src/share/vm/gc_interface/collectedHeap.hpp

Print this page
rev 4773 : 8005849: JEP 167: Event-Based JVM Tracing
Reviewed-by: acorn, coleenp, sla
Contributed-by: Karen Kinnear <karen.kinnear@oracle.com>, Bengt Rutisson <bengt.rutisson@oracle.com>, Calvin Cheung <calvin.cheung@oracle.com>, Erik Gahlin <erik.gahlin@oracle.com>, Erik Helin <erik.helin@oracle.com>, Jesper Wilhelmsson <jesper.wilhelmsson@oracle.com>, Keith McGuigan <keith.mcguigan@oracle.com>, Mattias Tobiasson <mattias.tobiasson@oracle.com>, Markus Gronlund <markus.gronlund@oracle.com>, Mikael Auno <mikael.auno@oracle.com>, Nils Eliasson <nils.eliasson@oracle.com>, Nils Loodin <nils.loodin@oracle.com>, Rickard Backman <rickard.backman@oracle.com>, Staffan Larsen <staffan.larsen@oracle.com>, Stefan Karlsson <stefan.karlsson@oracle.com>, Yekaterina Kantserova <yekaterina.kantserova@oracle.com>
   1 /*
   2  * Copyright (c) 2001, 2012, 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_INTERFACE_COLLECTEDHEAP_HPP
  26 #define SHARE_VM_GC_INTERFACE_COLLECTEDHEAP_HPP
  27 
  28 #include "gc_interface/gcCause.hpp"

  29 #include "memory/allocation.hpp"
  30 #include "memory/barrierSet.hpp"
  31 #include "runtime/handles.hpp"
  32 #include "runtime/perfData.hpp"
  33 #include "runtime/safepoint.hpp"
  34 #include "utilities/events.hpp"
  35 
  36 // A "CollectedHeap" is an implementation of a java heap for HotSpot.  This
  37 // is an abstract class: there may be many different kinds of heaps.  This
  38 // class defines the functions that a heap must implement, and contains
  39 // infrastructure common to all heaps.
  40 
  41 class BarrierSet;
  42 class ThreadClosure;
  43 class AdaptiveSizePolicy;
  44 class Thread;
  45 class CollectorPolicy;







  46 
  47 class GCMessage : public FormatBuffer<1024> {
  48  public:
  49   bool is_before;
  50 
  51  public:
  52   GCMessage() {}
  53 };
  54 
  55 class GCHeapLog : public EventLogBase<GCMessage> {
  56  private:
  57   void log_heap(bool before);
  58 
  59  public:
  60   GCHeapLog() : EventLogBase<GCMessage>("GC Heap History") {}
  61 
  62   void log_heap_before() {
  63     log_heap(true);
  64   }
  65   void log_heap_after() {


 111   // Constructor
 112   CollectedHeap();
 113 
 114   // Do common initializations that must follow instance construction,
 115   // for example, those needing virtual calls.
 116   // This code could perhaps be moved into initialize() but would
 117   // be slightly more awkward because we want the latter to be a
 118   // pure virtual.
 119   void pre_initialize();
 120 
 121   // Create a new tlab. All TLAB allocations must go through this.
 122   virtual HeapWord* allocate_new_tlab(size_t size);
 123 
 124   // Accumulate statistics on all tlabs.
 125   virtual void accumulate_statistics_all_tlabs();
 126 
 127   // Reinitialize tlabs before resuming mutators.
 128   virtual void resize_all_tlabs();
 129 
 130   // Allocate from the current thread's TLAB, with broken-out slow path.
 131   inline static HeapWord* allocate_from_tlab(Thread* thread, size_t size);
 132   static HeapWord* allocate_from_tlab_slow(Thread* thread, size_t size);
 133 
 134   // Allocate an uninitialized block of the given size, or returns NULL if
 135   // this is impossible.
 136   inline static HeapWord* common_mem_allocate_noinit(size_t size, TRAPS);
 137 
 138   // Like allocate_init, but the block returned by a successful allocation
 139   // is guaranteed initialized to zeros.
 140   inline static HeapWord* common_mem_allocate_init(size_t size, TRAPS);
 141 
 142   // Helper functions for (VM) allocation.
 143   inline static void post_allocation_setup_common(KlassHandle klass, HeapWord* obj);
 144   inline static void post_allocation_setup_no_klass_install(KlassHandle klass,
 145                                                             HeapWord* objPtr);
 146 
 147   inline static void post_allocation_setup_obj(KlassHandle klass, HeapWord* obj);
 148 
 149   inline static void post_allocation_setup_array(KlassHandle klass,
 150                                                  HeapWord* obj, int length);
 151 
 152   // Clears an allocated object.
 153   inline static void init_obj(HeapWord* obj, size_t size);
 154 
 155   // Filler object utilities.
 156   static inline size_t filler_array_hdr_size();
 157   static inline size_t filler_array_min_size();
 158 
 159   DEBUG_ONLY(static void fill_args_check(HeapWord* start, size_t words);)
 160   DEBUG_ONLY(static void zap_filler_array(HeapWord* start, size_t words, bool zap = true);)
 161 
 162   // Fill with a single array; caller must ensure filler_array_min_size() <=
 163   // words <= filler_array_max_size().
 164   static inline void fill_with_array(HeapWord* start, size_t words, bool zap = true);
 165 
 166   // Fill with a single object (either an int array or a java.lang.Object).
 167   static inline void fill_with_object_impl(HeapWord* start, size_t words, bool zap = true);
 168 


 169   // Verification functions
 170   virtual void check_for_bad_heap_word_value(HeapWord* addr, size_t size)
 171     PRODUCT_RETURN;
 172   virtual void check_for_non_bad_heap_word_value(HeapWord* addr, size_t size)
 173     PRODUCT_RETURN;
 174   debug_only(static void check_for_valid_allocation_state();)
 175 
 176  public:
 177   enum Name {
 178     Abstract,
 179     SharedHeap,
 180     GenCollectedHeap,
 181     ParallelScavengeHeap,
 182     G1CollectedHeap
 183   };
 184 
 185   static inline size_t filler_array_max_size() {
 186     return _filler_array_max_size;
 187   }
 188 
 189   virtual CollectedHeap::Name kind() const { return CollectedHeap::Abstract; }
 190 
 191   /**
 192    * Returns JNI error code JNI_ENOMEM if memory could not be allocated,
 193    * and JNI_OK on success.
 194    */
 195   virtual jint initialize() = 0;
 196 
 197   // In many heaps, there will be a need to perform some initialization activities
 198   // after the Universe is fully formed, but before general heap allocation is allowed.
 199   // This is the correct place to place such initialization methods.
 200   virtual void post_initialize() = 0;
 201 
 202   MemRegion reserved_region() const { return _reserved; }
 203   address base() const { return (address)reserved_region().start(); }
 204 
 205   // Future cleanup here. The following functions should specify bytes or
 206   // heapwords as part of their signature.
 207   virtual size_t capacity() const = 0;
 208   virtual size_t used() const = 0;
 209 
 210   // Return "true" if the part of the heap that allocates Java
 211   // objects has reached the maximal committed limit that it can
 212   // reach, without a garbage collection.
 213   virtual bool is_maximal_no_gc() const = 0;
 214 
 215   // Support for java.lang.Runtime.maxMemory():  return the maximum amount of
 216   // memory that the vm could make available for storing 'normal' java objects.
 217   // This is based on the reserved address space, but should not include space
 218   // that the vm uses internally for bookkeeping or temporary storage
 219   // (e.g., in the case of the young gen, one of the survivor
 220   // spaces).
 221   virtual size_t max_capacity() const = 0;
 222 
 223   // Returns "TRUE" if "p" points into the reserved area of the heap.
 224   bool is_in_reserved(const void* p) const {
 225     return _reserved.contains(p);
 226   }


 533   // non-object.
 534   virtual HeapWord* block_start(const void* addr) const = 0;
 535 
 536   // Requires "addr" to be the start of a chunk, and returns its size.
 537   // "addr + size" is required to be the start of a new chunk, or the end
 538   // of the active area of the heap.
 539   virtual size_t block_size(const HeapWord* addr) const = 0;
 540 
 541   // Requires "addr" to be the start of a block, and returns "TRUE" iff
 542   // the block is an object.
 543   virtual bool block_is_obj(const HeapWord* addr) const = 0;
 544 
 545   // Returns the longest time (in ms) that has elapsed since the last
 546   // time that any part of the heap was examined by a garbage collection.
 547   virtual jlong millis_since_last_gc() = 0;
 548 
 549   // Perform any cleanup actions necessary before allowing a verification.
 550   virtual void prepare_for_verify() = 0;
 551 
 552   // Generate any dumps preceding or following a full gc
 553   void pre_full_gc_dump();
 554   void post_full_gc_dump();





 555 
 556   // Print heap information on the given outputStream.
 557   virtual void print_on(outputStream* st) const = 0;
 558   // The default behavior is to call print_on() on tty.
 559   virtual void print() const {
 560     print_on(tty);
 561   }
 562   // Print more detailed heap information on the given
 563   // outputStream. The default behaviour is to call print_on(). It is
 564   // up to each subclass to override it and add any additional output
 565   // it needs.
 566   virtual void print_extended_on(outputStream* st) const {
 567     print_on(st);
 568   }
 569 
 570   virtual void print_on_error(outputStream* st) const {
 571     st->print_cr("Heap:");
 572     print_extended_on(st);
 573     st->cr();
 574 
 575     _barrier_set->print_on(st);
 576   }
 577 
 578   // Print all GC threads (other than the VM thread)
 579   // used by this heap.
 580   virtual void print_gc_threads_on(outputStream* st) const = 0;
 581   // The default behavior is to call print_gc_threads_on() on tty.
 582   void print_gc_threads() {
 583     print_gc_threads_on(tty);
 584   }
 585   // Iterator for all GC threads (other than VM thread)
 586   virtual void gc_threads_do(ThreadClosure* tc) const = 0;
 587 
 588   // Print any relevant tracing info that flags imply.
 589   // Default implementation does nothing.
 590   virtual void print_tracing_info() const = 0;
 591 
 592   // If PrintHeapAtGC is set call the appropriate routi
 593   void print_heap_before_gc() {
 594     if (PrintHeapAtGC) {
 595       Universe::print_heap_before_gc();
 596     }
 597     if (_gc_heap_log != NULL) {
 598       _gc_heap_log->log_heap_before();
 599     }
 600   }
 601   void print_heap_after_gc() {
 602     if (PrintHeapAtGC) {
 603       Universe::print_heap_after_gc();
 604     }
 605     if (_gc_heap_log != NULL) {
 606       _gc_heap_log->log_heap_after();
 607     }
 608   }
 609 
 610   // Heap verification
 611   virtual void verify(bool silent, VerifyOption option) = 0;
 612 
 613   // Non product verification and debugging.
 614 #ifndef PRODUCT
 615   // Support for PromotionFailureALot.  Return true if it's time to cause a
 616   // promotion failure.  The no-argument version uses
 617   // this->_promotion_failure_alot_count as the counter.
 618   inline bool promotion_should_fail(volatile size_t* count);
 619   inline bool promotion_should_fail();
 620 
 621   // Reset the PromotionFailureALot counters.  Should be called at the end of a
 622   // GC in which promotion failure ocurred.
 623   inline void reset_promotion_should_fail(volatile size_t* count);
 624   inline void reset_promotion_should_fail();
 625 #endif  // #ifndef PRODUCT
 626 
 627 #ifdef ASSERT
 628   static int fired_fake_oom() {
 629     return (CIFireOOMAt > 1 && _fire_out_of_memory_count >= CIFireOOMAt);
 630   }
 631 #endif
 632 
 633  public:
 634   // This is a convenience method that is used in cases where
 635   // the actual number of GC worker threads is not pertinent but
 636   // only whether there more than 0.  Use of this method helps
 637   // reduce the occurrence of ParallelGCThreads to uses where the
 638   // actual number may be germane.
 639   static bool use_parallel_gc_threads() { return ParallelGCThreads > 0; }
 640 
 641   /////////////// Unit tests ///////////////
 642 


   1 /*
   2  * Copyright (c) 2001, 2013, 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_INTERFACE_COLLECTEDHEAP_HPP
  26 #define SHARE_VM_GC_INTERFACE_COLLECTEDHEAP_HPP
  27 
  28 #include "gc_interface/gcCause.hpp"
  29 #include "gc_implementation/shared/gcWhen.hpp"
  30 #include "memory/allocation.hpp"
  31 #include "memory/barrierSet.hpp"
  32 #include "runtime/handles.hpp"
  33 #include "runtime/perfData.hpp"
  34 #include "runtime/safepoint.hpp"
  35 #include "utilities/events.hpp"
  36 
  37 // A "CollectedHeap" is an implementation of a java heap for HotSpot.  This
  38 // is an abstract class: there may be many different kinds of heaps.  This
  39 // class defines the functions that a heap must implement, and contains
  40 // infrastructure common to all heaps.
  41 


  42 class AdaptiveSizePolicy;
  43 class BarrierSet;
  44 class CollectorPolicy;
  45 class GCHeapSummary;
  46 class GCTimer;
  47 class GCTracer;
  48 class MetaspaceSummary;
  49 class Thread;
  50 class ThreadClosure;
  51 class VirtualSpaceSummary;
  52 
  53 class GCMessage : public FormatBuffer<1024> {
  54  public:
  55   bool is_before;
  56 
  57  public:
  58   GCMessage() {}
  59 };
  60 
  61 class GCHeapLog : public EventLogBase<GCMessage> {
  62  private:
  63   void log_heap(bool before);
  64 
  65  public:
  66   GCHeapLog() : EventLogBase<GCMessage>("GC Heap History") {}
  67 
  68   void log_heap_before() {
  69     log_heap(true);
  70   }
  71   void log_heap_after() {


 117   // Constructor
 118   CollectedHeap();
 119 
 120   // Do common initializations that must follow instance construction,
 121   // for example, those needing virtual calls.
 122   // This code could perhaps be moved into initialize() but would
 123   // be slightly more awkward because we want the latter to be a
 124   // pure virtual.
 125   void pre_initialize();
 126 
 127   // Create a new tlab. All TLAB allocations must go through this.
 128   virtual HeapWord* allocate_new_tlab(size_t size);
 129 
 130   // Accumulate statistics on all tlabs.
 131   virtual void accumulate_statistics_all_tlabs();
 132 
 133   // Reinitialize tlabs before resuming mutators.
 134   virtual void resize_all_tlabs();
 135 
 136   // Allocate from the current thread's TLAB, with broken-out slow path.
 137   inline static HeapWord* allocate_from_tlab(KlassHandle klass, Thread* thread, size_t size);
 138   static HeapWord* allocate_from_tlab_slow(KlassHandle klass, Thread* thread, size_t size);
 139 
 140   // Allocate an uninitialized block of the given size, or returns NULL if
 141   // this is impossible.
 142   inline static HeapWord* common_mem_allocate_noinit(KlassHandle klass, size_t size, TRAPS);
 143 
 144   // Like allocate_init, but the block returned by a successful allocation
 145   // is guaranteed initialized to zeros.
 146   inline static HeapWord* common_mem_allocate_init(KlassHandle klass, size_t size, TRAPS);
 147 
 148   // Helper functions for (VM) allocation.
 149   inline static void post_allocation_setup_common(KlassHandle klass, HeapWord* obj);
 150   inline static void post_allocation_setup_no_klass_install(KlassHandle klass,
 151                                                             HeapWord* objPtr);
 152 
 153   inline static void post_allocation_setup_obj(KlassHandle klass, HeapWord* obj);
 154 
 155   inline static void post_allocation_setup_array(KlassHandle klass,
 156                                                  HeapWord* obj, int length);
 157 
 158   // Clears an allocated object.
 159   inline static void init_obj(HeapWord* obj, size_t size);
 160 
 161   // Filler object utilities.
 162   static inline size_t filler_array_hdr_size();
 163   static inline size_t filler_array_min_size();
 164 
 165   DEBUG_ONLY(static void fill_args_check(HeapWord* start, size_t words);)
 166   DEBUG_ONLY(static void zap_filler_array(HeapWord* start, size_t words, bool zap = true);)
 167 
 168   // Fill with a single array; caller must ensure filler_array_min_size() <=
 169   // words <= filler_array_max_size().
 170   static inline void fill_with_array(HeapWord* start, size_t words, bool zap = true);
 171 
 172   // Fill with a single object (either an int array or a java.lang.Object).
 173   static inline void fill_with_object_impl(HeapWord* start, size_t words, bool zap = true);
 174 
 175   virtual void trace_heap(GCWhen::Type when, GCTracer* tracer);
 176 
 177   // Verification functions
 178   virtual void check_for_bad_heap_word_value(HeapWord* addr, size_t size)
 179     PRODUCT_RETURN;
 180   virtual void check_for_non_bad_heap_word_value(HeapWord* addr, size_t size)
 181     PRODUCT_RETURN;
 182   debug_only(static void check_for_valid_allocation_state();)
 183 
 184  public:
 185   enum Name {
 186     Abstract,
 187     SharedHeap,
 188     GenCollectedHeap,
 189     ParallelScavengeHeap,
 190     G1CollectedHeap
 191   };
 192 
 193   static inline size_t filler_array_max_size() {
 194     return _filler_array_max_size;
 195   }
 196 
 197   virtual CollectedHeap::Name kind() const { return CollectedHeap::Abstract; }
 198 
 199   /**
 200    * Returns JNI error code JNI_ENOMEM if memory could not be allocated,
 201    * and JNI_OK on success.
 202    */
 203   virtual jint initialize() = 0;
 204 
 205   // In many heaps, there will be a need to perform some initialization activities
 206   // after the Universe is fully formed, but before general heap allocation is allowed.
 207   // This is the correct place to place such initialization methods.
 208   virtual void post_initialize() = 0;
 209 
 210   MemRegion reserved_region() const { return _reserved; }
 211   address base() const { return (address)reserved_region().start(); }
 212 


 213   virtual size_t capacity() const = 0;
 214   virtual size_t used() const = 0;
 215 
 216   // Return "true" if the part of the heap that allocates Java
 217   // objects has reached the maximal committed limit that it can
 218   // reach, without a garbage collection.
 219   virtual bool is_maximal_no_gc() const = 0;
 220 
 221   // Support for java.lang.Runtime.maxMemory():  return the maximum amount of
 222   // memory that the vm could make available for storing 'normal' java objects.
 223   // This is based on the reserved address space, but should not include space
 224   // that the vm uses internally for bookkeeping or temporary storage
 225   // (e.g., in the case of the young gen, one of the survivor
 226   // spaces).
 227   virtual size_t max_capacity() const = 0;
 228 
 229   // Returns "TRUE" if "p" points into the reserved area of the heap.
 230   bool is_in_reserved(const void* p) const {
 231     return _reserved.contains(p);
 232   }


 539   // non-object.
 540   virtual HeapWord* block_start(const void* addr) const = 0;
 541 
 542   // Requires "addr" to be the start of a chunk, and returns its size.
 543   // "addr + size" is required to be the start of a new chunk, or the end
 544   // of the active area of the heap.
 545   virtual size_t block_size(const HeapWord* addr) const = 0;
 546 
 547   // Requires "addr" to be the start of a block, and returns "TRUE" iff
 548   // the block is an object.
 549   virtual bool block_is_obj(const HeapWord* addr) const = 0;
 550 
 551   // Returns the longest time (in ms) that has elapsed since the last
 552   // time that any part of the heap was examined by a garbage collection.
 553   virtual jlong millis_since_last_gc() = 0;
 554 
 555   // Perform any cleanup actions necessary before allowing a verification.
 556   virtual void prepare_for_verify() = 0;
 557 
 558   // Generate any dumps preceding or following a full gc
 559   void pre_full_gc_dump(GCTimer* timer);
 560   void post_full_gc_dump(GCTimer* timer);
 561 
 562   VirtualSpaceSummary create_heap_space_summary();
 563   GCHeapSummary create_heap_summary();
 564 
 565   MetaspaceSummary create_metaspace_summary();
 566 
 567   // Print heap information on the given outputStream.
 568   virtual void print_on(outputStream* st) const = 0;
 569   // The default behavior is to call print_on() on tty.
 570   virtual void print() const {
 571     print_on(tty);
 572   }
 573   // Print more detailed heap information on the given
 574   // outputStream. The default behavior is to call print_on(). It is
 575   // up to each subclass to override it and add any additional output
 576   // it needs.
 577   virtual void print_extended_on(outputStream* st) const {
 578     print_on(st);
 579   }
 580 
 581   virtual void print_on_error(outputStream* st) const {
 582     st->print_cr("Heap:");
 583     print_extended_on(st);
 584     st->cr();
 585 
 586     _barrier_set->print_on(st);
 587   }
 588 
 589   // Print all GC threads (other than the VM thread)
 590   // used by this heap.
 591   virtual void print_gc_threads_on(outputStream* st) const = 0;
 592   // The default behavior is to call print_gc_threads_on() on tty.
 593   void print_gc_threads() {
 594     print_gc_threads_on(tty);
 595   }
 596   // Iterator for all GC threads (other than VM thread)
 597   virtual void gc_threads_do(ThreadClosure* tc) const = 0;
 598 
 599   // Print any relevant tracing info that flags imply.
 600   // Default implementation does nothing.
 601   virtual void print_tracing_info() const = 0;
 602 
 603   void print_heap_before_gc();
 604   void print_heap_after_gc();
 605 
 606   void trace_heap_before_gc(GCTracer* gc_tracer);
 607   void trace_heap_after_gc(GCTracer* gc_tracer);












 608 
 609   // Heap verification
 610   virtual void verify(bool silent, VerifyOption option) = 0;
 611 
 612   // Non product verification and debugging.
 613 #ifndef PRODUCT
 614   // Support for PromotionFailureALot.  Return true if it's time to cause a
 615   // promotion failure.  The no-argument version uses
 616   // this->_promotion_failure_alot_count as the counter.
 617   inline bool promotion_should_fail(volatile size_t* count);
 618   inline bool promotion_should_fail();
 619 
 620   // Reset the PromotionFailureALot counters.  Should be called at the end of a
 621   // GC in which promotion failure occurred.
 622   inline void reset_promotion_should_fail(volatile size_t* count);
 623   inline void reset_promotion_should_fail();
 624 #endif  // #ifndef PRODUCT
 625 
 626 #ifdef ASSERT
 627   static int fired_fake_oom() {
 628     return (CIFireOOMAt > 1 && _fire_out_of_memory_count >= CIFireOOMAt);
 629   }
 630 #endif
 631 
 632  public:
 633   // This is a convenience method that is used in cases where
 634   // the actual number of GC worker threads is not pertinent but
 635   // only whether there more than 0.  Use of this method helps
 636   // reduce the occurrence of ParallelGCThreads to uses where the
 637   // actual number may be germane.
 638   static bool use_parallel_gc_threads() { return ParallelGCThreads > 0; }
 639 
 640   /////////////// Unit tests ///////////////
 641