< prev index next >

src/share/vm/gc/shared/referenceProcessor.cpp

Print this page




  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 #include "precompiled.hpp"
  26 #include "classfile/javaClasses.hpp"
  27 #include "classfile/systemDictionary.hpp"
  28 #include "gc/shared/collectedHeap.hpp"
  29 #include "gc/shared/collectedHeap.inline.hpp"
  30 #include "gc/shared/gcTimer.hpp"
  31 #include "gc/shared/gcTraceTime.hpp"
  32 #include "gc/shared/referencePolicy.hpp"
  33 #include "gc/shared/referenceProcessor.hpp"

  34 #include "memory/allocation.hpp"
  35 #include "oops/oop.inline.hpp"
  36 #include "runtime/java.hpp"
  37 #include "runtime/jniHandles.hpp"
  38 
  39 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
  40 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy      = NULL;
  41 jlong            ReferenceProcessor::_soft_ref_timestamp_clock = 0;
  42 
  43 void referenceProcessor_init() {
  44   ReferenceProcessor::init_statics();
  45 }
  46 
  47 void ReferenceProcessor::init_statics() {
  48   // We need a monotonically non-decreasing time in ms but
  49   // os::javaTimeMillis() does not guarantee monotonicity.
  50   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
  51 
  52   // Initialize the soft ref timestamp clock.
  53   _soft_ref_timestamp_clock = now;


 169   // javaTimeNanos(), which is guaranteed to be monotonically
 170   // non-decreasing provided the underlying platform provides such
 171   // a time source (and it is bug free).
 172   // In product mode, however, protect ourselves from non-monotonicity.
 173   if (now > _soft_ref_timestamp_clock) {
 174     _soft_ref_timestamp_clock = now;
 175     java_lang_ref_SoftReference::set_clock(now);
 176   }
 177   // Else leave clock stalled at its old value until time progresses
 178   // past clock value.
 179 }
 180 
 181 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
 182   size_t total = 0;
 183   for (uint i = 0; i < _max_num_q; ++i) {
 184     total += lists[i].length();
 185   }
 186   return total;
 187 }
 188 
 189 static void log_ref_count(size_t count, bool doit) {
 190   if (doit) {
 191     gclog_or_tty->print(", " SIZE_FORMAT " refs", count);
 192   }
 193 }
 194 
 195 class GCRefTraceTime : public StackObj {
 196   GCTraceTimeImpl _gc_trace_time;
 197  public:
 198   GCRefTraceTime(const char* title, bool doit, GCTimer* timer, size_t count) :
 199     _gc_trace_time(title, doit, false, timer) {
 200     log_ref_count(count, doit);
 201   }
 202 };
 203 
 204 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 205   BoolObjectClosure*           is_alive,
 206   OopClosure*                  keep_alive,
 207   VoidClosure*                 complete_gc,
 208   AbstractRefProcTaskExecutor* task_executor,
 209   GCTimer*                     gc_timer) {
 210 
 211   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 212   // Stop treating discovered references specially.
 213   disable_discovery();
 214 
 215   // If discovery was concurrent, someone could have modified
 216   // the value of the static field in the j.l.r.SoftReference
 217   // class that holds the soft reference timestamp clock using
 218   // reflection or Unsafe between when discovery was enabled and
 219   // now. Unconditionally update the static field in ReferenceProcessor
 220   // here so that we use the new value during processing of the
 221   // discovered soft refs.
 222 
 223   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 224 
 225   bool trace_time = PrintGCDetails && PrintReferenceGC;
 226 
 227   // Include cleaners in phantom statistics.  We expect Cleaner
 228   // references to be temporary, and don't want to deal with
 229   // possible incompatibilities arising from making it more visible.
 230   ReferenceProcessorStats stats(
 231       total_count(_discoveredSoftRefs),
 232       total_count(_discoveredWeakRefs),
 233       total_count(_discoveredFinalRefs),
 234       total_count(_discoveredPhantomRefs) + total_count(_discoveredCleanerRefs));
 235 
 236   // Soft references
 237   {
 238     GCRefTraceTime tt("SoftReference", trace_time, gc_timer, stats.soft_count());
 239     process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 240                                is_alive, keep_alive, complete_gc, task_executor);
 241   }
 242 
 243   update_soft_ref_master_clock();
 244 
 245   // Weak references
 246   {
 247     GCRefTraceTime tt("WeakReference", trace_time, gc_timer, stats.weak_count());
 248     process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 249                                is_alive, keep_alive, complete_gc, task_executor);
 250   }
 251 
 252   // Final references
 253   {
 254     GCRefTraceTime tt("FinalReference", trace_time, gc_timer, stats.final_count());
 255     process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 256                                is_alive, keep_alive, complete_gc, task_executor);
 257   }
 258 
 259   // Phantom references
 260   {
 261     GCRefTraceTime tt("PhantomReference", trace_time, gc_timer, stats.phantom_count());
 262     process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
 263                                is_alive, keep_alive, complete_gc, task_executor);
 264 
 265     // Process cleaners, but include them in phantom timing.  We expect
 266     // Cleaner references to be temporary, and don't want to deal with
 267     // possible incompatibilities arising from making it more visible.
 268     process_discovered_reflist(_discoveredCleanerRefs, NULL, true,
 269                                  is_alive, keep_alive, complete_gc, task_executor);
 270   }
 271 
 272   // Weak global JNI references. It would make more sense (semantically) to
 273   // traverse these simultaneously with the regular weak references above, but
 274   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 275   // thus use JNI weak references to circumvent the phantom references and
 276   // resurrect a "post-mortem" object.
 277   {
 278     GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer);
 279     NOT_PRODUCT(log_ref_count(count_jni_refs(), trace_time);)
 280     if (task_executor != NULL) {
 281       task_executor->set_single_threaded_mode();
 282     }
 283     process_phaseJNI(is_alive, keep_alive, complete_gc);
 284   }
 285 




 286   return stats;
 287 }
 288 
 289 #ifndef PRODUCT
 290 // Calculate the number of jni handles.
 291 uint ReferenceProcessor::count_jni_refs() {
 292   class AlwaysAliveClosure: public BoolObjectClosure {
 293   public:
 294     virtual bool do_object_b(oop obj) { return true; }
 295   };
 296 
 297   class CountHandleClosure: public OopClosure {
 298   private:
 299     int _count;
 300   public:
 301     CountHandleClosure(): _count(0) {}
 302     void do_oop(oop* unused)       { _count++; }
 303     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 304     int count() { return _count; }
 305   };
 306   CountHandleClosure global_handle_count;
 307   AlwaysAliveClosure always_alive;
 308   JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
 309   return global_handle_count.count();
 310 }
 311 #endif
 312 
 313 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 314                                           OopClosure*        keep_alive,
 315                                           VoidClosure*       complete_gc) {
 316   JNIHandles::weak_oops_do(is_alive, keep_alive);
 317   complete_gc->do_void();
 318 }
 319 
 320 
 321 template <class T>
 322 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
 323                                    AbstractRefProcTaskExecutor* task_executor) {
 324 


 345     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
 346   } else {
 347     return enqueue_discovered_ref_helper<oop>(this, task_executor);
 348   }
 349 }
 350 
 351 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
 352                                                     HeapWord* pending_list_addr) {
 353   // Given a list of refs linked through the "discovered" field
 354   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 355   // thus distinguishing them from active References, then
 356   // prepend them to the pending list.
 357   //
 358   // The Java threads will see the Reference objects linked together through
 359   // the discovered field. Instead of trying to do the write barrier updates
 360   // in all places in the reference processor where we manipulate the discovered
 361   // field we make sure to do the barrier here where we anyway iterate through
 362   // all linked Reference objects. Note that it is important to not dirty any
 363   // cards during reference processing since this will cause card table
 364   // verification to fail for G1.
 365   if (TraceReferenceGC && PrintGCDetails) {
 366     gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
 367                            INTPTR_FORMAT, p2i(refs_list.head()));
 368   }
 369 
 370   oop obj = NULL;
 371   oop next_d = refs_list.head();
 372   // Walk down the list, self-looping the next field
 373   // so that the References are not considered active.
 374   while (obj != next_d) {
 375     obj = next_d;
 376     assert(obj->is_instance(), "should be an instance object");
 377     assert(InstanceKlass::cast(obj->klass())->is_reference_instance_klass(), "should be reference object");
 378     next_d = java_lang_ref_Reference::discovered(obj);
 379     if (TraceReferenceGC && PrintGCDetails) {
 380       gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 381                              p2i(obj), p2i(next_d));
 382     }
 383     assert(java_lang_ref_Reference::next(obj) == NULL,
 384            "Reference not active; should not be discovered");
 385     // Self-loop next, so as to make Ref not active.
 386     java_lang_ref_Reference::set_next_raw(obj, obj);
 387     if (next_d != obj) {
 388       oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
 389     } else {
 390       // This is the last object.
 391       // Swap refs_list into pending_list_addr and
 392       // set obj's discovered to what we read from pending_list_addr.
 393       oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 394       // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
 395       java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
 396       oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
 397     }
 398   }
 399 }
 400 
 401 // Parallel enqueue task
 402 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {


 500 // a single general iterator invocation that receives appropriate
 501 // closures that accomplish this work.
 502 
 503 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 504 // referents are not alive, but that should be kept alive for policy reasons.
 505 // Keep alive the transitive closure of all such referents.
 506 void
 507 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 508                                    ReferencePolicy*   policy,
 509                                    BoolObjectClosure* is_alive,
 510                                    OopClosure*        keep_alive,
 511                                    VoidClosure*       complete_gc) {
 512   assert(policy != NULL, "Must have a non-NULL policy");
 513   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 514   // Decide which softly reachable refs should be kept alive.
 515   while (iter.has_next()) {
 516     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 517     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 518     if (referent_is_dead &&
 519         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 520       if (TraceReferenceGC) {
 521         gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 522                                p2i(iter.obj()), iter.obj()->klass()->internal_name());
 523       }
 524       // Remove Reference object from list
 525       iter.remove();
 526       // keep the referent around
 527       iter.make_referent_alive();
 528       iter.move_to_next();
 529     } else {
 530       iter.next();
 531     }
 532   }
 533   // Close the reachable set
 534   complete_gc->do_void();
 535   NOT_PRODUCT(
 536     if (PrintGCDetails && TraceReferenceGC) {
 537       gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT
 538         " discovered Refs by policy, from list " INTPTR_FORMAT,
 539         iter.removed(), iter.processed(), p2i(refs_list.head()));
 540     }
 541   )
 542 }
 543 
 544 // Traverse the list and remove any Refs that are not active, or
 545 // whose referents are either alive or NULL.
 546 void
 547 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 548                              BoolObjectClosure* is_alive,
 549                              OopClosure*        keep_alive) {
 550   assert(discovery_is_atomic(), "Error");
 551   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 552   while (iter.has_next()) {
 553     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 554     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 555     assert(next == NULL, "Should not discover inactive Reference");
 556     if (iter.is_referent_alive()) {
 557       if (TraceReferenceGC) {
 558         gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 559                                p2i(iter.obj()), iter.obj()->klass()->internal_name());
 560       }
 561       // The referent is reachable after all.
 562       // Remove Reference object from list.
 563       iter.remove();
 564       // Update the referent pointer as necessary: Note that this
 565       // should not entail any recursive marking because the
 566       // referent must already have been traversed.
 567       iter.make_referent_alive();
 568       iter.move_to_next();
 569     } else {
 570       iter.next();
 571     }
 572   }
 573   NOT_PRODUCT(
 574     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 575       gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 576         " Refs in discovered list " INTPTR_FORMAT,
 577         iter.removed(), iter.processed(), p2i(refs_list.head()));
 578     }
 579   )
 580 }
 581 
 582 void
 583 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 584                                                   BoolObjectClosure* is_alive,
 585                                                   OopClosure*        keep_alive,
 586                                                   VoidClosure*       complete_gc) {
 587   assert(!discovery_is_atomic(), "Error");
 588   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 589   while (iter.has_next()) {
 590     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 591     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 592     oop next = java_lang_ref_Reference::next(iter.obj());
 593     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 594          next != NULL)) {
 595       assert(next->is_oop_or_null(), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next));
 596       // Remove Reference object from list
 597       iter.remove();
 598       // Trace the cohorts
 599       iter.make_referent_alive();
 600       if (UseCompressedOops) {
 601         keep_alive->do_oop((narrowOop*)next_addr);
 602       } else {
 603         keep_alive->do_oop((oop*)next_addr);
 604       }
 605       iter.move_to_next();
 606     } else {
 607       iter.next();
 608     }
 609   }
 610   // Now close the newly reachable set
 611   complete_gc->do_void();
 612   NOT_PRODUCT(
 613     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 614       gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 615         " Refs in discovered list " INTPTR_FORMAT,
 616         iter.removed(), iter.processed(), p2i(refs_list.head()));
 617     }
 618   )
 619 }
 620 
 621 // Traverse the list and process the referents, by either
 622 // clearing them or keeping them (and their reachable
 623 // closure) alive.
 624 void
 625 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 626                                    bool               clear_referent,
 627                                    BoolObjectClosure* is_alive,
 628                                    OopClosure*        keep_alive,
 629                                    VoidClosure*       complete_gc) {
 630   ResourceMark rm;
 631   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 632   while (iter.has_next()) {
 633     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 634     if (clear_referent) {
 635       // NULL out referent pointer
 636       iter.clear_referent();
 637     } else {
 638       // keep the referent around
 639       iter.make_referent_alive();
 640     }
 641     if (TraceReferenceGC) {
 642       gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 643                              clear_referent ? "cleared " : "",
 644                              p2i(iter.obj()), iter.obj()->klass()->internal_name());
 645     }
 646     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
 647     iter.next();
 648   }
 649   // Close the reachable set
 650   complete_gc->do_void();
 651 }
 652 
 653 void
 654 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 655   oop obj = NULL;
 656   oop next = refs_list.head();
 657   while (next != obj) {
 658     obj = next;
 659     next = java_lang_ref_Reference::discovered(obj);
 660     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 661   }
 662   refs_list.set_head(NULL);
 663   refs_list.set_length(0);
 664 }
 665 
 666 void ReferenceProcessor::abandon_partial_discovery() {
 667   // loop over the lists
 668   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 669     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 670       gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
 671     }
 672     clear_discovered_references(_discovered_refs[i]);
 673   }
 674 }
 675 
 676 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 677 public:
 678   RefProcPhase1Task(ReferenceProcessor& ref_processor,
 679                     DiscoveredList      refs_lists[],
 680                     ReferencePolicy*    policy,
 681                     bool                marks_oops_alive)
 682     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 683       _policy(policy)
 684   { }
 685   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 686                     OopClosure& keep_alive,
 687                     VoidClosure& complete_gc)
 688   {
 689     Thread* thr = Thread::current();
 690     int refs_list_index = ((WorkerThread*)thr)->id();


 719                     bool                marks_oops_alive)
 720     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 721       _clear_referent(clear_referent)
 722   { }
 723   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 724                     OopClosure& keep_alive,
 725                     VoidClosure& complete_gc)
 726   {
 727     // Don't use "refs_list_index" calculated in this way because
 728     // balance_queues() has moved the Ref's into the first n queues.
 729     // Thread* thr = Thread::current();
 730     // int refs_list_index = ((WorkerThread*)thr)->id();
 731     // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
 732     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 733                                   &is_alive, &keep_alive, &complete_gc);
 734   }
 735 private:
 736   bool _clear_referent;
 737 };
 738 














 739 // Balances reference queues.
 740 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 741 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 742 // corresponding to the active workers will be processed.
 743 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 744 {
 745   // calculate total length
 746   size_t total_refs = 0;
 747   if (TraceReferenceGC && PrintGCDetails) {
 748     gclog_or_tty->print_cr("\nBalance ref_lists ");
 749   }
 750 
 751   for (uint i = 0; i < _max_num_q; ++i) {
 752     total_refs += ref_lists[i].length();
 753     if (TraceReferenceGC && PrintGCDetails) {
 754       gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());
 755     }
 756   }
 757   if (TraceReferenceGC && PrintGCDetails) {
 758     gclog_or_tty->print_cr(" = " SIZE_FORMAT, total_refs);
 759   }

 760   size_t avg_refs = total_refs / _num_q + 1;
 761   uint to_idx = 0;
 762   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
 763     bool move_all = false;
 764     if (from_idx >= _num_q) {
 765       move_all = ref_lists[from_idx].length() > 0;
 766     }
 767     while ((ref_lists[from_idx].length() > avg_refs) ||
 768            move_all) {
 769       assert(to_idx < _num_q, "Sanity Check!");
 770       if (ref_lists[to_idx].length() < avg_refs) {
 771         // move superfluous refs
 772         size_t refs_to_move;
 773         // Move all the Ref's if the from queue will not be processed.
 774         if (move_all) {
 775           refs_to_move = MIN2(ref_lists[from_idx].length(),
 776                               avg_refs - ref_lists[to_idx].length());
 777         } else {
 778           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 779                               avg_refs - ref_lists[to_idx].length());


 803         // Remove the chain from the from list.
 804         if (move_tail == new_head) {
 805           // We found the end of the from list.
 806           ref_lists[from_idx].set_head(NULL);
 807         } else {
 808           ref_lists[from_idx].set_head(new_head);
 809         }
 810         ref_lists[from_idx].dec_length(refs_to_move);
 811         if (ref_lists[from_idx].length() == 0) {
 812           break;
 813         }
 814       } else {
 815         to_idx = (to_idx + 1) % _num_q;
 816       }
 817     }
 818   }
 819 #ifdef ASSERT
 820   size_t balanced_total_refs = 0;
 821   for (uint i = 0; i < _max_num_q; ++i) {
 822     balanced_total_refs += ref_lists[i].length();
 823     if (TraceReferenceGC && PrintGCDetails) {
 824       gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());
 825     }
 826   }
 827   if (TraceReferenceGC && PrintGCDetails) {
 828     gclog_or_tty->print_cr(" = " SIZE_FORMAT, balanced_total_refs);
 829     gclog_or_tty->flush();
 830   }

 831   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 832 #endif
 833 }
 834 
 835 void ReferenceProcessor::balance_all_queues() {
 836   balance_queues(_discoveredSoftRefs);
 837   balance_queues(_discoveredWeakRefs);
 838   balance_queues(_discoveredFinalRefs);
 839   balance_queues(_discoveredPhantomRefs);
 840   balance_queues(_discoveredCleanerRefs);
 841 }
 842 
 843 void ReferenceProcessor::process_discovered_reflist(
 844   DiscoveredList               refs_lists[],
 845   ReferencePolicy*             policy,
 846   bool                         clear_referent,
 847   BoolObjectClosure*           is_alive,
 848   OopClosure*                  keep_alive,
 849   VoidClosure*                 complete_gc,
 850   AbstractRefProcTaskExecutor* task_executor)


 933     case REF_SOFT:
 934       list = &_discoveredSoftRefs[id];
 935       break;
 936     case REF_WEAK:
 937       list = &_discoveredWeakRefs[id];
 938       break;
 939     case REF_FINAL:
 940       list = &_discoveredFinalRefs[id];
 941       break;
 942     case REF_PHANTOM:
 943       list = &_discoveredPhantomRefs[id];
 944       break;
 945     case REF_CLEANER:
 946       list = &_discoveredCleanerRefs[id];
 947       break;
 948     case REF_NONE:
 949       // we should not reach here if we are an InstanceRefKlass
 950     default:
 951       ShouldNotReachHere();
 952   }
 953   if (TraceReferenceGC && PrintGCDetails) {
 954     gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
 955   }
 956   return list;
 957 }
 958 
 959 inline void
 960 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
 961                                               oop             obj,
 962                                               HeapWord*       discovered_addr) {
 963   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
 964   // First we must make sure this object is only enqueued once. CAS in a non null
 965   // discovered_addr.
 966   oop current_head = refs_list.head();
 967   // The last ref must have its discovered field pointing to itself.
 968   oop next_discovered = (current_head != NULL) ? current_head : obj;
 969 
 970   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
 971                                                     NULL);
 972   if (retest == NULL) {
 973     // This thread just won the right to enqueue the object.
 974     // We have separate lists for enqueueing, so no synchronization
 975     // is necessary.
 976     refs_list.set_head(obj);
 977     refs_list.inc_length(1);
 978 
 979     if (TraceReferenceGC) {
 980       gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
 981                              p2i(obj), obj->klass()->internal_name());
 982     }
 983   } else {
 984     // If retest was non NULL, another thread beat us to it:
 985     // The reference has already been discovered...
 986     if (TraceReferenceGC) {
 987       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
 988                              p2i(obj), obj->klass()->internal_name());
 989     }
 990   }
 991 }
 992 
 993 #ifndef PRODUCT
 994 // Non-atomic (i.e. concurrent) discovery might allow us
 995 // to observe j.l.References with NULL referents, being those
 996 // cleared concurrently by mutators during (or after) discovery.
 997 void ReferenceProcessor::verify_referent(oop obj) {
 998   bool da = discovery_is_atomic();
 999   oop referent = java_lang_ref_Reference::referent(obj);
1000   assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1001          "Bad referent " INTPTR_FORMAT " found in Reference "
1002          INTPTR_FORMAT " during %satomic discovery ",
1003          p2i(referent), p2i(obj), da ? "" : "non-");
1004 }
1005 #endif
1006 
1007 // We mention two of several possible choices here:
1008 // #0: if the reference object is not in the "originating generation"
1009 //     (or part of the heap being collected, indicated by our "span"
1010 //     we don't treat it specially (i.e. we scan it as we would
1011 //     a normal oop, treating its references as strong references).


1061   if (rt == REF_SOFT) {
1062     // For soft refs we can decide now if these are not
1063     // current candidates for clearing, in which case we
1064     // can mark through them now, rather than delaying that
1065     // to the reference-processing phase. Since all current
1066     // time-stamp policies advance the soft-ref clock only
1067     // at a full collection cycle, this is always currently
1068     // accurate.
1069     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1070       return false;
1071     }
1072   }
1073 
1074   ResourceMark rm;      // Needed for tracing.
1075 
1076   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1077   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1078   assert(discovered->is_oop_or_null(), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
1079   if (discovered != NULL) {
1080     // The reference has already been discovered...
1081     if (TraceReferenceGC) {
1082       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1083                              p2i(obj), obj->klass()->internal_name());
1084     }
1085     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1086       // assumes that an object is not processed twice;
1087       // if it's been already discovered it must be on another
1088       // generation's discovered list; so we won't discover it.
1089       return false;
1090     } else {
1091       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1092              "Unrecognized policy");
1093       // Check assumption that an object is not potentially
1094       // discovered twice except by concurrent collectors that potentially
1095       // trace the same Reference object twice.
1096       assert(UseConcMarkSweepGC || UseG1GC,
1097              "Only possible with a concurrent marking collector");
1098       return true;
1099     }
1100   }
1101 
1102   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1103     verify_referent(obj);
1104     // Discover if and only if EITHER:


1119   // Get the right type of discovered queue head.
1120   DiscoveredList* list = get_discovered_list(rt);
1121   if (list == NULL) {
1122     return false;   // nothing special needs to be done
1123   }
1124 
1125   if (_discovery_is_mt) {
1126     add_to_discovered_list_mt(*list, obj, discovered_addr);
1127   } else {
1128     // We do a raw store here: the field will be visited later when processing
1129     // the discovered references.
1130     oop current_head = list->head();
1131     // The last ref must have its discovered field pointing to itself.
1132     oop next_discovered = (current_head != NULL) ? current_head : obj;
1133 
1134     assert(discovered == NULL, "control point invariant");
1135     oop_store_raw(discovered_addr, next_discovered);
1136     list->set_head(obj);
1137     list->inc_length(1);
1138 
1139     if (TraceReferenceGC) {
1140       gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1141                                 p2i(obj), obj->klass()->internal_name());
1142     }
1143   }
1144   assert(obj->is_oop(), "Discovered a bad reference");
1145   verify_referent(obj);
1146   return true;
1147 }
1148 
1149 // Preclean the discovered references by removing those
1150 // whose referents are alive, and by marking from those that
1151 // are not active. These lists can be handled here
1152 // in any order and, indeed, concurrently.
1153 void ReferenceProcessor::preclean_discovered_references(
1154   BoolObjectClosure* is_alive,
1155   OopClosure* keep_alive,
1156   VoidClosure* complete_gc,
1157   YieldClosure* yield,
1158   GCTimer* gc_timer) {
1159 
1160   // Soft references
1161   {
1162     GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1163               false, gc_timer);
1164     for (uint i = 0; i < _max_num_q; i++) {
1165       if (yield->should_return()) {
1166         return;
1167       }
1168       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1169                                   keep_alive, complete_gc, yield);
1170     }
1171   }
1172 
1173   // Weak references
1174   {
1175     GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1176               false, gc_timer);
1177     for (uint i = 0; i < _max_num_q; i++) {
1178       if (yield->should_return()) {
1179         return;
1180       }
1181       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1182                                   keep_alive, complete_gc, yield);
1183     }
1184   }
1185 
1186   // Final references
1187   {
1188     GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1189               false, gc_timer);
1190     for (uint i = 0; i < _max_num_q; i++) {
1191       if (yield->should_return()) {
1192         return;
1193       }
1194       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1195                                   keep_alive, complete_gc, yield);
1196     }
1197   }
1198 
1199   // Phantom references
1200   {
1201     GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1202               false, gc_timer);
1203     for (uint i = 0; i < _max_num_q; i++) {
1204       if (yield->should_return()) {
1205         return;
1206       }
1207       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1208                                   keep_alive, complete_gc, yield);
1209     }
1210 
1211     // Cleaner references.  Included in timing for phantom references.  We
1212     // expect Cleaner references to be temporary, and don't want to deal with
1213     // possible incompatibilities arising from making it more visible.
1214     for (uint i = 0; i < _max_num_q; i++) {
1215       if (yield->should_return()) {
1216         return;
1217       }
1218       preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1219                                   keep_alive, complete_gc, yield);
1220     }
1221   }
1222 }


1227 // thus precleaning the ref lists (which happens single-threaded today),
1228 // we do not disable refs discovery to honor the correct semantics of
1229 // java.lang.Reference. As a result, we need to be careful below
1230 // that ref removal steps interleave safely with ref discovery steps
1231 // (in this thread).
1232 void
1233 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1234                                                 BoolObjectClosure* is_alive,
1235                                                 OopClosure*        keep_alive,
1236                                                 VoidClosure*       complete_gc,
1237                                                 YieldClosure*      yield) {
1238   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1239   while (iter.has_next()) {
1240     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1241     oop obj = iter.obj();
1242     oop next = java_lang_ref_Reference::next(obj);
1243     if (iter.referent() == NULL || iter.is_referent_alive() ||
1244         next != NULL) {
1245       // The referent has been cleared, or is alive, or the Reference is not
1246       // active; we need to trace and mark its cohort.
1247       if (TraceReferenceGC) {
1248         gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1249                                p2i(iter.obj()), iter.obj()->klass()->internal_name());
1250       }
1251       // Remove Reference object from list
1252       iter.remove();
1253       // Keep alive its cohort.
1254       iter.make_referent_alive();
1255       if (UseCompressedOops) {
1256         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1257         keep_alive->do_oop(next_addr);
1258       } else {
1259         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1260         keep_alive->do_oop(next_addr);
1261       }
1262       iter.move_to_next();
1263     } else {
1264       iter.next();
1265     }
1266   }
1267   // Close the reachable set
1268   complete_gc->do_void();
1269 
1270   NOT_PRODUCT(
1271     if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1272       gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT
1273         " Refs in discovered list " INTPTR_FORMAT,
1274         iter.removed(), iter.processed(), p2i(refs_list.head()));
1275     }
1276   )
1277 }
1278 
1279 const char* ReferenceProcessor::list_name(uint i) {
1280    assert(i <= _max_num_q * number_of_subclasses_of_ref(),
1281           "Out of bounds index");
1282 
1283    int j = i / _max_num_q;
1284    switch (j) {
1285      case 0: return "SoftRef";
1286      case 1: return "WeakRef";
1287      case 2: return "FinalRef";
1288      case 3: return "PhantomRef";
1289      case 4: return "CleanerRef";
1290    }
1291    ShouldNotReachHere();
1292    return NULL;
1293 }


  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 #include "precompiled.hpp"
  26 #include "classfile/javaClasses.hpp"
  27 #include "classfile/systemDictionary.hpp"
  28 #include "gc/shared/collectedHeap.hpp"
  29 #include "gc/shared/collectedHeap.inline.hpp"
  30 #include "gc/shared/gcTimer.hpp"
  31 #include "gc/shared/gcTraceTime.hpp"
  32 #include "gc/shared/referencePolicy.hpp"
  33 #include "gc/shared/referenceProcessor.hpp"
  34 #include "logging/log.hpp"
  35 #include "memory/allocation.hpp"
  36 #include "oops/oop.inline.hpp"
  37 #include "runtime/java.hpp"
  38 #include "runtime/jniHandles.hpp"
  39 
  40 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
  41 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy      = NULL;
  42 jlong            ReferenceProcessor::_soft_ref_timestamp_clock = 0;
  43 
  44 void referenceProcessor_init() {
  45   ReferenceProcessor::init_statics();
  46 }
  47 
  48 void ReferenceProcessor::init_statics() {
  49   // We need a monotonically non-decreasing time in ms but
  50   // os::javaTimeMillis() does not guarantee monotonicity.
  51   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
  52 
  53   // Initialize the soft ref timestamp clock.
  54   _soft_ref_timestamp_clock = now;


 170   // javaTimeNanos(), which is guaranteed to be monotonically
 171   // non-decreasing provided the underlying platform provides such
 172   // a time source (and it is bug free).
 173   // In product mode, however, protect ourselves from non-monotonicity.
 174   if (now > _soft_ref_timestamp_clock) {
 175     _soft_ref_timestamp_clock = now;
 176     java_lang_ref_SoftReference::set_clock(now);
 177   }
 178   // Else leave clock stalled at its old value until time progresses
 179   // past clock value.
 180 }
 181 
 182 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
 183   size_t total = 0;
 184   for (uint i = 0; i < _max_num_q; ++i) {
 185     total += lists[i].length();
 186   }
 187   return total;
 188 }
 189 















 190 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 191   BoolObjectClosure*           is_alive,
 192   OopClosure*                  keep_alive,
 193   VoidClosure*                 complete_gc,
 194   AbstractRefProcTaskExecutor* task_executor,
 195   GCTimer*                     gc_timer) {
 196 
 197   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 198   // Stop treating discovered references specially.
 199   disable_discovery();
 200 
 201   // If discovery was concurrent, someone could have modified
 202   // the value of the static field in the j.l.r.SoftReference
 203   // class that holds the soft reference timestamp clock using
 204   // reflection or Unsafe between when discovery was enabled and
 205   // now. Unconditionally update the static field in ReferenceProcessor
 206   // here so that we use the new value during processing of the
 207   // discovered soft refs.
 208 
 209   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 210 


 211   // Include cleaners in phantom statistics.  We expect Cleaner
 212   // references to be temporary, and don't want to deal with
 213   // possible incompatibilities arising from making it more visible.
 214   ReferenceProcessorStats stats(
 215       total_count(_discoveredSoftRefs),
 216       total_count(_discoveredWeakRefs),
 217       total_count(_discoveredFinalRefs),
 218       total_count(_discoveredPhantomRefs) + total_count(_discoveredCleanerRefs));
 219 
 220   // Soft references
 221   {
 222     GCTraceTime(Debug, gc, ref) tt("SoftReference", gc_timer);
 223     process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 224                                is_alive, keep_alive, complete_gc, task_executor);
 225   }
 226 
 227   update_soft_ref_master_clock();
 228 
 229   // Weak references
 230   {
 231     GCTraceTime(Debug, gc, ref) tt("WeakReference", gc_timer);
 232     process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 233                                is_alive, keep_alive, complete_gc, task_executor);
 234   }
 235 
 236   // Final references
 237   {
 238         GCTraceTime(Debug, gc, ref) tt("FinalReference", gc_timer);
 239     process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 240                                is_alive, keep_alive, complete_gc, task_executor);
 241   }
 242 
 243   // Phantom references
 244   {
 245     GCTraceTime(Debug, gc, ref) tt("PhantomReference", gc_timer);
 246     process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
 247                                is_alive, keep_alive, complete_gc, task_executor);
 248 
 249     // Process cleaners, but include them in phantom timing.  We expect
 250     // Cleaner references to be temporary, and don't want to deal with
 251     // possible incompatibilities arising from making it more visible.
 252     process_discovered_reflist(_discoveredCleanerRefs, NULL, true,
 253                                  is_alive, keep_alive, complete_gc, task_executor);
 254   }
 255 
 256   // Weak global JNI references. It would make more sense (semantically) to
 257   // traverse these simultaneously with the regular weak references above, but
 258   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 259   // thus use JNI weak references to circumvent the phantom references and
 260   // resurrect a "post-mortem" object.
 261   {
 262     GCTraceTime(Debug, gc, ref) tt("JNI Weak Reference", gc_timer);

 263     if (task_executor != NULL) {
 264       task_executor->set_single_threaded_mode();
 265     }
 266     process_phaseJNI(is_alive, keep_alive, complete_gc);
 267   }
 268 
 269   log_debug(gc, ref)("Ref Counts: Soft " SIZE_FORMAT " Weak: " SIZE_FORMAT " Final: " SIZE_FORMAT " Phantom: " SIZE_FORMAT,
 270                   stats.soft_count(), stats.weak_count(), stats.final_count(), stats.phantom_count());
 271   log_develop(gc, ref)("JNI Weak Reference count: " SIZE_FORMAT, count_jni_refs());
 272 
 273   return stats;
 274 }
 275 
 276 #ifndef PRODUCT
 277 // Calculate the number of jni handles.
 278 size_t ReferenceProcessor::count_jni_refs() {
 279   class AlwaysAliveClosure: public BoolObjectClosure {
 280   public:
 281     virtual bool do_object_b(oop obj) { return true; }
 282   };
 283 
 284   class CountHandleClosure: public OopClosure {
 285   private:
 286     size_t _count;
 287   public:
 288     CountHandleClosure(): _count(0) {}
 289     void do_oop(oop* unused)       { _count++; }
 290     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 291     size_t count() { return _count; }
 292   };
 293   CountHandleClosure global_handle_count;
 294   AlwaysAliveClosure always_alive;
 295   JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
 296   return global_handle_count.count();
 297 }
 298 #endif
 299 
 300 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 301                                           OopClosure*        keep_alive,
 302                                           VoidClosure*       complete_gc) {
 303   JNIHandles::weak_oops_do(is_alive, keep_alive);
 304   complete_gc->do_void();
 305 }
 306 
 307 
 308 template <class T>
 309 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
 310                                    AbstractRefProcTaskExecutor* task_executor) {
 311 


 332     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
 333   } else {
 334     return enqueue_discovered_ref_helper<oop>(this, task_executor);
 335   }
 336 }
 337 
 338 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
 339                                                     HeapWord* pending_list_addr) {
 340   // Given a list of refs linked through the "discovered" field
 341   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 342   // thus distinguishing them from active References, then
 343   // prepend them to the pending list.
 344   //
 345   // The Java threads will see the Reference objects linked together through
 346   // the discovered field. Instead of trying to do the write barrier updates
 347   // in all places in the reference processor where we manipulate the discovered
 348   // field we make sure to do the barrier here where we anyway iterate through
 349   // all linked Reference objects. Note that it is important to not dirty any
 350   // cards during reference processing since this will cause card table
 351   // verification to fail for G1.
 352   log_develop(gc, ref)("ReferenceProcessor::enqueue_discovered_reflist list " INTPTR_FORMAT, p2i(refs_list.head()));



 353 
 354   oop obj = NULL;
 355   oop next_d = refs_list.head();
 356   // Walk down the list, self-looping the next field
 357   // so that the References are not considered active.
 358   while (obj != next_d) {
 359     obj = next_d;
 360     assert(obj->is_instance(), "should be an instance object");
 361     assert(InstanceKlass::cast(obj->klass())->is_reference_instance_klass(), "should be reference object");
 362     next_d = java_lang_ref_Reference::discovered(obj);
 363     log_develop(gc, ref)("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, p2i(obj), p2i(next_d));



 364     assert(java_lang_ref_Reference::next(obj) == NULL,
 365            "Reference not active; should not be discovered");
 366     // Self-loop next, so as to make Ref not active.
 367     java_lang_ref_Reference::set_next_raw(obj, obj);
 368     if (next_d != obj) {
 369       oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
 370     } else {
 371       // This is the last object.
 372       // Swap refs_list into pending_list_addr and
 373       // set obj's discovered to what we read from pending_list_addr.
 374       oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 375       // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
 376       java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
 377       oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
 378     }
 379   }
 380 }
 381 
 382 // Parallel enqueue task
 383 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {


 481 // a single general iterator invocation that receives appropriate
 482 // closures that accomplish this work.
 483 
 484 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 485 // referents are not alive, but that should be kept alive for policy reasons.
 486 // Keep alive the transitive closure of all such referents.
 487 void
 488 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 489                                    ReferencePolicy*   policy,
 490                                    BoolObjectClosure* is_alive,
 491                                    OopClosure*        keep_alive,
 492                                    VoidClosure*       complete_gc) {
 493   assert(policy != NULL, "Must have a non-NULL policy");
 494   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 495   // Decide which softly reachable refs should be kept alive.
 496   while (iter.has_next()) {
 497     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 498     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 499     if (referent_is_dead &&
 500         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 501       log_develop(gc, ref)("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",

 502                                p2i(iter.obj()), iter.obj()->klass()->internal_name());

 503       // Remove Reference object from list
 504       iter.remove();
 505       // keep the referent around
 506       iter.make_referent_alive();
 507       iter.move_to_next();
 508     } else {
 509       iter.next();
 510     }
 511   }
 512   // Close the reachable set
 513   complete_gc->do_void();
 514   log_develop(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT,



 515         iter.removed(), iter.processed(), p2i(refs_list.head()));
 516     }


 517 
 518 // Traverse the list and remove any Refs that are not active, or
 519 // whose referents are either alive or NULL.
 520 void
 521 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 522                              BoolObjectClosure* is_alive,
 523                              OopClosure*        keep_alive) {
 524   assert(discovery_is_atomic(), "Error");
 525   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 526   while (iter.has_next()) {
 527     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 528     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 529     assert(next == NULL, "Should not discover inactive Reference");
 530     if (iter.is_referent_alive()) {
 531       log_develop(gc, ref)("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",

 532                                p2i(iter.obj()), iter.obj()->klass()->internal_name());

 533       // The referent is reachable after all.
 534       // Remove Reference object from list.
 535       iter.remove();
 536       // Update the referent pointer as necessary: Note that this
 537       // should not entail any recursive marking because the
 538       // referent must already have been traversed.
 539       iter.make_referent_alive();
 540       iter.move_to_next();
 541     } else {
 542       iter.next();
 543     }
 544   }
 545   NOT_PRODUCT(
 546     if (iter.processed() > 0) {
 547       log_develop(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 548         " Refs in discovered list " INTPTR_FORMAT,
 549         iter.removed(), iter.processed(), p2i(refs_list.head()));
 550     }
 551   )
 552 }
 553 
 554 void
 555 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 556                                                   BoolObjectClosure* is_alive,
 557                                                   OopClosure*        keep_alive,
 558                                                   VoidClosure*       complete_gc) {
 559   assert(!discovery_is_atomic(), "Error");
 560   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 561   while (iter.has_next()) {
 562     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 563     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 564     oop next = java_lang_ref_Reference::next(iter.obj());
 565     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 566          next != NULL)) {
 567       assert(next->is_oop_or_null(), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next));
 568       // Remove Reference object from list
 569       iter.remove();
 570       // Trace the cohorts
 571       iter.make_referent_alive();
 572       if (UseCompressedOops) {
 573         keep_alive->do_oop((narrowOop*)next_addr);
 574       } else {
 575         keep_alive->do_oop((oop*)next_addr);
 576       }
 577       iter.move_to_next();
 578     } else {
 579       iter.next();
 580     }
 581   }
 582   // Now close the newly reachable set
 583   complete_gc->do_void();
 584   NOT_PRODUCT(
 585     if (iter.processed() > 0) {
 586       log_develop(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 587         " Refs in discovered list " INTPTR_FORMAT,
 588         iter.removed(), iter.processed(), p2i(refs_list.head()));
 589     }
 590   )
 591 }
 592 
 593 // Traverse the list and process the referents, by either
 594 // clearing them or keeping them (and their reachable
 595 // closure) alive.
 596 void
 597 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 598                                    bool               clear_referent,
 599                                    BoolObjectClosure* is_alive,
 600                                    OopClosure*        keep_alive,
 601                                    VoidClosure*       complete_gc) {
 602   ResourceMark rm;
 603   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 604   while (iter.has_next()) {
 605     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 606     if (clear_referent) {
 607       // NULL out referent pointer
 608       iter.clear_referent();
 609     } else {
 610       // keep the referent around
 611       iter.make_referent_alive();
 612     }
 613     log_develop(gc, ref)("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 614                          clear_referent ? "cleared " : "", p2i(iter.obj()), iter.obj()->klass()->internal_name());



 615     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
 616     iter.next();
 617   }
 618   // Close the reachable set
 619   complete_gc->do_void();
 620 }
 621 
 622 void
 623 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 624   oop obj = NULL;
 625   oop next = refs_list.head();
 626   while (next != obj) {
 627     obj = next;
 628     next = java_lang_ref_Reference::discovered(obj);
 629     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 630   }
 631   refs_list.set_head(NULL);
 632   refs_list.set_length(0);
 633 }
 634 
 635 void ReferenceProcessor::abandon_partial_discovery() {
 636   // loop over the lists
 637   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 638     if ((i % _max_num_q) == 0) {
 639       log_develop(gc, ref)("\nAbandoning %s discovered list", list_name(i));
 640     }
 641     clear_discovered_references(_discovered_refs[i]);
 642   }
 643 }
 644 
 645 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 646 public:
 647   RefProcPhase1Task(ReferenceProcessor& ref_processor,
 648                     DiscoveredList      refs_lists[],
 649                     ReferencePolicy*    policy,
 650                     bool                marks_oops_alive)
 651     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 652       _policy(policy)
 653   { }
 654   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 655                     OopClosure& keep_alive,
 656                     VoidClosure& complete_gc)
 657   {
 658     Thread* thr = Thread::current();
 659     int refs_list_index = ((WorkerThread*)thr)->id();


 688                     bool                marks_oops_alive)
 689     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 690       _clear_referent(clear_referent)
 691   { }
 692   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 693                     OopClosure& keep_alive,
 694                     VoidClosure& complete_gc)
 695   {
 696     // Don't use "refs_list_index" calculated in this way because
 697     // balance_queues() has moved the Ref's into the first n queues.
 698     // Thread* thr = Thread::current();
 699     // int refs_list_index = ((WorkerThread*)thr)->id();
 700     // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
 701     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 702                                   &is_alive, &keep_alive, &complete_gc);
 703   }
 704 private:
 705   bool _clear_referent;
 706 };
 707 
 708 #ifndef PRODUCT
 709 void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], size_t total_refs) {
 710   if (!Log<LogTag::gc, LogTag::ref>::is_level(LogLevel::Trace)) {
 711     return;
 712   }
 713 
 714   stringStream st;
 715   for (uint i = 0; i < _max_num_q; ++i) {
 716     st.print(SIZE_FORMAT " ", ref_lists[i].length());
 717   }
 718   log_develop(gc, ref)("%s= " SIZE_FORMAT, st.as_string(), total_refs);
 719 }
 720 #endif
 721 
 722 // Balances reference queues.
 723 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 724 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 725 // corresponding to the active workers will be processed.
 726 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 727 {
 728   // calculate total length
 729   size_t total_refs = 0;
 730   log_develop(gc, ref)("Balance ref_lists ");


 731 
 732   for (uint i = 0; i < _max_num_q; ++i) {
 733     total_refs += ref_lists[i].length();






 734     }
 735   log_reflist_counts(ref_lists, total_refs);
 736   size_t avg_refs = total_refs / _num_q + 1;
 737   uint to_idx = 0;
 738   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
 739     bool move_all = false;
 740     if (from_idx >= _num_q) {
 741       move_all = ref_lists[from_idx].length() > 0;
 742     }
 743     while ((ref_lists[from_idx].length() > avg_refs) ||
 744            move_all) {
 745       assert(to_idx < _num_q, "Sanity Check!");
 746       if (ref_lists[to_idx].length() < avg_refs) {
 747         // move superfluous refs
 748         size_t refs_to_move;
 749         // Move all the Ref's if the from queue will not be processed.
 750         if (move_all) {
 751           refs_to_move = MIN2(ref_lists[from_idx].length(),
 752                               avg_refs - ref_lists[to_idx].length());
 753         } else {
 754           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 755                               avg_refs - ref_lists[to_idx].length());


 779         // Remove the chain from the from list.
 780         if (move_tail == new_head) {
 781           // We found the end of the from list.
 782           ref_lists[from_idx].set_head(NULL);
 783         } else {
 784           ref_lists[from_idx].set_head(new_head);
 785         }
 786         ref_lists[from_idx].dec_length(refs_to_move);
 787         if (ref_lists[from_idx].length() == 0) {
 788           break;
 789         }
 790       } else {
 791         to_idx = (to_idx + 1) % _num_q;
 792       }
 793     }
 794   }
 795 #ifdef ASSERT
 796   size_t balanced_total_refs = 0;
 797   for (uint i = 0; i < _max_num_q; ++i) {
 798     balanced_total_refs += ref_lists[i].length();







 799     }
 800   log_reflist_counts(ref_lists, balanced_total_refs);
 801   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 802 #endif
 803 }
 804 
 805 void ReferenceProcessor::balance_all_queues() {
 806   balance_queues(_discoveredSoftRefs);
 807   balance_queues(_discoveredWeakRefs);
 808   balance_queues(_discoveredFinalRefs);
 809   balance_queues(_discoveredPhantomRefs);
 810   balance_queues(_discoveredCleanerRefs);
 811 }
 812 
 813 void ReferenceProcessor::process_discovered_reflist(
 814   DiscoveredList               refs_lists[],
 815   ReferencePolicy*             policy,
 816   bool                         clear_referent,
 817   BoolObjectClosure*           is_alive,
 818   OopClosure*                  keep_alive,
 819   VoidClosure*                 complete_gc,
 820   AbstractRefProcTaskExecutor* task_executor)


 903     case REF_SOFT:
 904       list = &_discoveredSoftRefs[id];
 905       break;
 906     case REF_WEAK:
 907       list = &_discoveredWeakRefs[id];
 908       break;
 909     case REF_FINAL:
 910       list = &_discoveredFinalRefs[id];
 911       break;
 912     case REF_PHANTOM:
 913       list = &_discoveredPhantomRefs[id];
 914       break;
 915     case REF_CLEANER:
 916       list = &_discoveredCleanerRefs[id];
 917       break;
 918     case REF_NONE:
 919       // we should not reach here if we are an InstanceRefKlass
 920     default:
 921       ShouldNotReachHere();
 922   }
 923   log_develop(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));


 924   return list;
 925 }
 926 
 927 inline void
 928 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
 929                                               oop             obj,
 930                                               HeapWord*       discovered_addr) {
 931   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
 932   // First we must make sure this object is only enqueued once. CAS in a non null
 933   // discovered_addr.
 934   oop current_head = refs_list.head();
 935   // The last ref must have its discovered field pointing to itself.
 936   oop next_discovered = (current_head != NULL) ? current_head : obj;
 937 
 938   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
 939                                                     NULL);
 940   if (retest == NULL) {
 941     // This thread just won the right to enqueue the object.
 942     // We have separate lists for enqueueing, so no synchronization
 943     // is necessary.
 944     refs_list.set_head(obj);
 945     refs_list.inc_length(1);
 946 
 947     log_develop(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",

 948         p2i(obj), obj->klass()->internal_name());

 949   } else {
 950     // If retest was non NULL, another thread beat us to it:
 951     // The reference has already been discovered...
 952         log_develop(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",

 953             p2i(obj), obj->klass()->internal_name());
 954     }
 955   }

 956 
 957 #ifndef PRODUCT
 958 // Non-atomic (i.e. concurrent) discovery might allow us
 959 // to observe j.l.References with NULL referents, being those
 960 // cleared concurrently by mutators during (or after) discovery.
 961 void ReferenceProcessor::verify_referent(oop obj) {
 962   bool da = discovery_is_atomic();
 963   oop referent = java_lang_ref_Reference::referent(obj);
 964   assert(da ? referent->is_oop() : referent->is_oop_or_null(),
 965          "Bad referent " INTPTR_FORMAT " found in Reference "
 966          INTPTR_FORMAT " during %satomic discovery ",
 967          p2i(referent), p2i(obj), da ? "" : "non-");
 968 }
 969 #endif
 970 
 971 // We mention two of several possible choices here:
 972 // #0: if the reference object is not in the "originating generation"
 973 //     (or part of the heap being collected, indicated by our "span"
 974 //     we don't treat it specially (i.e. we scan it as we would
 975 //     a normal oop, treating its references as strong references).


1025   if (rt == REF_SOFT) {
1026     // For soft refs we can decide now if these are not
1027     // current candidates for clearing, in which case we
1028     // can mark through them now, rather than delaying that
1029     // to the reference-processing phase. Since all current
1030     // time-stamp policies advance the soft-ref clock only
1031     // at a full collection cycle, this is always currently
1032     // accurate.
1033     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1034       return false;
1035     }
1036   }
1037 
1038   ResourceMark rm;      // Needed for tracing.
1039 
1040   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1041   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1042   assert(discovered->is_oop_or_null(), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
1043   if (discovered != NULL) {
1044     // The reference has already been discovered...
1045         log_develop(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",

1046                              p2i(obj), obj->klass()->internal_name());

1047     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1048       // assumes that an object is not processed twice;
1049       // if it's been already discovered it must be on another
1050       // generation's discovered list; so we won't discover it.
1051       return false;
1052     } else {
1053       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1054              "Unrecognized policy");
1055       // Check assumption that an object is not potentially
1056       // discovered twice except by concurrent collectors that potentially
1057       // trace the same Reference object twice.
1058       assert(UseConcMarkSweepGC || UseG1GC,
1059              "Only possible with a concurrent marking collector");
1060       return true;
1061     }
1062   }
1063 
1064   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1065     verify_referent(obj);
1066     // Discover if and only if EITHER:


1081   // Get the right type of discovered queue head.
1082   DiscoveredList* list = get_discovered_list(rt);
1083   if (list == NULL) {
1084     return false;   // nothing special needs to be done
1085   }
1086 
1087   if (_discovery_is_mt) {
1088     add_to_discovered_list_mt(*list, obj, discovered_addr);
1089   } else {
1090     // We do a raw store here: the field will be visited later when processing
1091     // the discovered references.
1092     oop current_head = list->head();
1093     // The last ref must have its discovered field pointing to itself.
1094     oop next_discovered = (current_head != NULL) ? current_head : obj;
1095 
1096     assert(discovered == NULL, "control point invariant");
1097     oop_store_raw(discovered_addr, next_discovered);
1098     list->set_head(obj);
1099     list->inc_length(1);
1100 
1101     log_develop(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name());



1102   }
1103   assert(obj->is_oop(), "Discovered a bad reference");
1104   verify_referent(obj);
1105   return true;
1106 }
1107 
1108 // Preclean the discovered references by removing those
1109 // whose referents are alive, and by marking from those that
1110 // are not active. These lists can be handled here
1111 // in any order and, indeed, concurrently.
1112 void ReferenceProcessor::preclean_discovered_references(
1113   BoolObjectClosure* is_alive,
1114   OopClosure* keep_alive,
1115   VoidClosure* complete_gc,
1116   YieldClosure* yield,
1117   GCTimer* gc_timer) {
1118 
1119   // Soft references
1120   {
1121     GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer);

1122     for (uint i = 0; i < _max_num_q; i++) {
1123       if (yield->should_return()) {
1124         return;
1125       }
1126       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1127                                   keep_alive, complete_gc, yield);
1128     }
1129   }
1130 
1131   // Weak references
1132   {
1133     GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer);

1134     for (uint i = 0; i < _max_num_q; i++) {
1135       if (yield->should_return()) {
1136         return;
1137       }
1138       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1139                                   keep_alive, complete_gc, yield);
1140     }
1141   }
1142 
1143   // Final references
1144   {
1145     GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer);

1146     for (uint i = 0; i < _max_num_q; i++) {
1147       if (yield->should_return()) {
1148         return;
1149       }
1150       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1151                                   keep_alive, complete_gc, yield);
1152     }
1153   }
1154 
1155   // Phantom references
1156   {
1157     GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer);

1158     for (uint i = 0; i < _max_num_q; i++) {
1159       if (yield->should_return()) {
1160         return;
1161       }
1162       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1163                                   keep_alive, complete_gc, yield);
1164     }
1165 
1166     // Cleaner references.  Included in timing for phantom references.  We
1167     // expect Cleaner references to be temporary, and don't want to deal with
1168     // possible incompatibilities arising from making it more visible.
1169     for (uint i = 0; i < _max_num_q; i++) {
1170       if (yield->should_return()) {
1171         return;
1172       }
1173       preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1174                                   keep_alive, complete_gc, yield);
1175     }
1176   }
1177 }


1182 // thus precleaning the ref lists (which happens single-threaded today),
1183 // we do not disable refs discovery to honor the correct semantics of
1184 // java.lang.Reference. As a result, we need to be careful below
1185 // that ref removal steps interleave safely with ref discovery steps
1186 // (in this thread).
1187 void
1188 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1189                                                 BoolObjectClosure* is_alive,
1190                                                 OopClosure*        keep_alive,
1191                                                 VoidClosure*       complete_gc,
1192                                                 YieldClosure*      yield) {
1193   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1194   while (iter.has_next()) {
1195     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1196     oop obj = iter.obj();
1197     oop next = java_lang_ref_Reference::next(obj);
1198     if (iter.referent() == NULL || iter.is_referent_alive() ||
1199         next != NULL) {
1200       // The referent has been cleared, or is alive, or the Reference is not
1201       // active; we need to trace and mark its cohort.
1202       log_develop(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)",

1203                                p2i(iter.obj()), iter.obj()->klass()->internal_name());

1204       // Remove Reference object from list
1205       iter.remove();
1206       // Keep alive its cohort.
1207       iter.make_referent_alive();
1208       if (UseCompressedOops) {
1209         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1210         keep_alive->do_oop(next_addr);
1211       } else {
1212         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1213         keep_alive->do_oop(next_addr);
1214       }
1215       iter.move_to_next();
1216     } else {
1217       iter.next();
1218     }
1219   }
1220   // Close the reachable set
1221   complete_gc->do_void();
1222 
1223   NOT_PRODUCT(
1224     if (iter.processed() > 0) {
1225       log_develop(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT,

1226         iter.removed(), iter.processed(), p2i(refs_list.head()));
1227     }
1228   )
1229 }
1230 
1231 const char* ReferenceProcessor::list_name(uint i) {
1232    assert(i <= _max_num_q * number_of_subclasses_of_ref(),
1233           "Out of bounds index");
1234 
1235    int j = i / _max_num_q;
1236    switch (j) {
1237      case 0: return "SoftRef";
1238      case 1: return "WeakRef";
1239      case 2: return "FinalRef";
1240      case 3: return "PhantomRef";
1241      case 4: return "CleanerRef";
1242    }
1243    ShouldNotReachHere();
1244    return NULL;
1245 }
< prev index next >