoid
......@@ -240,92 +224,68 @@ public class ForkJoinWorkerThread extends Thread {
* of them (usually the most current). Declared non-volatile,
* relying on other prevailing sync to keep reasonably current.
*/
private int stealHint;
/**
* Run state of this worker. In addition to the usual run levels,
* tracks if this worker is suspended as a spare, and if it was
* killed (trimmed) while suspended. However, "active" status is
* maintained separately and modified only in conjunction with
* CASes of the pool's runState (which are currently sadly
* manually inlined for performance.) Accessed directly by pool
* to simplify checks for normal (zero) status.
*/
volatile int runState;
private static final int TERMINATING = 0x01;
private static final int TERMINATED = 0x02;
private static final int SUSPENDED = 0x04; // inactive spare
private static final int TRIMMED = 0x08; // killed while suspended
int stealHint;
/**
* Number of steals. Directly accessed (and reset) by
* pool.tryAccumulateStealCount when idle.
* Index of this worker in pool array. Set once by pool before
* running, and accessed directly by pool to locate this worker in
* its workers array.
*/
int stealCount;
final int poolIndex;
/**
* Seed for random number generator for choosing steal victims.
* Uses Marsaglia xorshift. Must be initialized as nonzero.
* Encoded record for pool task waits. Usages are always
* surrounded by volatile reads/writes
*/
private int seed;
int nextWait;
/**
* Activity status. When true, this worker is considered active.
* Accessed directly by pool. Must be false upon construction.
* Complement of poolIndex, offset by count of entries of task
* waits. Accessed by ForkJoinPool to manage event waiters.
*/
boolean active;
volatile int eventCount;
/**
* True if use local fifo, not default lifo, for local polling.
* Shadows value from ForkJoinPool.
* Seed for random number generator for choosing steal victims.
* Uses Marsaglia xorshift. Must be initialized as nonzero.
*/
private final boolean locallyFifo;
int seed;
/**
* Index of this worker in pool array. Set once by pool before
* running, and accessed directly by pool to locate this worker in
* its workers array.
* Number of steals. Directly accessed (and reset) by pool when
* idle.
*/
int poolIndex;
int stealCount;
/**
* The last pool event waited for. Accessed only by pool in
* callback methods invoked within this thread.
* True if this worker should or did terminate
*/
int lastEventCount;
volatile boolean terminate;
/**
* Encoded index and event count of next event waiter. Accessed
* only by ForkJoinPool for managing event waiters.
* Set to true before LockSupport.park; false on return
*/
volatile long nextWaiter;
volatile boolean parked;
/**
* Number of times this thread suspended as spare. Accessed only
* by pool.
* True if use local fifo, not default lifo, for local polling.
* Shadows value from ForkJoinPool.
*/
int spareCount;
final boolean locallyFifo;
/**
* Encoded index and count of next spare waiter. Accessed only
* by ForkJoinPool for managing spares.
* The task most recently stolen from another worker (or
* submission queue). All uses are surrounded by enough volatile
* reads/writes to maintain as non-volatile.
*/
volatile int nextSpare;
ForkJoinTask<?> currentSteal;
/**
* The task currently being joined, set only when actively trying
* to help other stealers in helpJoinTask. Written only by this
* thread, but read by others.
* to help other stealers in helpJoinTask. All uses are surrounded
* by enough volatile reads/writes to maintain as non-volatile.
*/
private volatile ForkJoinTask<?> currentJoin;
/**
* The task most recently stolen from another worker (or
* submission queue). Written only by this thread, but read by
* others.
*/
private volatile ForkJoinTask<?> currentSteal;
ForkJoinTask<?> currentJoin;
/**
* Creates a ForkJoinWorkerThread operating in the given pool.
......@@ -334,24 +294,19 @@ public class ForkJoinWorkerThread extends Thread {
* @throws NullPointerException if pool is null
*/
protected ForkJoinWorkerThread(ForkJoinPool pool) {
super(pool.nextWorkerName());
this.pool = pool;
this.locallyFifo = pool.locallyFifo;
setDaemon(true);
// To avoid exposing construction details to subclasses,
// remaining initialization is in start() and onStart()
}
/**
* Performs additional initialization and starts this thread.
*/
final void start(int poolIndex, UncaughtExceptionHandler ueh) {
this.poolIndex = poolIndex;
int k = pool.registerWorker(this);
poolIndex = k;
eventCount = ~k & SMASK; // clear wait count
locallyFifo = pool.locallyFifo;
Thread.UncaughtExceptionHandler ueh = pool.ueh;
if (ueh != null)
setUncaughtExceptionHandler(ueh);
start();
setDaemon(true);
}
// Public/protected methods
// Public methods
/**
* Returns the pool hosting this thread.
......@@ -375,6 +330,25 @@ public class ForkJoinWorkerThread extends Thread {
return poolIndex;
}
// Randomization
/**
* Computes next value for random victim probes and backoffs.
* 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 FJP.scan() to avoid
* writes inside busy loops.
*/
private int nextSeed() {
int r = seed;
r ^= r << 13;
r ^= r >>> 17;
r ^= r << 5;
return seed = r;
}
// Run State management
/**
* Initializes internal state after construction but before
* processing any tasks. If you override this method, you must
......@@ -385,15 +359,9 @@ public class ForkJoinWorkerThread extends Thread {
* processing tasks.
*/
protected void onStart() {
int rs = seedGenerator.nextInt();
seed = (rs == 0) ? 1 : rs; // seed must be nonzero
// Allocate name string and arrays in this thread
String pid = Integer.toString(pool.getPoolNumber());
String wid = Integer.toString(poolIndex);
setName("ForkJoinPool-" + pid + "-worker-" + wid);
queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
int r = pool.workerSeedGenerator.nextInt();
seed = (r == 0)? 1 : r; // must be nonzero
}
/**
......@@ -406,16 +374,9 @@ public class ForkJoinWorkerThread extends Thread {
*/
protected void onTermination(Throwable exception) {
try {
ForkJoinPool p = pool;
if (active) {
int a; // inline p.tryDecrementActiveCount
active = false;
do {} while (!UNSAFE.compareAndSwapInt
(p, poolRunStateOffset, a = p.runState, a - 1));
}
terminate = true;
cancelTasks();
setTerminated();
p.workerTerminated(this);
pool.deregisterWorker(this, exception);
} catch (Throwable ex) { // Shouldn't ever happen
if (exception == null) // but if so, at least rethrown
exception = ex;
......@@ -434,7 +395,7 @@ public class ForkJoinWorkerThread extends Thread {
Throwable exception = null;
try {
onStart();
mainLoop();
pool.work(this);
} catch (Throwable ex) {
exception = ex;
} finally {
......@@ -442,81 +403,6 @@ public class ForkJoinWorkerThread extends Thread {
}
}
// helpers for run()
/**
* Finds and executes tasks, and checks status while running.
*/
private void mainLoop() {
boolean ran = false; // true if ran a task on last step
ForkJoinPool p = pool;
for (;;) {
p.preStep(this, ran);
if (runState != 0)
break;
ran = tryExecSteal() || tryExecSubmission();
}
}
/**
* Tries to steal a task and execute it.
*
* @return true if ran a task
*/
private boolean tryExecSteal() {
ForkJoinTask<?> t;
if ((t = scan()) != null) {
t.quietlyExec();
UNSAFE.putOrderedObject(this, currentStealOffset, null);
if (sp != base)
execLocalTasks();
return true;
}
return false;
}
/**
* If a submission exists, try to activate and run it.
*
* @return true if ran a task
*/
private boolean tryExecSubmission() {
ForkJoinPool p = pool;
// This loop is needed in case attempt to activate fails, in
// which case we only retry if there still appears to be a
// submission.
while (p.hasQueuedSubmissions()) {
ForkJoinTask<?> t; int a;
if (active || // inline p.tryIncrementActiveCount
(active = UNSAFE.compareAndSwapInt(p, poolRunStateOffset,
a = p.runState, a + 1))) {
if ((t = p.pollSubmission()) != null) {
UNSAFE.putOrderedObject(this, currentStealOffset, t);
t.quietlyExec();
UNSAFE.putOrderedObject(this, currentStealOffset, null);
if (sp != base)
execLocalTasks();
return true;
}
}
}
return false;
}
/**
* Runs local tasks until queue is empty or shut down. Call only
* while active.
*/
private void execLocalTasks() {
while (runState == 0) {
ForkJoinTask<?> t = locallyFifo ? locallyDeqTask() : popTask();
if (t != null)
t.quietlyExec();
else if (sp == base)
break;
}
}
/*
* Intrinsics-based atomic writes for queue slots. These are
* basically the same as methods in AtomicReferenceArray, but
......@@ -528,10 +414,20 @@ public class ForkJoinWorkerThread extends Thread {
* because they are protected by other volatile reads and are
* confirmed by CASes.
*
* Most uses don't actually call these methods, but instead contain
* inlined forms that enable more predictable optimization. We
* don't define the version of write used in pushTask at all, but
* instead inline there a store-fenced array slot write.
* Most uses don't actually call these methods, but instead
* contain inlined forms that enable more predictable
* optimization. We don't define the version of write used in
* pushTask at all, but instead inline there a store-fenced array
* slot write.
*
* Also in most methods, as a performance (not correctness) issue,
* we'd like to encourage compilers not to arbitrarily postpone
* setting queueTop after writing slot. Currently there is no
* intrinsic for arranging this, but using Unsafe putOrderedInt
* may be a preferable strategy on some compilers even though its
* main effect is a pre-, not post- fence. To simplify possible
* changes, the option is left in comments next to the associated
* assignments.
*/
/**
......@@ -540,7 +436,7 @@ public class ForkJoinWorkerThread extends Thread {
*/
private static final boolean casSlotNull(ForkJoinTask<?>[] q, int i,
ForkJoinTask<?> t) {
return UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null);
return UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE, t, null);
}
/**
......@@ -550,7 +446,7 @@ public class ForkJoinWorkerThread extends Thread {
*/
private static final void writeSlot(ForkJoinTask<?>[] q, int i,
ForkJoinTask<?> t) {
UNSAFE.putObjectVolatile(q, (i << qShift) + qBase, t);
UNSAFE.putObjectVolatile(q, (i << ASHIFT) + ABASE, t);
}
// queue methods
......@@ -561,14 +457,43 @@ public class ForkJoinWorkerThread extends Thread {
* @param t the task. Caller must ensure non-null.
*/
final void pushTask(ForkJoinTask<?> t) {
ForkJoinTask<?>[] q = queue;
int mask = q.length - 1; // implicit assert q != null
int s = sp++; // ok to increment sp before slot write
UNSAFE.putOrderedObject(q, ((s & mask) << qShift) + qBase, t);
if ((s -= base) == 0)
pool.signalWork(); // was empty
else if (s == mask)
growQueue(); // is full
ForkJoinTask<?>[] q; int s, m;
if ((q = queue) != null) { // ignore if queue removed
long u = (((s = queueTop) & (m = q.length - 1)) << ASHIFT) + ABASE;
UNSAFE.putOrderedObject(q, u, t);
queueTop = s + 1; // or use putOrderedInt
if ((s -= queueBase) <= 2)
pool.signalWork();
else if (s == m)
growQueue();
}
}
/**
* Creates or doubles queue array. Transfers elements by
* emulating steals (deqs) from old array and placing, oldest
* first, into new array.
*/
private void growQueue() {
ForkJoinTask<?>[] oldQ = queue;
int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY;
if (size > MAXIMUM_QUEUE_CAPACITY)
throw new RejectedExecutionException("Queue capacity exceeded");
if (size < INITIAL_QUEUE_CAPACITY)
size = INITIAL_QUEUE_CAPACITY;
ForkJoinTask<?>[] q = queue = new ForkJoinTask<?>[size];
int mask = size - 1;
int top = queueTop;
int oldMask;
if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) {
for (int b = queueBase; b != top; ++b) {
long u = ((b & oldMask) << ASHIFT) + ABASE;
Object x = UNSAFE.getObjectVolatile(oldQ, u);
if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null))
UNSAFE.putObjectVolatile
(q, ((b & mask) << ASHIFT) + ABASE, x);
}
}
}
/**
......@@ -579,35 +504,34 @@ public class ForkJoinWorkerThread extends Thread {
* @return a task, or null if none or contended
*/
final ForkJoinTask<?> deqTask() {
ForkJoinTask<?> t;
ForkJoinTask<?>[] q;
int b, i;
if (sp != (b = base) &&
ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
if (queueTop != (b = queueBase) &&
(q = queue) != null && // must read q after b
(t = q[i = (q.length - 1) & b]) != null && base == b &&
UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) {
base = b + 1;
(i = (q.length - 1) & b) >= 0 &&
(t = q[i]) != null && queueBase == b &&
UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE, t, null)) {
queueBase = b + 1;
return t;
}
return null;
}
/**
* Tries to take a task from the base of own queue. Assumes active
* status. Called only by this thread.
* Tries to take a task from the base of own queue. Called only
* by this thread.
*
* @return a task, or null if none
*/
final ForkJoinTask<?> locallyDeqTask() {
ForkJoinTask<?> t; int m, b, i;
ForkJoinTask<?>[] q = queue;
if (q != null) {
ForkJoinTask<?> t;
int b, i;
while (sp != (b = base)) {
if ((t = q[i = (q.length - 1) & b]) != null && base == b &&
UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase,
if (q != null && (m = q.length - 1) >= 0) {
while (queueTop != (b = queueBase)) {
if ((t = q[i = m & b]) != null &&
queueBase == b &&
UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE,
t, null)) {
base = b + 1;
queueBase = b + 1;
return t;
}
}
......@@ -616,35 +540,21 @@ public class ForkJoinWorkerThread extends Thread {
}
/**
* Returns a popped task, or null if empty. Assumes active status.
* Returns a popped task, or null if empty.
* Called only by this thread.
*/
private ForkJoinTask<?> popTask() {
int m;
ForkJoinTask<?>[] q = queue;
if (q != null) {
int s;
while ((s = sp) != base) {
int i = (q.length - 1) & --s;
long u = (i << qShift) + qBase; // raw offset
if (q != null && (m = q.length - 1) >= 0) {
for (int s; (s = queueTop) != queueBase;) {
int i = m & --s;
long u = (i << ASHIFT) + ABASE; // raw offset
ForkJoinTask<?> t = q[i];
if (t == null) // lost to stealer
break;
if (UNSAFE.compareAndSwapObject(q, u, t, null)) {
/*
* Note: here and in related methods, as a
* performance (not correctness) issue, we'd like
* to encourage compiler not to arbitrarily
* postpone setting sp after successful CAS.
* Currently there is no intrinsic for arranging
* this, but using Unsafe putOrderedInt may be a
* preferable strategy on some compilers even
* though its main effect is a pre-, not post-
* fence. To simplify possible changes, the option
* is left in comments next to the associated
* assignments.
*/
sp = s; // putOrderedInt may encourage more timely write
// UNSAFE.putOrderedInt(this, spOffset, s);
queueTop = s; // or putOrderedInt
return t;
}
}
......@@ -654,18 +564,17 @@ public class ForkJoinWorkerThread extends Thread {
/**
* Specialized version of popTask to pop only if topmost element
* is the given task. Called only by this thread while active.
* is the given task. Called only by this thread.
*
* @param t the task. Caller must ensure non-null.
*/
final boolean unpushTask(ForkJoinTask<?> t) {
ForkJoinTask<?>[] q;
int s;
ForkJoinTask<?>[] q = queue;
if ((s = sp) != base && q != null &&
if ((q = queue) != null && (s = queueTop) != queueBase &&
UNSAFE.compareAndSwapObject
(q, (((q.length - 1) & --s) << qShift) + qBase, t, null)) {
sp = s; // putOrderedInt may encourage more timely write
// UNSAFE.putOrderedInt(this, spOffset, s);
(q, (((q.length - 1) & --s) << ASHIFT) + ABASE, t, null)) {
queueTop = s; // or putOrderedInt
return true;
}
return false;
......@@ -675,222 +584,30 @@ public class ForkJoinWorkerThread extends Thread {
* Returns next task, or null if empty or contended.
*/
final ForkJoinTask<?> peekTask() {
int m;
ForkJoinTask<?>[] q = queue;
if (q == null)
return null;
int mask = q.length - 1;
int i = locallyFifo ? base : (sp - 1);
return q[i & mask];
}
/**
* Doubles queue array size. Transfers elements by emulating
* steals (deqs) from old array and placing, oldest first, into
* new array.
*/
private void growQueue() {
ForkJoinTask<?>[] oldQ = queue;
int oldSize = oldQ.length;
int newSize = oldSize << 1;
if (newSize > MAXIMUM_QUEUE_CAPACITY)
throw new RejectedExecutionException("Queue capacity exceeded");
ForkJoinTask<?>[] newQ = queue = new ForkJoinTask<?>[newSize];
int b = base;
int bf = b + oldSize;
int oldMask = oldSize - 1;
int newMask = newSize - 1;
do {
int oldIndex = b & oldMask;
ForkJoinTask<?> t = oldQ[oldIndex];
if (t != null && !casSlotNull(oldQ, oldIndex, t))
t = null;
writeSlot(newQ, b & newMask, t);
} while (++b != bf);
pool.signalWork();
}
/**
* Computes next value for random victim probe in scan(). 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 scan().
*/
private static final int xorShift(int r) {
r ^= r << 13;
r ^= r >>> 17;
return r ^ (r << 5);
}
/**
* Tries to steal a task from another worker. Starts at a random
* index of workers array, and probes workers until finding one
* with non-empty queue or finding that all are empty. It
* randomly selects the first n probes. If these are empty, it
* resorts to a circular sweep, which is necessary to accurately
* set active status. (The circular sweep uses steps of
* approximately half the array size plus 1, to avoid bias
* stemming from leftmost packing of the array in ForkJoinPool.)
*
* This method must be both fast and quiet -- usually avoiding
* memory accesses that could disrupt cache sharing etc other than
* those needed to check for and take tasks (or to activate if not
* already active). This accounts for, among other things,
* updating random seed in place without storing it until exit.
*
* @return a task, or null if none found
*/
private ForkJoinTask<?> scan() {
ForkJoinPool p = pool;
ForkJoinWorkerThread[] ws; // worker array
int n; // upper bound of #workers
if ((ws = p.workers) != null && (n = ws.length) > 1) {
boolean canSteal = active; // shadow active status
int r = seed; // extract seed once
int mask = n - 1;
int j = -n; // loop counter
int k = r; // worker index, random if j < 0
for (;;) {
ForkJoinWorkerThread v = ws[k & mask];
r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // inline xorshift
ForkJoinTask<?>[] q; ForkJoinTask<?> t; int b, a;
if (v != null && (b = v.base) != v.sp &&
(q = v.queue) != null) {
int i = (q.length - 1) & b;
long u = (i << qShift) + qBase; // raw offset
int pid = poolIndex;
if ((t = q[i]) != null) {
if (!canSteal && // inline p.tryIncrementActiveCount
UNSAFE.compareAndSwapInt(p, poolRunStateOffset,
a = p.runState, a + 1))
canSteal = active = true;
if (canSteal && v.base == b++ &&
UNSAFE.compareAndSwapObject(q, u, t, null)) {
v.base = b;
v.stealHint = pid;
UNSAFE.putOrderedObject(this,
currentStealOffset, t);
seed = r;
++stealCount;
return t;
}
}
j = -n;
k = r; // restart on contention
}
else if (++j <= 0)
k = r;
else if (j <= n)
k += (n >>> 1) | 1;
else
break;
}
}
if (q == null || (m = q.length - 1) < 0)
return null;
int i = locallyFifo ? queueBase : (queueTop - 1);
return q[i & m];
}
// Run State management
// status check methods used mainly by ForkJoinPool
final boolean isRunning() { return runState == 0; }
final boolean isTerminated() { return (runState & TERMINATED) != 0; }
final boolean isSuspended() { return (runState & SUSPENDED) != 0; }
final boolean isTrimmed() { return (runState & TRIMMED) != 0; }
final boolean isTerminating() {
if ((runState & TERMINATING) != 0)
return true;
if (pool.isAtLeastTerminating()) { // propagate pool state
shutdown();
return true;
}
return false;
}
// Support methods for ForkJoinPool
/**
* Sets state to TERMINATING. Does NOT unpark or interrupt
* to wake up if currently blocked. Callers must do so if desired.
* Runs the given task, plus any local tasks until queue is empty
*/
final void shutdown() {
final void execTask(ForkJoinTask<?> t) {
currentSteal = t;
for (;;) {
int s = runState;
if ((s & (TERMINATING|TERMINATED)) != 0)
break;
if ((s & SUSPENDED) != 0) { // kill and wakeup if suspended
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
(s & ~SUSPENDED) |
(TRIMMED|TERMINATING)))
break;
}
else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
s | TERMINATING))
break;
}
}
/**
* Sets state to TERMINATED. Called only by onTermination().
*/
private void setTerminated() {
int s;
do {} while (!UNSAFE.compareAndSwapInt(this, runStateOffset,
s = runState,
s | (TERMINATING|TERMINATED)));
}
/**
* If suspended, tries to set status to unsuspended.
* Does NOT wake up if blocked.
*
* @return true if successful
*/
final boolean tryUnsuspend() {
int s;
while (((s = runState) & SUSPENDED) != 0) {
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
s & ~SUSPENDED))
return true;
}
return false;
}
/**
* Sets suspended status and blocks as spare until resumed
* or shutdown.
*/
final void suspendAsSpare() {
for (;;) { // set suspended unless terminating
int s = runState;
if ((s & TERMINATING) != 0) { // must kill
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
s | (TRIMMED | TERMINATING)))
return;
}
else if (UNSAFE.compareAndSwapInt(this, runStateOffset, s,
s | SUSPENDED))
break;
}
ForkJoinPool p = pool;
p.pushSpare(this);
while ((runState & SUSPENDED) != 0) {
if (p.tryAccumulateStealCount(this)) {
interrupted(); // clear/ignore interrupts
if ((runState & SUSPENDED) == 0)
if (t != null)
t.doExec();
if (queueTop == queueBase)
break;
LockSupport.park(this);
}
}
t = locallyFifo ? locallyDeqTask() : popTask();
}
// Misc support methods for ForkJoinPool
/**
* Returns an estimate of the number of tasks in the queue. Also
* used by ForkJoinTask.
*/
final int getQueueSize() {
int n; // external calls must read base first
return (n = -base + sp) <= 0 ? 0 : n;
++stealCount;
currentSteal = null;
}
/**
......@@ -899,17 +616,12 @@ public class ForkJoinWorkerThread extends Thread {
*/
final void cancelTasks() {
ForkJoinTask<?> cj = currentJoin; // try to cancel ongoing tasks
if (cj != null && cj.status >= 0) {
if (cj != null && cj.status >= 0)
cj.cancelIgnoringExceptions();
try {
this.interrupt(); // awaken wait
} catch (SecurityException ignore) {
}
}
ForkJoinTask<?> cs = currentSteal;
if (cs != null && cs.status >= 0)
cs.cancelIgnoringExceptions();
while (base != sp) {
while (queueBase != queueTop) {
ForkJoinTask<?> t = deqTask();
if (t != null)
t.cancelIgnoringExceptions();
......@@ -923,7 +635,7 @@ public class ForkJoinWorkerThread extends Thread {
*/
final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
int n = 0;
while (base != sp) {
while (queueBase != queueTop) {
ForkJoinTask<?> t = deqTask();
if (t != null) {
c.add(t);
......@@ -935,22 +647,21 @@ public class ForkJoinWorkerThread extends Thread {
// Support methods for ForkJoinTask
/**
* Returns an estimate of the number of tasks in the queue.
*/
final int getQueueSize() {
return queueTop - queueBase;
}
/**
* Gets and removes a local task.
*
* @return a task, if available
*/
final ForkJoinTask<?> pollLocalTask() {
ForkJoinPool p = pool;
while (sp != base) {
int a; // inline p.tryIncrementActiveCount
if (active ||
(active = UNSAFE.compareAndSwapInt(p, poolRunStateOffset,
a = p.runState, a + 1)))
return locallyFifo ? locallyDeqTask() : popTask();
}
return null;
}
/**
* Gets and removes a local or stolen task.
......@@ -958,172 +669,205 @@ public class ForkJoinWorkerThread extends Thread {
* @return a task, if available
*/
final ForkJoinTask<?> pollTask() {
ForkJoinWorkerThread[] ws;
ForkJoinTask<?> t = pollLocalTask();
if (t == null) {
t = scan();
// cannot retain/track/help steal
UNSAFE.putOrderedObject(this, currentStealOffset, null);
}
if (t != null || (ws = pool.workers) == null)
return t;
int n = ws.length; // cheap version of FJP.scan
int steps = n << 1;
int r = nextSeed();
int i = 0;
while (i < steps) {
ForkJoinWorkerThread w = ws[(i++ + r) & (n - 1)];
if (w != null && w.queueBase != w.queueTop && w.queue != null) {
if ((t = w.deqTask()) != null)
return t;
i = 0;
}
}
return null;
}
/**
* The maximum stolen->joining link depth allowed in helpJoinTask,
* as well as the maximum number of retries (allowing on average
* one staleness retry per level) per attempt to instead try
* compensation. Depths for legitimate chains are unbounded, but
* we use a fixed constant to avoid (otherwise unchecked) cycles
* and bound staleness of traversal parameters at the expense of
* sometimes blocking when we could be helping.
*/
private static final int MAX_HELP = 16;
/**
* Possibly runs some tasks and/or blocks, until task is done.
* Possibly runs some tasks and/or blocks, until joinMe is done.
*
* @param joinMe the task to join
* @param timed true if use timed wait
* @param nanos wait time if timed
* @return completion status on exit
*/
final void joinTask(ForkJoinTask<?> joinMe, boolean timed, long nanos) {
// currentJoin only written by this thread; only need ordered store
final int joinTask(ForkJoinTask<?> joinMe) {
ForkJoinTask<?> prevJoin = currentJoin;
UNSAFE.putOrderedObject(this, currentJoinOffset, joinMe);
pool.awaitJoin(joinMe, this, timed, nanos);
UNSAFE.putOrderedObject(this, currentJoinOffset, prevJoin);
currentJoin = joinMe;
for (int s, retries = MAX_HELP;;) {
if ((s = joinMe.status) < 0) {
currentJoin = prevJoin;
return s;
}
if (retries > 0) {
if (queueTop != queueBase) {
if (!localHelpJoinTask(joinMe))
retries = 0; // cannot help
}
else if (retries == MAX_HELP >>> 1) {
--retries; // check uncommon case
if (tryDeqAndExec(joinMe) >= 0)
Thread.yield(); // for politeness
}
else
retries = helpJoinTask(joinMe)? MAX_HELP : retries - 1;
}
else {
retries = MAX_HELP; // restart if not done
pool.tryAwaitJoin(joinMe);
}
}
}
/**
* Tries to locate and help perform 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 implementation is very branchy to cope with potential
* inconsistencies or loops encountering chains that are stale,
* unknown, or of length greater than MAX_HELP_DEPTH links. All
* of these cases are dealt with by just returning back to the
* caller, who is expected to retry if other join mechanisms also
* don't work out.
* If present, pops and executes the given task, or any other
* cancelled task
*
* @param joinMe the task to join
* @param running if false, then must update pool count upon
* running a task
* @return value of running on exit
*/
final boolean helpJoinTask(ForkJoinTask<?> joinMe, boolean running) {
/*
* Initial checks to (1) abort if terminating; (2) clean out
* old cancelled tasks from local queue; (3) if joinMe is next
* task, run it; (4) omit scan if local queue nonempty (since
* it may contain non-descendents of joinMe).
* @return false if any other non-cancelled task exists in local queue
*/
ForkJoinPool p = pool;
for (;;) {
ForkJoinTask<?>[] q;
int s;
if (joinMe.status < 0)
return running;
else if ((runState & TERMINATING) != 0) {
joinMe.cancelIgnoringExceptions();
return running;
}
else if ((s = sp) == base || (q = queue) == null)
break; // queue empty
else {
int i = (q.length - 1) & --s;
long u = (i << qShift) + qBase; // raw offset
ForkJoinTask<?> t = q[i];
if (t == null)
break; // lost to a stealer
else if (t != joinMe && t.status >= 0)
return running; // cannot safely help
else if ((running ||
(running = p.tryIncrementRunningCount())) &&
UNSAFE.compareAndSwapObject(q, u, t, null)) {
sp = s; // putOrderedInt may encourage more timely write
// UNSAFE.putOrderedInt(this, spOffset, s);
t.quietlyExec();
private boolean localHelpJoinTask(ForkJoinTask<?> joinMe) {
int s, i; ForkJoinTask<?>[] q; ForkJoinTask<?> t;
if ((s = queueTop) != queueBase && (q = queue) != null &&
(i = (q.length - 1) & --s) >= 0 &&
(t = q[i]) != null) {
if (t != joinMe && t.status >= 0)
return false;
if (UNSAFE.compareAndSwapObject
(q, (i << ASHIFT) + ABASE, t, null)) {
queueTop = s; // or putOrderedInt
t.doExec();
}
}
return true;
}
int n; // worker array size
ForkJoinWorkerThread[] ws = p.workers;
if (ws != null && (n = ws.length) > 1) { // need at least 2 workers
/**
* 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 implementation is very
* branchy to cope with potential inconsistencies or loops
* encountering chains that are stale, unknown, or of length
* greater than MAX_HELP links. All of these cases are dealt with
* by just retrying by caller.
*
* @param joinMe the task to join
* @param canSteal true if local queue is empty
* @return true if ran a task
*/
private boolean helpJoinTask(ForkJoinTask<?> joinMe) {
boolean helped = false;
int m = pool.scanGuard & SMASK;
ForkJoinWorkerThread[] ws = pool.workers;
if (ws != null && ws.length > m && joinMe.status >= 0) {
int levels = MAX_HELP; // remaining chain length
ForkJoinTask<?> task = joinMe; // base of chain
ForkJoinWorkerThread thread = this; // thread with stolen task
outer:for (int d = 0; d < MAX_HELP_DEPTH; ++d) { // chain length
outer:for (ForkJoinWorkerThread thread = this;;) {
// Try to find v, the stealer of task, by first using hint
ForkJoinWorkerThread v = ws[thread.stealHint & (n - 1)];
ForkJoinWorkerThread v = ws[thread.stealHint & m];
if (v == null || v.currentSteal != task) {
for (int j = 0; ; ++j) { // search array
if (j < n) {
ForkJoinTask<?> vs;
if ((v = ws[j]) != null &&
(vs = v.currentSteal) != null) {
if (joinMe.status < 0)
break outer;
if (vs == task) {
if (task.status < 0)
break outer; // stale
for (int j = 0; ;) { // search array
if ((v = ws[j]) != null && v.currentSteal == task) {
thread.stealHint = j;
break; // save hint for next time
}
if (++j > m)
break outer; // can't find stealer
}
}
else
break outer; // no stealer
}
}
// Try to help v, using specialized form of deqTask
for (;;) {
ForkJoinTask<?>[] q; int b, i;
if (joinMe.status < 0)
break outer;
int b = v.base;
ForkJoinTask<?>[] q = v.queue;
if (b == v.sp || q == null)
if ((b = v.queueBase) == v.queueTop ||
(q = v.queue) == null ||
(i = (q.length-1) & b) < 0)
break; // empty
int i = (q.length - 1) & b;
long u = (i << qShift) + qBase;
long u = (i << ASHIFT) + ABASE;
ForkJoinTask<?> t = q[i];
if (task.status < 0)
break outer; // stale
if (t != null &&
(running ||
(running = p.tryIncrementRunningCount())) &&
v.base == b++ &&
if (t != null && v.queueBase == b &&
UNSAFE.compareAndSwapObject(q, u, t, null)) {
if (t != joinMe && joinMe.status < 0) {
UNSAFE.putObjectVolatile(q, u, t);
break outer; // joinMe cancelled; back out
}
v.base = b;
if (t.status >= 0) {
v.queueBase = b + 1;
v.stealHint = poolIndex;
ForkJoinTask<?> ps = currentSteal;
int pid = poolIndex;
v.stealHint = pid;
UNSAFE.putOrderedObject(this,
currentStealOffset, t);
t.quietlyExec();
UNSAFE.putOrderedObject(this,
currentStealOffset, ps);
}
}
else if ((runState & TERMINATING) != 0) {
joinMe.cancelIgnoringExceptions();
break outer;
currentSteal = t;
t.doExec();
currentSteal = ps;
helped = true;
}
}
// Try to descend to find v's stealer
ForkJoinTask<?> next = v.currentJoin;
if (task.status < 0 || next == null || next == task ||
joinMe.status < 0)
break; // done, stale, dead-end, or cyclic
if (--levels > 0 && task.status >= 0 &&
next != null && next != task) {
task = next;
thread = v;
}
else
break; // max levels, stale, dead-end, or cyclic
}
}
return running;
return helped;
}
/**
* Implements ForkJoinTask.getSurplusQueuedTaskCount().
* Returns an estimate of the number of tasks, offset by a
* function of number of idle workers.
* Performs an uncommon case for joinTask: If task t is at base of
* some workers queue, steals and executes it.
*
* @param t the task
* @return t's status
*/
private int tryDeqAndExec(ForkJoinTask<?> t) {
int m = pool.scanGuard & SMASK;
ForkJoinWorkerThread[] ws = pool.workers;
if (ws != null && ws.length > m && t.status >= 0) {
for (int j = 0; j <= m; ++j) {
ForkJoinTask<?>[] q; int b, i;
ForkJoinWorkerThread v = ws[j];
if (v != null &&
(b = v.queueBase) != v.queueTop &&
(q = v.queue) != null &&
(i = (q.length - 1) & b) >= 0 &&
q[i] == t) {
long u = (i << ASHIFT) + ABASE;
if (v.queueBase == b &&
UNSAFE.compareAndSwapObject(q, u, t, null)) {
v.queueBase = b + 1;
v.stealHint = poolIndex;
ForkJoinTask<?> ps = currentSteal;
currentSteal = t;
t.doExec();
currentSteal = ps;
}
break;
}
}
}
return t.status;
}
/**
* Implements ForkJoinTask.getSurplusQueuedTaskCount(). Returns
* an estimate of the number of tasks, offset by a function of
* number of idle workers.
*
* This method provides a cheap heuristic guide for task
* partitioning when programmers, frameworks, tools, or languages
......@@ -1159,82 +903,96 @@ public class ForkJoinWorkerThread extends Thread {
* 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 (although note that (sp - base)
* can be an overestimate because of stealers lagging increments
* of base). 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
* just use estimated queue length (although note that (queueTop -
* queueBase) can be an overestimate because of stealers lagging
* increments of queueBase). 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.
*/
final int getEstimatedSurplusTaskCount() {
return sp - base - pool.idlePerActive();
return queueTop - queueBase - pool.idlePerActive();
}
/**
* Runs tasks until {@code pool.isQuiescent()}.
* Runs tasks until {@code pool.isQuiescent()}. We piggyback on
* pool's active count ctl maintenance, but rather than blocking
* when tasks cannot be found, we rescan until all others cannot
* find tasks either. The bracketing by pool quiescerCounts
* updates suppresses pool auto-shutdown mechanics that could
* otherwise prematurely terminate the pool because all threads
* appear to be inactive.
*/
final void helpQuiescePool() {
boolean active = true;
ForkJoinTask<?> ps = currentSteal; // to restore below
ForkJoinPool p = pool;
p.addQuiescerCount(1);
for (;;) {
ForkJoinTask<?> t = pollLocalTask();
if (t != null || (t = scan()) != null)
t.quietlyExec();
ForkJoinWorkerThread[] ws = p.workers;
ForkJoinWorkerThread v = null;
int n;
if (queueTop != queueBase)
v = this;
else if (ws != null && (n = ws.length) > 1) {
ForkJoinWorkerThread w;
int r = nextSeed(); // cheap version of FJP.scan
int steps = n << 1;
for (int i = 0; i < steps; ++i) {
if ((w = ws[(i + r) & (n - 1)]) != null &&
w.queueBase != w.queueTop) {
v = w;
break;
}
}
}
if (v != null) {
ForkJoinTask<?> t;
if (!active) {
active = true;
p.addActiveCount(1);
}
if ((t = (v != this) ? v.deqTask() :
locallyFifo? locallyDeqTask() : popTask()) != null) {
currentSteal = t;
t.doExec();
currentSteal = ps;
}
}
else {
ForkJoinPool p = pool;
int a; // to inline CASes
if (active) {
if (!UNSAFE.compareAndSwapInt
(p, poolRunStateOffset, a = p.runState, a - 1))
continue; // retry later
active = false; // inactivate
UNSAFE.putOrderedObject(this, currentStealOffset, ps);
active = false;
p.addActiveCount(-1);
}
if (p.isQuiescent()) {
active = true; // re-activate
do {} while (!UNSAFE.compareAndSwapInt
(p, poolRunStateOffset, a = p.runState, a+1));
return;
p.addActiveCount(1);
p.addQuiescerCount(-1);
break;
}
}
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long spOffset =
objectFieldOffset("sp", ForkJoinWorkerThread.class);
private static final long runStateOffset =
objectFieldOffset("runState", ForkJoinWorkerThread.class);
private static final long currentJoinOffset =
objectFieldOffset("currentJoin", ForkJoinWorkerThread.class);
private static final long currentStealOffset =
objectFieldOffset("currentSteal", ForkJoinWorkerThread.class);
private static final long qBase =
UNSAFE.arrayBaseOffset(ForkJoinTask[].class);
private static final long poolRunStateOffset = // to inline CAS
objectFieldOffset("runState", ForkJoinPool.class);
private static final int qShift;
private static final sun.misc.Unsafe UNSAFE;
private static final long ABASE;
private static final int ASHIFT;
static {
int s = UNSAFE.arrayIndexScale(ForkJoinTask[].class);
int s;
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class a = ForkJoinTask[].class;
ABASE = UNSAFE.arrayBaseOffset(a);
s = UNSAFE.arrayIndexScale(a);
} catch (Exception e) {
throw new Error(e);
}
if ((s & (s-1)) != 0)
throw new Error("data type scale not a power of two");
qShift = 31 - Integer.numberOfLeadingZeros(s);
MAXIMUM_QUEUE_CAPACITY = 1 << (31 - qShift);
ASHIFT = 31 - Integer.numberOfLeadingZeros(s);
}
private static long objectFieldOffset(String field, Class<?> klazz) {
try {
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
} catch (NoSuchFieldException e) {
// Convert Exception to corresponding Error
NoSuchFieldError error = new NoSuchFieldError(field);
error.initCause(e);
throw error;
}
}
}
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