/* * Copyright (c) 2001, 2010, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. * */ class ConcurrentMarkSweepGeneration; class CMSCollector; // The Concurrent Mark Sweep GC Thread (could be several in the future). class ConcurrentMarkSweepThread: public ConcurrentGCThread { friend class VMStructs; friend class ConcurrentMarkSweepGeneration; // XXX should remove friendship friend class CMSCollector; public: virtual void run(); private: static ConcurrentMarkSweepThread* _cmst; static CMSCollector* _collector; static SurrogateLockerThread* _slt; static SurrogateLockerThread::SLT_msg_type _sltBuffer; static Monitor* _sltMonitor; ConcurrentMarkSweepThread* _next; static bool _should_terminate; enum CMS_flag_type { CMS_nil = NoBits, CMS_cms_wants_token = nth_bit(0), CMS_cms_has_token = nth_bit(1), CMS_vm_wants_token = nth_bit(2), CMS_vm_has_token = nth_bit(3) }; static int _CMS_flag; static bool CMS_flag_is_set(int b) { return (_CMS_flag & b) != 0; } static bool set_CMS_flag(int b) { return (_CMS_flag |= b) != 0; } static bool clear_CMS_flag(int b) { return (_CMS_flag &= ~b) != 0; } void sleepBeforeNextCycle(); // CMS thread should yield for a young gen collection, direct allocation, // and iCMS activity. static char _pad_1[64 - sizeof(jint)]; // prevent cache-line sharing static volatile jint _pending_yields; static volatile jint _pending_decrements; // decrements to _pending_yields static char _pad_2[64 - sizeof(jint)]; // prevent cache-line sharing // Tracing messages, enabled by CMSTraceThreadState. static inline void trace_state(const char* desc); static volatile bool _icms_enabled; // iCMS enabled? static volatile bool _should_run; // iCMS may run static volatile bool _should_stop; // iCMS should stop // debugging void verify_ok_to_terminate() const PRODUCT_RETURN; public: // Constructor ConcurrentMarkSweepThread(CMSCollector* collector); static void makeSurrogateLockerThread(TRAPS); static SurrogateLockerThread* slt() { return _slt; } // Tester bool is_ConcurrentGC_thread() const { return true; } static void threads_do(ThreadClosure* tc); // Printing void print_on(outputStream* st) const; void print() const { print_on(tty); } static void print_all_on(outputStream* st); static void print_all() { print_all_on(tty); } // Returns the CMS Thread static ConcurrentMarkSweepThread* cmst() { return _cmst; } static CMSCollector* collector() { return _collector; } // Create and start the CMS Thread, or stop it on shutdown static ConcurrentMarkSweepThread* start(CMSCollector* collector); static void stop(); static bool should_terminate() { return _should_terminate; } // Synchronization using CMS token static void synchronize(bool is_cms_thread); static void desynchronize(bool is_cms_thread); static bool vm_thread_has_cms_token() { return CMS_flag_is_set(CMS_vm_has_token); } static bool cms_thread_has_cms_token() { return CMS_flag_is_set(CMS_cms_has_token); } static bool vm_thread_wants_cms_token() { return CMS_flag_is_set(CMS_vm_wants_token); } static bool cms_thread_wants_cms_token() { return CMS_flag_is_set(CMS_cms_wants_token); } // Wait on CMS lock until the next synchronous GC // or given timeout, whichever is earlier. A timeout value // of 0 indicates that there is no upper bound on the wait time. // A concurrent full gc request terminates the wait. void wait_on_cms_lock(long t_millis); // The CMS thread will yield during the work portion of its cycle // only when requested to. Both synchronous and asychronous requests // are provided: // (1) A synchronous request is used for young gen collections and // for direct allocations. The requesting thread increments // _pending_yields at the beginning of an operation, and decrements // _pending_yields when that operation is completed. // In turn, the CMS thread yields when _pending_yields is positive, // and continues to yield until the value reverts to 0. // (2) An asynchronous request, on the other hand, is used by iCMS // for the stop_icms() operation. A single yield satisfies all of // the outstanding asynch yield requests, of which there may // occasionally be several in close succession. To accomplish // this, an asynch-requesting thread atomically increments both // _pending_yields and _pending_decrements. An asynchr requesting // thread does not wait and "acknowledge" completion of an operation // and deregister the request, like the synchronous version described // above does. In turn, after yielding, the CMS thread decrements both // _pending_yields and _pending_decrements by the value seen in // _pending_decrements before the decrement. // NOTE: The above scheme is isomorphic to having two request counters, // one for async requests and one for sync requests, and for the CMS thread // to check the sum of the two counters to decide whether it should yield // and to clear only the async counter when it yields. However, it turns out // to be more efficient for CMS code to just check a single counter // _pending_yields that holds the sum (of both sync and async requests), and // a second counter _pending_decrements that only holds the async requests, // for greater efficiency, since in a typical CMS run, there are many more // pontential (i.e. static) yield points than there are actual // (i.e. dynamic) yields because of requests, which are few and far between. // // Note that, while "_pending_yields >= _pending_decrements" is an invariant, // we cannot easily test that invariant, since the counters are manipulated via // atomic instructions without explicit locking and we cannot read // the two counters atomically together: one suggestion is to // use (for example) 16-bit counters so as to be able to read the // two counters atomically even on 32-bit platforms. Notice that // the second assert in acknowledge_yield_request() below does indeed // check a form of the above invariant, albeit indirectly. static void increment_pending_yields() { Atomic::inc(&_pending_yields); assert(_pending_yields >= 0, "can't be negative"); } static void decrement_pending_yields() { Atomic::dec(&_pending_yields); assert(_pending_yields >= 0, "can't be negative"); } static void asynchronous_yield_request() { assert(CMSIncrementalMode, "Currently only used w/iCMS"); increment_pending_yields(); Atomic::inc(&_pending_decrements); assert(_pending_decrements >= 0, "can't be negative"); } static void acknowledge_yield_request() { jint decrement = _pending_decrements; if (decrement > 0) { assert(CMSIncrementalMode, "Currently only used w/iCMS"); // Order important to preserve: _pending_yields >= _pending_decrements Atomic::add(-decrement, &_pending_decrements); Atomic::add(-decrement, &_pending_yields); assert(_pending_decrements >= 0, "can't be negative"); assert(_pending_yields >= 0, "can't be negative"); } } static bool should_yield() { return _pending_yields > 0; } // CMS incremental mode. static void start_icms(); // notify thread to start a quantum of work static void stop_icms(); // request thread to stop working void icms_wait(); // if asked to stop, wait until notified to start // Incremental mode is enabled globally by the flag CMSIncrementalMode. It // must also be enabled/disabled dynamically to allow foreground collections. static inline void enable_icms() { _icms_enabled = true; } static inline void disable_icms() { _icms_enabled = false; } static inline void set_icms_enabled(bool val) { _icms_enabled = val; } static inline bool icms_enabled() { return _icms_enabled; } }; inline void ConcurrentMarkSweepThread::trace_state(const char* desc) { if (CMSTraceThreadState) { char buf[128]; TimeStamp& ts = gclog_or_tty->time_stamp(); if (!ts.is_updated()) { ts.update(); } jio_snprintf(buf, sizeof(buf), " [%.3f: CMSThread %s] ", ts.seconds(), desc); buf[sizeof(buf) - 1] = '\0'; gclog_or_tty->print(buf); } } // For scoped increment/decrement of (synchronous) yield requests class CMSSynchronousYieldRequest: public StackObj { public: CMSSynchronousYieldRequest() { ConcurrentMarkSweepThread::increment_pending_yields(); } ~CMSSynchronousYieldRequest() { ConcurrentMarkSweepThread::decrement_pending_yields(); } }; // Used to emit a warning in case of unexpectedly excessive // looping (in "apparently endless loops") in CMS code. class CMSLoopCountWarn: public StackObj { private: const char* _src; const char* _msg; const intx _threshold; intx _ticks; public: inline CMSLoopCountWarn(const char* src, const char* msg, const intx threshold) : _src(src), _msg(msg), _threshold(threshold), _ticks(0) { } inline void tick() { _ticks++; if (CMSLoopWarn && _ticks % _threshold == 0) { warning("%s has looped %d times %s", _src, _ticks, _msg); } } };