1 /*
   2  * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #ifndef SHARE_VM_GC_IMPLEMENTATION_CONCURRENTMARKSWEEP_CONCURRENTMARKSWEEPTHREAD_HPP
  26 #define SHARE_VM_GC_IMPLEMENTATION_CONCURRENTMARKSWEEP_CONCURRENTMARKSWEEPTHREAD_HPP
  27 
  28 #include "gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.hpp"
  29 #include "gc_implementation/shared/concurrentGCThread.hpp"
  30 #include "runtime/thread.inline.hpp"
  31 
  32 class ConcurrentMarkSweepGeneration;
  33 class CMSCollector;
  34 
  35 // The Concurrent Mark Sweep GC Thread
  36 class ConcurrentMarkSweepThread: public ConcurrentGCThread {
  37   friend class VMStructs;
  38   friend class ConcurrentMarkSweepGeneration;   // XXX should remove friendship
  39   friend class CMSCollector;
  40  public:
  41   virtual void run();
  42 
  43  private:
  44   static ConcurrentMarkSweepThread*     _cmst;
  45   static CMSCollector*                  _collector;
  46   static SurrogateLockerThread*         _slt;
  47   static SurrogateLockerThread::SLT_msg_type _sltBuffer;
  48   static Monitor*                       _sltMonitor;
  49 
  50   static bool _should_terminate;
  51 
  52   enum CMS_flag_type {
  53     CMS_nil             = NoBits,
  54     CMS_cms_wants_token = nth_bit(0),
  55     CMS_cms_has_token   = nth_bit(1),
  56     CMS_vm_wants_token  = nth_bit(2),
  57     CMS_vm_has_token    = nth_bit(3)
  58   };
  59 
  60   static int _CMS_flag;
  61 
  62   static bool CMS_flag_is_set(int b)        { return (_CMS_flag & b) != 0;   }
  63   static bool set_CMS_flag(int b)           { return (_CMS_flag |= b) != 0;  }
  64   static bool clear_CMS_flag(int b)         { return (_CMS_flag &= ~b) != 0; }
  65   void sleepBeforeNextCycle();
  66 
  67   // CMS thread should yield for a young gen collection, direct allocation,
  68   // and iCMS activity.
  69   static char _pad_1[64 - sizeof(jint)];    // prevent cache-line sharing
  70   static volatile jint _pending_yields;
  71   static volatile jint _pending_decrements; // decrements to _pending_yields
  72   static char _pad_2[64 - sizeof(jint)];    // prevent cache-line sharing
  73 
  74   // Tracing messages, enabled by CMSTraceThreadState.
  75   static inline void trace_state(const char* desc);
  76 
  77   static volatile int _icms_disabled;   // a counter to track #iCMS disable & enable
  78   static volatile bool _should_run;     // iCMS may run
  79   static volatile bool _should_stop;    // iCMS should stop
  80 
  81   // debugging
  82   void verify_ok_to_terminate() const PRODUCT_RETURN;
  83 
  84  public:
  85   // Constructor
  86   ConcurrentMarkSweepThread(CMSCollector* collector);
  87 
  88   static void makeSurrogateLockerThread(TRAPS);
  89   static SurrogateLockerThread* slt() { return _slt; }
  90 
  91   // Tester
  92   bool is_ConcurrentGC_thread() const { return true;       }
  93 
  94   static void threads_do(ThreadClosure* tc);
  95 
  96   // Printing
  97   static void print_all_on(outputStream* st);
  98   static void print_all()                             { print_all_on(tty); }
  99 
 100   // Returns the CMS Thread
 101   static ConcurrentMarkSweepThread* cmst()    { return _cmst; }
 102   static CMSCollector*         collector()    { return _collector;  }
 103 
 104   // Create and start the CMS Thread, or stop it on shutdown
 105   static ConcurrentMarkSweepThread* start(CMSCollector* collector);
 106   static void stop();
 107   static bool should_terminate() { return _should_terminate; }
 108 
 109   // Synchronization using CMS token
 110   static void synchronize(bool is_cms_thread);
 111   static void desynchronize(bool is_cms_thread);
 112   static bool vm_thread_has_cms_token() {
 113     return CMS_flag_is_set(CMS_vm_has_token);
 114   }
 115   static bool cms_thread_has_cms_token() {
 116     return CMS_flag_is_set(CMS_cms_has_token);
 117   }
 118   static bool vm_thread_wants_cms_token() {
 119     return CMS_flag_is_set(CMS_vm_wants_token);
 120   }
 121   static bool cms_thread_wants_cms_token() {
 122     return CMS_flag_is_set(CMS_cms_wants_token);
 123   }
 124 
 125   // Wait on CMS lock until the next synchronous GC
 126   // or given timeout, whichever is earlier. A timeout value
 127   // of 0 indicates that there is no upper bound on the wait time.
 128   // A concurrent full gc request terminates the wait.
 129   void wait_on_cms_lock(long t_millis);
 130 
 131   // Wait on CMS lock until the next synchronous GC
 132   // or given timeout, whichever is earlier. A timeout value
 133   // of 0 indicates that there is no upper bound on the wait time.
 134   // A concurrent full gc request terminates the wait.
 135   void wait_on_cms_lock_for_scavenge(long t_millis);
 136 
 137   // The CMS thread will yield during the work portion of its cycle
 138   // only when requested to.  Both synchronous and asychronous requests
 139   // are provided:
 140   // (1) A synchronous request is used for young gen collections and
 141   //     for direct allocations.  The requesting thread increments
 142   //     _pending_yields at the beginning of an operation, and decrements
 143   //     _pending_yields when that operation is completed.
 144   //     In turn, the CMS thread yields when _pending_yields is positive,
 145   //     and continues to yield until the value reverts to 0.
 146   // (2) An asynchronous request, on the other hand, is used by iCMS
 147   //     for the stop_icms() operation. A single yield satisfies all of
 148   //     the outstanding asynch yield requests, of which there may
 149   //     occasionally be several in close succession. To accomplish
 150   //     this, an asynch-requesting thread atomically increments both
 151   //     _pending_yields and _pending_decrements. An asynchr requesting
 152   //     thread does not wait and "acknowledge" completion of an operation
 153   //     and deregister the request, like the synchronous version described
 154   //     above does. In turn, after yielding, the CMS thread decrements both
 155   //     _pending_yields and _pending_decrements by the value seen in
 156   //     _pending_decrements before the decrement.
 157   //  NOTE: The above scheme is isomorphic to having two request counters,
 158   //  one for async requests and one for sync requests, and for the CMS thread
 159   //  to check the sum of the two counters to decide whether it should yield
 160   //  and to clear only the async counter when it yields. However, it turns out
 161   //  to be more efficient for CMS code to just check a single counter
 162   //  _pending_yields that holds the sum (of both sync and async requests), and
 163   //  a second counter _pending_decrements that only holds the async requests,
 164   //  for greater efficiency, since in a typical CMS run, there are many more
 165   //  potential (i.e. static) yield points than there are actual
 166   //  (i.e. dynamic) yields because of requests, which are few and far between.
 167   //
 168   // Note that, while "_pending_yields >= _pending_decrements" is an invariant,
 169   // we cannot easily test that invariant, since the counters are manipulated via
 170   // atomic instructions without explicit locking and we cannot read
 171   // the two counters atomically together: one suggestion is to
 172   // use (for example) 16-bit counters so as to be able to read the
 173   // two counters atomically even on 32-bit platforms. Notice that
 174   // the second assert in acknowledge_yield_request() below does indeed
 175   // check a form of the above invariant, albeit indirectly.
 176 
 177   static void increment_pending_yields()   {
 178     Atomic::inc(&_pending_yields);
 179     assert(_pending_yields >= 0, "can't be negative");
 180   }
 181   static void decrement_pending_yields()   {
 182     Atomic::dec(&_pending_yields);
 183     assert(_pending_yields >= 0, "can't be negative");
 184   }
 185   static void asynchronous_yield_request() {
 186     assert(CMSIncrementalMode, "Currently only used w/iCMS");
 187     increment_pending_yields();
 188     Atomic::inc(&_pending_decrements);
 189     assert(_pending_decrements >= 0, "can't be negative");
 190   }
 191   static void acknowledge_yield_request() {
 192     jint decrement = _pending_decrements;
 193     if (decrement > 0) {
 194       assert(CMSIncrementalMode, "Currently only used w/iCMS");
 195       // Order important to preserve: _pending_yields >= _pending_decrements
 196       Atomic::add(-decrement, &_pending_decrements);
 197       Atomic::add(-decrement, &_pending_yields);
 198       assert(_pending_decrements >= 0, "can't be negative");
 199       assert(_pending_yields >= 0, "can't be negative");
 200     }
 201   }
 202   static bool should_yield()   { return _pending_yields > 0; }
 203 
 204   // CMS incremental mode.
 205   static void start_icms(); // notify thread to start a quantum of work
 206   static void stop_icms();  // request thread to stop working
 207   void icms_wait();         // if asked to stop, wait until notified to start
 208 
 209   // Incremental mode is enabled globally by the flag CMSIncrementalMode.  It
 210   // must also be enabled/disabled dynamically to allow foreground collections.
 211 #define ICMS_ENABLING_ASSERT                                                      \
 212           assert((CMSIncrementalMode  && _icms_disabled >= 0) ||                  \
 213                  (!CMSIncrementalMode && _icms_disabled <= 0), "Error")
 214 
 215   static inline void enable_icms() {
 216     ICMS_ENABLING_ASSERT;
 217     Atomic::dec(&_icms_disabled);
 218   }
 219   static inline void disable_icms() {
 220    ICMS_ENABLING_ASSERT;
 221    Atomic::inc(&_icms_disabled);
 222   }
 223   static inline bool icms_is_disabled() {
 224    ICMS_ENABLING_ASSERT;
 225    return _icms_disabled > 0;
 226   }
 227   static inline bool icms_is_enabled() {
 228    return !icms_is_disabled();
 229   }
 230 };
 231 
 232 inline void ConcurrentMarkSweepThread::trace_state(const char* desc) {
 233   if (CMSTraceThreadState) {
 234     char buf[128];
 235     TimeStamp& ts = gclog_or_tty->time_stamp();
 236     if (!ts.is_updated()) {
 237       ts.update();
 238     }
 239     jio_snprintf(buf, sizeof(buf), " [%.3f:  CMSThread %s] ",
 240                  ts.seconds(), desc);
 241     buf[sizeof(buf) - 1] = '\0';
 242     gclog_or_tty->print("%s", buf);
 243   }
 244 }
 245 
 246 // For scoped increment/decrement of (synchronous) yield requests
 247 class CMSSynchronousYieldRequest: public StackObj {
 248  public:
 249   CMSSynchronousYieldRequest() {
 250     ConcurrentMarkSweepThread::increment_pending_yields();
 251   }
 252   ~CMSSynchronousYieldRequest() {
 253     ConcurrentMarkSweepThread::decrement_pending_yields();
 254   }
 255 };
 256 
 257 // Used to emit a warning in case of unexpectedly excessive
 258 // looping (in "apparently endless loops") in CMS code.
 259 class CMSLoopCountWarn: public StackObj {
 260  private:
 261   const char* _src;
 262   const char* _msg;
 263   const intx  _threshold;
 264   intx        _ticks;
 265 
 266  public:
 267   inline CMSLoopCountWarn(const char* src, const char* msg,
 268                           const intx threshold) :
 269     _src(src), _msg(msg), _threshold(threshold), _ticks(0) { }
 270 
 271   inline void tick() {
 272     _ticks++;
 273     if (CMSLoopWarn && _ticks % _threshold == 0) {
 274       warning("%s has looped " INTX_FORMAT " times %s", _src, _ticks, _msg);
 275     }
 276   }
 277 };
 278 
 279 #endif // SHARE_VM_GC_IMPLEMENTATION_CONCURRENTMARKSWEEP_CONCURRENTMARKSWEEPTHREAD_HPP