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hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepThread.hpp

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+/*
+ * Copyright 2001-2006 Sun Microsystems, Inc.  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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.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.
+  void    wait_on_cms_lock(long t); // milliseconds
+
+  // The CMS thread will yield during the work portion of it's cycle
+  // only when requested to.  Both synchronous and asychronous requests
+  // are provided.  A synchronous request is used for young gen
+  // collections and direct allocations.  The requesting thread increments
+  // pending_yields at the beginning of an operation, and decrements it when
+  // the operation is completed.  The CMS thread yields when pending_yields
+  // is positive.  An asynchronous request is used by iCMS in the stop_icms()
+  // operation. A single yield satisfies the outstanding asynch yield requests.
+  // The requesting thread increments both pending_yields and pending_decrements.
+  // After yielding, the CMS thread decrements both by the amount in
+  // pending_decrements.
+  // 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() 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() {
+    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) {
+      // 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 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);
+    }
+  }
+};