Mercurial > jdk8-mips64-public > hotspot / file revision

 /*

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  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 #include "precompiled.hpp"

 #include "classfile/symbolTable.hpp"

 #include "classfile/systemDictionary.hpp"

 #include "classfile/vmSymbols.hpp"

 #include "code/codeCache.hpp"

 #include "code/icBuffer.hpp"

 #include "gc_implementation/shared/collectorCounters.hpp"

 #include "gc_implementation/shared/gcTrace.hpp"

 #include "gc_implementation/shared/gcTraceTime.hpp"

 #include "gc_implementation/shared/vmGCOperations.hpp"

 #include "gc_interface/collectedHeap.inline.hpp"

 #include "memory/filemap.hpp"

 #include "memory/gcLocker.inline.hpp"

 #include "memory/genCollectedHeap.hpp"

 #include "memory/genOopClosures.inline.hpp"

 #include "memory/generation.inline.hpp"

 #include "memory/generationSpec.hpp"

 #include "memory/resourceArea.hpp"

 #include "memory/sharedHeap.hpp"

 #include "memory/space.hpp"

 #include "oops/oop.inline.hpp"

 #include "oops/oop.inline2.hpp"

 #include "runtime/biasedLocking.hpp"

 #include "runtime/fprofiler.hpp"

 #include "runtime/handles.hpp"

 #include "runtime/handles.inline.hpp"

 #include "runtime/java.hpp"

 #include "runtime/vmThread.hpp"

 #include "services/management.hpp"

 #include "services/memoryService.hpp"

 #include "utilities/vmError.hpp"

 #include "utilities/workgroup.hpp"

 #include "utilities/macros.hpp"

 #if INCLUDE_ALL_GCS

 #include "gc_implementation/concurrentMarkSweep/concurrentMarkSweepThread.hpp"

 #include "gc_implementation/concurrentMarkSweep/vmCMSOperations.hpp"

 #endif // INCLUDE_ALL_GCS

 GenCollectedHeap* GenCollectedHeap::_gch;

 NOT_PRODUCT(size_t GenCollectedHeap::_skip_header_HeapWords = 0;)

 // The set of potentially parallel tasks in root scanning.

 enum GCH_strong_roots_tasks {

   GCH_PS_Universe_oops_do,

   GCH_PS_JNIHandles_oops_do,

   GCH_PS_ObjectSynchronizer_oops_do,

   GCH_PS_FlatProfiler_oops_do,

   GCH_PS_Management_oops_do,

   GCH_PS_SystemDictionary_oops_do,

   GCH_PS_ClassLoaderDataGraph_oops_do,

   GCH_PS_jvmti_oops_do,

   GCH_PS_CodeCache_oops_do,

   GCH_PS_younger_gens,

   // Leave this one last.

   GCH_PS_NumElements

 };

 GenCollectedHeap::GenCollectedHeap(GenCollectorPolicy *policy) :

   SharedHeap(policy),

   _gen_policy(policy),

   _process_strong_tasks(new SubTasksDone(GCH_PS_NumElements)),

   _full_collections_completed(0)

 {

   assert(policy != NULL, "Sanity check");

 }

 jint GenCollectedHeap::initialize() {

   CollectedHeap::pre_initialize();

   int i;

   _n_gens = gen_policy()->number_of_generations();

   // While there are no constraints in the GC code that HeapWordSize

   // be any particular value, there are multiple other areas in the

   // system which believe this to be true (e.g. oop->object_size in some

   // cases incorrectly returns the size in wordSize units rather than

   // HeapWordSize).

   guarantee(HeapWordSize == wordSize, "HeapWordSize must equal wordSize");

   // The heap must be at least as aligned as generations.

   size_t gen_alignment = Generation::GenGrain;

   _gen_specs = gen_policy()->generations();

   // Make sure the sizes are all aligned.

   for (i = 0; i < _n_gens; i++) {

     _gen_specs[i]->align(gen_alignment);

   }

   // Allocate space for the heap.

   char* heap_address;

   size_t total_reserved = 0;

   int n_covered_regions = 0;

   ReservedSpace heap_rs;

   size_t heap_alignment = collector_policy()->heap_alignment();

   heap_address = allocate(heap_alignment, &total_reserved,

                           &n_covered_regions, &heap_rs);

   if (!heap_rs.is_reserved()) {

     vm_shutdown_during_initialization(

       "Could not reserve enough space for object heap");

     return JNI_ENOMEM;

   }

   _reserved = MemRegion((HeapWord*)heap_rs.base(),

                         (HeapWord*)(heap_rs.base() + heap_rs.size()));

   // It is important to do this in a way such that concurrent readers can't

   // temporarily think somethings in the heap.  (Seen this happen in asserts.)

   _reserved.set_word_size(0);

   _reserved.set_start((HeapWord*)heap_rs.base());

   size_t actual_heap_size = heap_rs.size();

   _reserved.set_end((HeapWord*)(heap_rs.base() + actual_heap_size));

   _rem_set = collector_policy()->create_rem_set(_reserved, n_covered_regions);

   set_barrier_set(rem_set()->bs());

   _gch = this;

   for (i = 0; i < _n_gens; i++) {

     ReservedSpace this_rs = heap_rs.first_part(_gen_specs[i]->max_size(), false, false);

     _gens[i] = _gen_specs[i]->init(this_rs, i, rem_set());

     heap_rs = heap_rs.last_part(_gen_specs[i]->max_size());

   }

   clear_incremental_collection_failed();

 #if INCLUDE_ALL_GCS

   // If we are running CMS, create the collector responsible

   // for collecting the CMS generations.

   if (collector_policy()->is_concurrent_mark_sweep_policy()) {

     bool success = create_cms_collector();

     if (!success) return JNI_ENOMEM;

   }

 #endif // INCLUDE_ALL_GCS

   return JNI_OK;

 }

 char* GenCollectedHeap::allocate(size_t alignment,

                                  size_t* _total_reserved,

                                  int* _n_covered_regions,

                                  ReservedSpace* heap_rs){

   const char overflow_msg[] = "The size of the object heap + VM data exceeds "

     "the maximum representable size";

   // Now figure out the total size.

   size_t total_reserved = 0;

   int n_covered_regions = 0;

   const size_t pageSize = UseLargePages ?

       os::large_page_size() : os::vm_page_size();

   assert(alignment % pageSize == 0, "Must be");

   for (int i = 0; i < _n_gens; i++) {

     total_reserved += _gen_specs[i]->max_size();

     if (total_reserved < _gen_specs[i]->max_size()) {

       vm_exit_during_initialization(overflow_msg);

     }

     n_covered_regions += _gen_specs[i]->n_covered_regions();

   }

   assert(total_reserved % alignment == 0,

          err_msg("Gen size; total_reserved=" SIZE_FORMAT ", alignment="

                  SIZE_FORMAT, total_reserved, alignment));

   // Needed until the cardtable is fixed to have the right number

   // of covered regions.

   n_covered_regions += 2;

   *_total_reserved = total_reserved;

   *_n_covered_regions = n_covered_regions;

   *heap_rs = Universe::reserve_heap(total_reserved, alignment);

   return heap_rs->base();

 }

 void GenCollectedHeap::post_initialize() {

   SharedHeap::post_initialize();

   TwoGenerationCollectorPolicy *policy =

     (TwoGenerationCollectorPolicy *)collector_policy();

   guarantee(policy->is_two_generation_policy(), "Illegal policy type");

   DefNewGeneration* def_new_gen = (DefNewGeneration*) get_gen(0);

   assert(def_new_gen->kind() == Generation::DefNew ||

          def_new_gen->kind() == Generation::ParNew ||

          def_new_gen->kind() == Generation::ASParNew,

          "Wrong generation kind");

   Generation* old_gen = get_gen(1);

   assert(old_gen->kind() == Generation::ConcurrentMarkSweep ||

          old_gen->kind() == Generation::ASConcurrentMarkSweep ||

          old_gen->kind() == Generation::MarkSweepCompact,

     "Wrong generation kind");

   policy->initialize_size_policy(def_new_gen->eden()->capacity(),

                                  old_gen->capacity(),

                                  def_new_gen->from()->capacity());

   policy->initialize_gc_policy_counters();

 }

 void GenCollectedHeap::ref_processing_init() {

   SharedHeap::ref_processing_init();

   for (int i = 0; i < _n_gens; i++) {

     _gens[i]->ref_processor_init();

   }

 }

 size_t GenCollectedHeap::capacity() const {

   size_t res = 0;

   for (int i = 0; i < _n_gens; i++) {

     res += _gens[i]->capacity();

   }

   return res;

 }

 size_t GenCollectedHeap::used() const {

   size_t res = 0;

   for (int i = 0; i < _n_gens; i++) {

     res += _gens[i]->used();

   }

   return res;

 }

 // Save the "used_region" for generations level and lower.

 void GenCollectedHeap::save_used_regions(int level) {

   assert(level < _n_gens, "Illegal level parameter");

   for (int i = level; i >= 0; i--) {

     _gens[i]->save_used_region();

   }

 }

 size_t GenCollectedHeap::max_capacity() const {

   size_t res = 0;

   for (int i = 0; i < _n_gens; i++) {

     res += _gens[i]->max_capacity();

   }

   return res;

 }

 // Update the _full_collections_completed counter

 // at the end of a stop-world full GC.

 unsigned int GenCollectedHeap::update_full_collections_completed() {

   MonitorLockerEx ml(FullGCCount_lock, Mutex::_no_safepoint_check_flag);

   assert(_full_collections_completed <= _total_full_collections,

          "Can't complete more collections than were started");

   _full_collections_completed = _total_full_collections;

   ml.notify_all();

   return _full_collections_completed;

 }

 // Update the _full_collections_completed counter, as appropriate,

 // at the end of a concurrent GC cycle. Note the conditional update

 // below to allow this method to be called by a concurrent collector

 // without synchronizing in any manner with the VM thread (which

 // may already have initiated a STW full collection "concurrently").

 unsigned int GenCollectedHeap::update_full_collections_completed(unsigned int count) {

   MonitorLockerEx ml(FullGCCount_lock, Mutex::_no_safepoint_check_flag);

   assert((_full_collections_completed <= _total_full_collections) &&

          (count <= _total_full_collections),

          "Can't complete more collections than were started");

   if (count > _full_collections_completed) {

     _full_collections_completed = count;

     ml.notify_all();

   }

   return _full_collections_completed;

 }

 #ifndef PRODUCT

 // Override of memory state checking method in CollectedHeap:

 // Some collectors (CMS for example) can't have badHeapWordVal written

 // in the first two words of an object. (For instance , in the case of

 // CMS these words hold state used to synchronize between certain

 // (concurrent) GC steps and direct allocating mutators.)

 // The skip_header_HeapWords() method below, allows us to skip

 // over the requisite number of HeapWord's. Note that (for

 // generational collectors) this means that those many words are

 // skipped in each object, irrespective of the generation in which

 // that object lives. The resultant loss of precision seems to be

 // harmless and the pain of avoiding that imprecision appears somewhat

 // higher than we are prepared to pay for such rudimentary debugging

 // support.

 void GenCollectedHeap::check_for_non_bad_heap_word_value(HeapWord* addr,

                                                          size_t size) {

   if (CheckMemoryInitialization && ZapUnusedHeapArea) {

     // We are asked to check a size in HeapWords,

     // but the memory is mangled in juint words.

     juint* start = (juint*) (addr + skip_header_HeapWords());

     juint* end   = (juint*) (addr + size);

     for (juint* slot = start; slot < end; slot += 1) {

       assert(*slot == badHeapWordVal,

              "Found non badHeapWordValue in pre-allocation check");

     }

   }

 }

 #endif

 HeapWord* GenCollectedHeap::attempt_allocation(size_t size,

                                                bool is_tlab,

                                                bool first_only) {

   HeapWord* res;

   for (int i = 0; i < _n_gens; i++) {

     if (_gens[i]->should_allocate(size, is_tlab)) {

       res = _gens[i]->allocate(size, is_tlab);

       if (res != NULL) return res;

       else if (first_only) break;

     }

   }

   // Otherwise...

   return NULL;

 }

 HeapWord* GenCollectedHeap::mem_allocate(size_t size,

                                          bool* gc_overhead_limit_was_exceeded) {

   return collector_policy()->mem_allocate_work(size,

                                                false /* is_tlab */,

                                                gc_overhead_limit_was_exceeded);

 }

 bool GenCollectedHeap::must_clear_all_soft_refs() {

   return _gc_cause == GCCause::_last_ditch_collection;

 }

 bool GenCollectedHeap::should_do_concurrent_full_gc(GCCause::Cause cause) {

   return UseConcMarkSweepGC &&

          ((cause == GCCause::_gc_locker && GCLockerInvokesConcurrent) ||

           (cause == GCCause::_java_lang_system_gc && ExplicitGCInvokesConcurrent));

 }

 void GenCollectedHeap::do_collection(bool  full,

                                      bool   clear_all_soft_refs,

                                      size_t size,

                                      bool   is_tlab,

                                      int    max_level) {

   bool prepared_for_verification = false;

   ResourceMark rm;

   DEBUG_ONLY(Thread* my_thread = Thread::current();)

   assert(SafepointSynchronize::is_at_safepoint(), "should be at safepoint");

   assert(my_thread->is_VM_thread() ||

          my_thread->is_ConcurrentGC_thread(),

          "incorrect thread type capability");

   assert(Heap_lock->is_locked(),

          "the requesting thread should have the Heap_lock");

   guarantee(!is_gc_active(), "collection is not reentrant");

   assert(max_level < n_gens(), "sanity check");

   if (GC_locker::check_active_before_gc()) {

     return; // GC is disabled (e.g. JNI GetXXXCritical operation)

   }

   const bool do_clear_all_soft_refs = clear_all_soft_refs ||

                           collector_policy()->should_clear_all_soft_refs();

   ClearedAllSoftRefs casr(do_clear_all_soft_refs, collector_policy());

   const size_t metadata_prev_used = MetaspaceAux::used_bytes();

   print_heap_before_gc();

   {

     FlagSetting fl(_is_gc_active, true);

     bool complete = full && (max_level == (n_gens()-1));

     const char* gc_cause_prefix = complete ? "Full GC" : "GC";

     TraceCPUTime tcpu(PrintGCDetails, true, gclog_or_tty);

     // The PrintGCDetails logging starts before we have incremented the GC id. We will do that later

     // so we can assume here that the next GC id is what we want.

     GCTraceTime t(GCCauseString(gc_cause_prefix, gc_cause()), PrintGCDetails, false, NULL, GCId::peek());

     gc_prologue(complete);

     increment_total_collections(complete);

     size_t gch_prev_used = used();

     int starting_level = 0;

     if (full) {

       // Search for the oldest generation which will collect all younger

       // generations, and start collection loop there.

       for (int i = max_level; i >= 0; i--) {

         if (_gens[i]->full_collects_younger_generations()) {

           starting_level = i;

           break;

         }

       }

     }

     bool must_restore_marks_for_biased_locking = false;

     int max_level_collected = starting_level;

     for (int i = starting_level; i <= max_level; i++) {

       if (_gens[i]->should_collect(full, size, is_tlab)) {

         if (i == n_gens() - 1) {  // a major collection is to happen

           if (!complete) {

             // The full_collections increment was missed above.

             increment_total_full_collections();

           }

           pre_full_gc_dump(NULL);    // do any pre full gc dumps

         }

         // Timer for individual generations. Last argument is false: no CR

         // FIXME: We should try to start the timing earlier to cover more of the GC pause

         // The PrintGCDetails logging starts before we have incremented the GC id. We will do that later

         // so we can assume here that the next GC id is what we want.

         GCTraceTime t1(_gens[i]->short_name(), PrintGCDetails, false, NULL, GCId::peek());

         TraceCollectorStats tcs(_gens[i]->counters());

         TraceMemoryManagerStats tmms(_gens[i]->kind(),gc_cause());

         size_t prev_used = _gens[i]->used();

         _gens[i]->stat_record()->invocations++;

         _gens[i]->stat_record()->accumulated_time.start();

         // Must be done anew before each collection because

         // a previous collection will do mangling and will

         // change top of some spaces.

         record_gen_tops_before_GC();

         if (PrintGC && Verbose) {

           gclog_or_tty->print("level=%d invoke=%d size=" SIZE_FORMAT,

                      i,

                      _gens[i]->stat_record()->invocations,

                      size*HeapWordSize);

         }

         if (VerifyBeforeGC && i >= VerifyGCLevel &&

             total_collections() >= VerifyGCStartAt) {

           HandleMark hm;  // Discard invalid handles created during verification

           if (!prepared_for_verification) {

             prepare_for_verify();

             prepared_for_verification = true;

           }

           Universe::verify(" VerifyBeforeGC:");

         }

         COMPILER2_PRESENT(DerivedPointerTable::clear());

         if (!must_restore_marks_for_biased_locking &&

             _gens[i]->performs_in_place_marking()) {

           // We perform this mark word preservation work lazily

           // because it's only at this point that we know whether we

           // absolutely have to do it; we want to avoid doing it for

           // scavenge-only collections where it's unnecessary

           must_restore_marks_for_biased_locking = true;

           BiasedLocking::preserve_marks();

         }

         // Do collection work

         {

           // Note on ref discovery: For what appear to be historical reasons,

           // GCH enables and disabled (by enqueing) refs discovery.

           // In the future this should be moved into the generation's

           // collect method so that ref discovery and enqueueing concerns

           // are local to a generation. The collect method could return

           // an appropriate indication in the case that notification on

           // the ref lock was needed. This will make the treatment of

           // weak refs more uniform (and indeed remove such concerns

           // from GCH). XXX

           HandleMark hm;  // Discard invalid handles created during gc

           save_marks();   // save marks for all gens

           // We want to discover references, but not process them yet.

           // This mode is disabled in process_discovered_references if the

           // generation does some collection work, or in

           // enqueue_discovered_references if the generation returns

           // without doing any work.

           ReferenceProcessor* rp = _gens[i]->ref_processor();

           // If the discovery of ("weak") refs in this generation is

           // atomic wrt other collectors in this configuration, we

           // are guaranteed to have empty discovered ref lists.

           if (rp->discovery_is_atomic()) {

             rp->enable_discovery(true /*verify_disabled*/, true /*verify_no_refs*/);

             rp->setup_policy(do_clear_all_soft_refs);

           } else {

             // collect() below will enable discovery as appropriate

           }

           _gens[i]->collect(full, do_clear_all_soft_refs, size, is_tlab);

           if (!rp->enqueuing_is_done()) {

             rp->enqueue_discovered_references();

           } else {

             rp->set_enqueuing_is_done(false);

           }

           rp->verify_no_references_recorded();

         }

         max_level_collected = i;

         // Determine if allocation request was met.

         if (size > 0) {

           if (!is_tlab || _gens[i]->supports_tlab_allocation()) {

             if (size*HeapWordSize <= _gens[i]->unsafe_max_alloc_nogc()) {

               size = 0;

             }

           }

         }

         COMPILER2_PRESENT(DerivedPointerTable::update_pointers());

         _gens[i]->stat_record()->accumulated_time.stop();

         update_gc_stats(i, full);

         if (VerifyAfterGC && i >= VerifyGCLevel &&

             total_collections() >= VerifyGCStartAt) {

           HandleMark hm;  // Discard invalid handles created during verification

           Universe::verify(" VerifyAfterGC:");

         }

         if (PrintGCDetails) {

           gclog_or_tty->print(":");

           _gens[i]->print_heap_change(prev_used);

         }

       }

     }

     // Update "complete" boolean wrt what actually transpired --

     // for instance, a promotion failure could have led to

     // a whole heap collection.

     complete = complete || (max_level_collected == n_gens() - 1);

     if (complete) { // We did a "major" collection

       // FIXME: See comment at pre_full_gc_dump call

       post_full_gc_dump(NULL);   // do any post full gc dumps

     }

     if (PrintGCDetails) {

       print_heap_change(gch_prev_used);

       // Print metaspace info for full GC with PrintGCDetails flag.

       if (complete) {

         MetaspaceAux::print_metaspace_change(metadata_prev_used);

       }

     }

     for (int j = max_level_collected; j >= 0; j -= 1) {

       // Adjust generation sizes.

       _gens[j]->compute_new_size();

     }

     if (complete) {

       // Delete metaspaces for unloaded class loaders and clean up loader_data graph

       ClassLoaderDataGraph::purge();

       MetaspaceAux::verify_metrics();

       // Resize the metaspace capacity after full collections

       MetaspaceGC::compute_new_size();

       update_full_collections_completed();

     }

     // Track memory usage and detect low memory after GC finishes

     MemoryService::track_memory_usage();

     gc_epilogue(complete);

     if (must_restore_marks_for_biased_locking) {

       BiasedLocking::restore_marks();

     }

   }

   AdaptiveSizePolicy* sp = gen_policy()->size_policy();

   AdaptiveSizePolicyOutput(sp, total_collections());

   print_heap_after_gc();

 #ifdef TRACESPINNING

   ParallelTaskTerminator::print_termination_counts();

 #endif

 }

 HeapWord* GenCollectedHeap::satisfy_failed_allocation(size_t size, bool is_tlab) {

   return collector_policy()->satisfy_failed_allocation(size, is_tlab);

 }

 void GenCollectedHeap::set_par_threads(uint t) {

   SharedHeap::set_par_threads(t);

   set_n_termination(t);

 }

 void GenCollectedHeap::set_n_termination(uint t) {

   _process_strong_tasks->set_n_threads(t);

 }

 #ifdef ASSERT

 class AssertNonScavengableClosure: public OopClosure {

 public:

   virtual void do_oop(oop* p) {

     assert(!Universe::heap()->is_in_partial_collection(*p),

       "Referent should not be scavengable.");  }

   virtual void do_oop(narrowOop* p) { ShouldNotReachHere(); }

 };

 static AssertNonScavengableClosure assert_is_non_scavengable_closure;

 #endif

 void GenCollectedHeap::process_roots(bool activate_scope,

                                      ScanningOption so,

                                      OopClosure* strong_roots,

                                      OopClosure* weak_roots,

                                      CLDClosure* strong_cld_closure,

                                      CLDClosure* weak_cld_closure,

                                      CodeBlobToOopClosure* code_roots) {

   StrongRootsScope srs(this, activate_scope);

   // General roots.

   assert(_strong_roots_parity != 0, "must have called prologue code");

   assert(code_roots != NULL, "code root closure should always be set");

   // _n_termination for _process_strong_tasks should be set up stream

   // in a method not running in a GC worker.  Otherwise the GC worker

   // could be trying to change the termination condition while the task

   // is executing in another GC worker.

   if (!_process_strong_tasks->is_task_claimed(GCH_PS_ClassLoaderDataGraph_oops_do)) {

     ClassLoaderDataGraph::roots_cld_do(strong_cld_closure, weak_cld_closure);

   }

   // Some CLDs contained in the thread frames should be considered strong.

   // Don't process them if they will be processed during the ClassLoaderDataGraph phase.

   CLDClosure* roots_from_clds_p = (strong_cld_closure != weak_cld_closure) ? strong_cld_closure : NULL;

   // Only process code roots from thread stacks if we aren't visiting the entire CodeCache anyway

   CodeBlobToOopClosure* roots_from_code_p = (so & SO_AllCodeCache) ? NULL : code_roots;

   Threads::possibly_parallel_oops_do(strong_roots, roots_from_clds_p, roots_from_code_p);

   if (!_process_strong_tasks->is_task_claimed(GCH_PS_Universe_oops_do)) {

     Universe::oops_do(strong_roots);

   }

   // Global (strong) JNI handles

   if (!_process_strong_tasks->is_task_claimed(GCH_PS_JNIHandles_oops_do)) {

     JNIHandles::oops_do(strong_roots);

   }

   if (!_process_strong_tasks->is_task_claimed(GCH_PS_ObjectSynchronizer_oops_do)) {

     ObjectSynchronizer::oops_do(strong_roots);

   }

   if (!_process_strong_tasks->is_task_claimed(GCH_PS_FlatProfiler_oops_do)) {

     FlatProfiler::oops_do(strong_roots);

   }

   if (!_process_strong_tasks->is_task_claimed(GCH_PS_Management_oops_do)) {

     Management::oops_do(strong_roots);

   }

   if (!_process_strong_tasks->is_task_claimed(GCH_PS_jvmti_oops_do)) {

     JvmtiExport::oops_do(strong_roots);

   }

   if (!_process_strong_tasks->is_task_claimed(GCH_PS_SystemDictionary_oops_do)) {

     SystemDictionary::roots_oops_do(strong_roots, weak_roots);

   }

   // All threads execute the following. A specific chunk of buckets

   // from the StringTable are the individual tasks.

   if (weak_roots != NULL) {

     if (CollectedHeap::use_parallel_gc_threads()) {

       StringTable::possibly_parallel_oops_do(weak_roots);

     } else {

       StringTable::oops_do(weak_roots);

     }

   }

   if (!_process_strong_tasks->is_task_claimed(GCH_PS_CodeCache_oops_do)) {

     if (so & SO_ScavengeCodeCache) {

       assert(code_roots != NULL, "must supply closure for code cache");

       // We only visit parts of the CodeCache when scavenging.

       CodeCache::scavenge_root_nmethods_do(code_roots);

     }

     if (so & SO_AllCodeCache) {

       assert(code_roots != NULL, "must supply closure for code cache");

       // CMSCollector uses this to do intermediate-strength collections.

       // We scan the entire code cache, since CodeCache::do_unloading is not called.

       CodeCache::blobs_do(code_roots);

     }

     // Verify that the code cache contents are not subject to

     // movement by a scavenging collection.

     DEBUG_ONLY(CodeBlobToOopClosure assert_code_is_non_scavengable(&assert_is_non_scavengable_closure, !CodeBlobToOopClosure::FixRelocations));

     DEBUG_ONLY(CodeCache::asserted_non_scavengable_nmethods_do(&assert_code_is_non_scavengable));

   }

 }

 void GenCollectedHeap::gen_process_roots(int level,

                                          bool younger_gens_as_roots,

                                          bool activate_scope,

                                          ScanningOption so,

                                          bool only_strong_roots,

                                          OopsInGenClosure* not_older_gens,

                                          OopsInGenClosure* older_gens,

                                          CLDClosure* cld_closure) {

   const bool is_adjust_phase = !only_strong_roots && !younger_gens_as_roots;

   bool is_moving_collection = false;

   if (level == 0 || is_adjust_phase) {

     // young collections are always moving

     is_moving_collection = true;

   }

   MarkingCodeBlobClosure mark_code_closure(not_older_gens, is_moving_collection);

   OopsInGenClosure* weak_roots = only_strong_roots ? NULL : not_older_gens;

   CLDClosure* weak_cld_closure = only_strong_roots ? NULL : cld_closure;

   process_roots(activate_scope, so,

                 not_older_gens, weak_roots,

                 cld_closure, weak_cld_closure,

                 &mark_code_closure);

   if (younger_gens_as_roots) {

     if (!_process_strong_tasks->is_task_claimed(GCH_PS_younger_gens)) {

       for (int i = 0; i < level; i++) {

         not_older_gens->set_generation(_gens[i]);

         _gens[i]->oop_iterate(not_older_gens);

       }

       not_older_gens->reset_generation();

     }

   }

   // When collection is parallel, all threads get to cooperate to do

   // older-gen scanning.

   for (int i = level+1; i < _n_gens; i++) {

     older_gens->set_generation(_gens[i]);

     rem_set()->younger_refs_iterate(_gens[i], older_gens);

     older_gens->reset_generation();

   }

   _process_strong_tasks->all_tasks_completed();

 }

 void GenCollectedHeap::gen_process_weak_roots(OopClosure* root_closure) {

   JNIHandles::weak_oops_do(root_closure);

   for (int i = 0; i < _n_gens; i++) {

     _gens[i]->ref_processor()->weak_oops_do(root_closure);

   }

 }

 #define GCH_SINCE_SAVE_MARKS_ITERATE_DEFN(OopClosureType, nv_suffix)    \

 void GenCollectedHeap::                                                 \

 oop_since_save_marks_iterate(int level,                                 \

                              OopClosureType* cur,                       \

                              OopClosureType* older) {                   \

   _gens[level]->oop_since_save_marks_iterate##nv_suffix(cur);           \

   for (int i = level+1; i < n_gens(); i++) {                            \

     _gens[i]->oop_since_save_marks_iterate##nv_suffix(older);           \

   }                                                                     \

 }

 ALL_SINCE_SAVE_MARKS_CLOSURES(GCH_SINCE_SAVE_MARKS_ITERATE_DEFN)

 #undef GCH_SINCE_SAVE_MARKS_ITERATE_DEFN

 bool GenCollectedHeap::no_allocs_since_save_marks(int level) {

   for (int i = level; i < _n_gens; i++) {

     if (!_gens[i]->no_allocs_since_save_marks()) return false;

   }

   return true;

 }

 bool GenCollectedHeap::supports_inline_contig_alloc() const {

   return _gens[0]->supports_inline_contig_alloc();

 }

 HeapWord** GenCollectedHeap::top_addr() const {

   return _gens[0]->top_addr();

 }

 HeapWord** GenCollectedHeap::end_addr() const {

   return _gens[0]->end_addr();

 }

 // public collection interfaces

 void GenCollectedHeap::collect(GCCause::Cause cause) {

   if (should_do_concurrent_full_gc(cause)) {

 #if INCLUDE_ALL_GCS

     // mostly concurrent full collection

     collect_mostly_concurrent(cause);

 #else  // INCLUDE_ALL_GCS

     ShouldNotReachHere();

 #endif // INCLUDE_ALL_GCS

   } else if (cause == GCCause::_wb_young_gc) {

     // minor collection for WhiteBox API

     collect(cause, 0);

   } else {

 #ifdef ASSERT

   if (cause == GCCause::_scavenge_alot) {

     // minor collection only

     collect(cause, 0);

   } else {

     // Stop-the-world full collection

     collect(cause, n_gens() - 1);

   }

 #else

     // Stop-the-world full collection

     collect(cause, n_gens() - 1);

 #endif

   }

 }

 void GenCollectedHeap::collect(GCCause::Cause cause, int max_level) {

   // The caller doesn't have the Heap_lock

   assert(!Heap_lock->owned_by_self(), "this thread should not own the Heap_lock");

   MutexLocker ml(Heap_lock);

   collect_locked(cause, max_level);

 }

 void GenCollectedHeap::collect_locked(GCCause::Cause cause) {

   // The caller has the Heap_lock

   assert(Heap_lock->owned_by_self(), "this thread should own the Heap_lock");

   collect_locked(cause, n_gens() - 1);

 }

 // this is the private collection interface

 // The Heap_lock is expected to be held on entry.

 void GenCollectedHeap::collect_locked(GCCause::Cause cause, int max_level) {

   // Read the GC count while holding the Heap_lock

   unsigned int gc_count_before      = total_collections();

   unsigned int full_gc_count_before = total_full_collections();

   {

     MutexUnlocker mu(Heap_lock);  // give up heap lock, execute gets it back

     VM_GenCollectFull op(gc_count_before, full_gc_count_before,

                          cause, max_level);

     VMThread::execute(&op);

   }

 }

 #if INCLUDE_ALL_GCS

 bool GenCollectedHeap::create_cms_collector() {

   assert(((_gens[1]->kind() == Generation::ConcurrentMarkSweep) ||

          (_gens[1]->kind() == Generation::ASConcurrentMarkSweep)),

          "Unexpected generation kinds");

   // Skip two header words in the block content verification

   NOT_PRODUCT(_skip_header_HeapWords = CMSCollector::skip_header_HeapWords();)

   CMSCollector* collector = new CMSCollector(

     (ConcurrentMarkSweepGeneration*)_gens[1],

     _rem_set->as_CardTableRS(),

     (ConcurrentMarkSweepPolicy*) collector_policy());

   if (collector == NULL || !collector->completed_initialization()) {

     if (collector) {

       delete collector;  // Be nice in embedded situation

     }

     vm_shutdown_during_initialization("Could not create CMS collector");

     return false;

   }

   return true;  // success

 }

 void GenCollectedHeap::collect_mostly_concurrent(GCCause::Cause cause) {

   assert(!Heap_lock->owned_by_self(), "Should not own Heap_lock");

   MutexLocker ml(Heap_lock);

   // Read the GC counts while holding the Heap_lock

   unsigned int full_gc_count_before = total_full_collections();

   unsigned int gc_count_before      = total_collections();

   {

     MutexUnlocker mu(Heap_lock);

     VM_GenCollectFullConcurrent op(gc_count_before, full_gc_count_before, cause);

     VMThread::execute(&op);

   }

 }

 #endif // INCLUDE_ALL_GCS

 void GenCollectedHeap::do_full_collection(bool clear_all_soft_refs) {

    do_full_collection(clear_all_soft_refs, _n_gens - 1);

 }

 void GenCollectedHeap::do_full_collection(bool clear_all_soft_refs,

                                           int max_level) {

   int local_max_level;

   if (!incremental_collection_will_fail(false /* don't consult_young */) &&

       gc_cause() == GCCause::_gc_locker) {

     local_max_level = 0;

   } else {

     local_max_level = max_level;

   }

   do_collection(true                 /* full */,

                 clear_all_soft_refs  /* clear_all_soft_refs */,

                 0                    /* size */,

                 false                /* is_tlab */,

                 local_max_level      /* max_level */);

   // Hack XXX FIX ME !!!

   // A scavenge may not have been attempted, or may have

   // been attempted and failed, because the old gen was too full

   if (local_max_level == 0 && gc_cause() == GCCause::_gc_locker &&

       incremental_collection_will_fail(false /* don't consult_young */)) {

     if (PrintGCDetails) {

       gclog_or_tty->print_cr("GC locker: Trying a full collection "

                              "because scavenge failed");

     }

     // This time allow the old gen to be collected as well

     do_collection(true                 /* full */,

                   clear_all_soft_refs  /* clear_all_soft_refs */,

                   0                    /* size */,

                   false                /* is_tlab */,

                   n_gens() - 1         /* max_level */);

   }

 }

 bool GenCollectedHeap::is_in_young(oop p) {

   bool result = ((HeapWord*)p) < _gens[_n_gens - 1]->reserved().start();

   assert(result == _gens[0]->is_in_reserved(p),

          err_msg("incorrect test - result=%d, p=" PTR_FORMAT, result, p2i((void*)p)));

   return result;

 }

 // Returns "TRUE" iff "p" points into the committed areas of the heap.

 bool GenCollectedHeap::is_in(const void* p) const {

   #ifndef ASSERT

   guarantee(VerifyBeforeGC      ||

             VerifyDuringGC      ||

             VerifyBeforeExit    ||

             VerifyDuringStartup ||

             PrintAssembly       ||

             tty->count() != 0   ||   // already printing

             VerifyAfterGC       ||

     VMError::fatal_error_in_progress(), "too expensive");

   #endif

   // This might be sped up with a cache of the last generation that

   // answered yes.

   for (int i = 0; i < _n_gens; i++) {

     if (_gens[i]->is_in(p)) return true;

   }

   // Otherwise...

   return false;

 }

 #ifdef ASSERT

 // Don't implement this by using is_in_young().  This method is used

 // in some cases to check that is_in_young() is correct.

 bool GenCollectedHeap::is_in_partial_collection(const void* p) {

   assert(is_in_reserved(p) || p == NULL,

     "Does not work if address is non-null and outside of the heap");

   return p < _gens[_n_gens - 2]->reserved().end() && p != NULL;

 }

 #endif

 void GenCollectedHeap::oop_iterate(ExtendedOopClosure* cl) {

   for (int i = 0; i < _n_gens; i++) {

     _gens[i]->oop_iterate(cl);

   }

 }

 void GenCollectedHeap::object_iterate(ObjectClosure* cl) {

   for (int i = 0; i < _n_gens; i++) {

     _gens[i]->object_iterate(cl);

   }

 }

 void GenCollectedHeap::safe_object_iterate(ObjectClosure* cl) {

   for (int i = 0; i < _n_gens; i++) {

     _gens[i]->safe_object_iterate(cl);

   }

 }

 Space* GenCollectedHeap::space_containing(const void* addr) const {

   for (int i = 0; i < _n_gens; i++) {

     Space* res = _gens[i]->space_containing(addr);

     if (res != NULL) return res;

   }

   // Otherwise...

   assert(false, "Could not find containing space");

   return NULL;

 }

 HeapWord* GenCollectedHeap::block_start(const void* addr) const {

   assert(is_in_reserved(addr), "block_start of address outside of heap");

   for (int i = 0; i < _n_gens; i++) {

     if (_gens[i]->is_in_reserved(addr)) {

       assert(_gens[i]->is_in(addr),

              "addr should be in allocated part of generation");

       return _gens[i]->block_start(addr);

     }

   }

   assert(false, "Some generation should contain the address");

   return NULL;

 }

 size_t GenCollectedHeap::block_size(const HeapWord* addr) const {

   assert(is_in_reserved(addr), "block_size of address outside of heap");

   for (int i = 0; i < _n_gens; i++) {

     if (_gens[i]->is_in_reserved(addr)) {

       assert(_gens[i]->is_in(addr),

              "addr should be in allocated part of generation");

       return _gens[i]->block_size(addr);

     }

   }

   assert(false, "Some generation should contain the address");

   return 0;

 }

 bool GenCollectedHeap::block_is_obj(const HeapWord* addr) const {

   assert(is_in_reserved(addr), "block_is_obj of address outside of heap");

   assert(block_start(addr) == addr, "addr must be a block start");

   for (int i = 0; i < _n_gens; i++) {

     if (_gens[i]->is_in_reserved(addr)) {

       return _gens[i]->block_is_obj(addr);

     }

   }

   assert(false, "Some generation should contain the address");

   return false;

 }

 bool GenCollectedHeap::supports_tlab_allocation() const {

   for (int i = 0; i < _n_gens; i += 1) {

     if (_gens[i]->supports_tlab_allocation()) {

       return true;

     }

   }

   return false;

 }

 size_t GenCollectedHeap::tlab_capacity(Thread* thr) const {

   size_t result = 0;

   for (int i = 0; i < _n_gens; i += 1) {

     if (_gens[i]->supports_tlab_allocation()) {

       result += _gens[i]->tlab_capacity();

     }

   }

   return result;

 }

 size_t GenCollectedHeap::tlab_used(Thread* thr) const {

   size_t result = 0;

   for (int i = 0; i < _n_gens; i += 1) {

     if (_gens[i]->supports_tlab_allocation()) {

       result += _gens[i]->tlab_used();

     }

   }

   return result;

 }

 size_t GenCollectedHeap::unsafe_max_tlab_alloc(Thread* thr) const {

   size_t result = 0;

   for (int i = 0; i < _n_gens; i += 1) {

     if (_gens[i]->supports_tlab_allocation()) {

       result += _gens[i]->unsafe_max_tlab_alloc();

     }

   }

   return result;

 }

 HeapWord* GenCollectedHeap::allocate_new_tlab(size_t size) {

   bool gc_overhead_limit_was_exceeded;

   return collector_policy()->mem_allocate_work(size /* size */,

                                                true /* is_tlab */,

                                                &gc_overhead_limit_was_exceeded);

 }

 // Requires "*prev_ptr" to be non-NULL.  Deletes and a block of minimal size

 // from the list headed by "*prev_ptr".

 static ScratchBlock *removeSmallestScratch(ScratchBlock **prev_ptr) {

   bool first = true;

   size_t min_size = 0;   // "first" makes this conceptually infinite.

   ScratchBlock **smallest_ptr, *smallest;

   ScratchBlock  *cur = *prev_ptr;

   while (cur) {

     assert(*prev_ptr == cur, "just checking");

     if (first || cur->num_words < min_size) {

       smallest_ptr = prev_ptr;

       smallest     = cur;

       min_size     = smallest->num_words;

       first        = false;

     }

     prev_ptr = &cur->next;

     cur     =  cur->next;

   }

   smallest      = *smallest_ptr;

   *smallest_ptr = smallest->next;

   return smallest;

 }

 // Sort the scratch block list headed by res into decreasing size order,

 // and set "res" to the result.

 static void sort_scratch_list(ScratchBlock*& list) {

   ScratchBlock* sorted = NULL;

   ScratchBlock* unsorted = list;

   while (unsorted) {

     ScratchBlock *smallest = removeSmallestScratch(&unsorted);

     smallest->next  = sorted;

     sorted          = smallest;

   }

   list = sorted;

 }

 ScratchBlock* GenCollectedHeap::gather_scratch(Generation* requestor,

                                                size_t max_alloc_words) {

   ScratchBlock* res = NULL;

   for (int i = 0; i < _n_gens; i++) {

     _gens[i]->contribute_scratch(res, requestor, max_alloc_words);

   }

   sort_scratch_list(res);

   return res;

 }

 void GenCollectedHeap::release_scratch() {

   for (int i = 0; i < _n_gens; i++) {

     _gens[i]->reset_scratch();

   }

 }

 class GenPrepareForVerifyClosure: public GenCollectedHeap::GenClosure {

   void do_generation(Generation* gen) {

     gen->prepare_for_verify();

   }

 };

 void GenCollectedHeap::prepare_for_verify() {

   ensure_parsability(false);        // no need to retire TLABs

   GenPrepareForVerifyClosure blk;

   generation_iterate(&blk, false);

 }

 void GenCollectedHeap::generation_iterate(GenClosure* cl,

                                           bool old_to_young) {

   if (old_to_young) {

     for (int i = _n_gens-1; i >= 0; i--) {

       cl->do_generation(_gens[i]);

     }

   } else {

     for (int i = 0; i < _n_gens; i++) {

       cl->do_generation(_gens[i]);

     }

   }

 }

 void GenCollectedHeap::space_iterate(SpaceClosure* cl) {

   for (int i = 0; i < _n_gens; i++) {

     _gens[i]->space_iterate(cl, true);

   }

 }

 bool GenCollectedHeap::is_maximal_no_gc() const {

   for (int i = 0; i < _n_gens; i++) {

     if (!_gens[i]->is_maximal_no_gc()) {

       return false;

     }

   }

   return true;

 }

 void GenCollectedHeap::save_marks() {

   for (int i = 0; i < _n_gens; i++) {

     _gens[i]->save_marks();

   }

 }

 GenCollectedHeap* GenCollectedHeap::heap() {

   assert(_gch != NULL, "Uninitialized access to GenCollectedHeap::heap()");

   assert(_gch->kind() == CollectedHeap::GenCollectedHeap, "not a generational heap");

   return _gch;

 }

 void GenCollectedHeap::prepare_for_compaction() {

   guarantee(_n_gens = 2, "Wrong number of generations");

   Generation* old_gen = _gens[1];

   // Start by compacting into same gen.

   CompactPoint cp(old_gen);

   old_gen->prepare_for_compaction(&cp);

   Generation* young_gen = _gens[0];

   young_gen->prepare_for_compaction(&cp);

 }

 GCStats* GenCollectedHeap::gc_stats(int level) const {

   return _gens[level]->gc_stats();

 }

 void GenCollectedHeap::verify(bool silent, VerifyOption option /* ignored */) {

   for (int i = _n_gens-1; i >= 0; i--) {

     Generation* g = _gens[i];

     if (!silent) {

       gclog_or_tty->print("%s", g->name());

       gclog_or_tty->print(" ");

     }

     g->verify();

   }

   if (!silent) {

     gclog_or_tty->print("remset ");

   }

   rem_set()->verify();

 }

 void GenCollectedHeap::print_on(outputStream* st) const {

   for (int i = 0; i < _n_gens; i++) {

     _gens[i]->print_on(st);

   }

   MetaspaceAux::print_on(st);

 }

 void GenCollectedHeap::gc_threads_do(ThreadClosure* tc) const {

   if (workers() != NULL) {

     workers()->threads_do(tc);

   }

 #if INCLUDE_ALL_GCS

   if (UseConcMarkSweepGC) {

     ConcurrentMarkSweepThread::threads_do(tc);

   }

 #endif // INCLUDE_ALL_GCS

 }

 void GenCollectedHeap::print_gc_threads_on(outputStream* st) const {

 #if INCLUDE_ALL_GCS

   if (UseParNewGC) {

     workers()->print_worker_threads_on(st);

   }

   if (UseConcMarkSweepGC) {

     ConcurrentMarkSweepThread::print_all_on(st);

   }

 #endif // INCLUDE_ALL_GCS

 }

 void GenCollectedHeap::print_on_error(outputStream* st) const {

   this->CollectedHeap::print_on_error(st);

 #if INCLUDE_ALL_GCS

   if (UseConcMarkSweepGC) {

     st->cr();

     CMSCollector::print_on_error(st);

   }

 #endif // INCLUDE_ALL_GCS

 }

 void GenCollectedHeap::print_tracing_info() const {

   if (TraceGen0Time) {

     get_gen(0)->print_summary_info();

   }

   if (TraceGen1Time) {

     get_gen(1)->print_summary_info();

   }

 }

 void GenCollectedHeap::print_heap_change(size_t prev_used) const {

   if (PrintGCDetails && Verbose) {

     gclog_or_tty->print(" "  SIZE_FORMAT

                         "->" SIZE_FORMAT

                         "("  SIZE_FORMAT ")",

                         prev_used, used(), capacity());

   } else {

     gclog_or_tty->print(" "  SIZE_FORMAT "K"

                         "->" SIZE_FORMAT "K"

                         "("  SIZE_FORMAT "K)",

                         prev_used / K, used() / K, capacity() / K);

   }

 }

 class GenGCPrologueClosure: public GenCollectedHeap::GenClosure {

  private:

   bool _full;

  public:

   void do_generation(Generation* gen) {

     gen->gc_prologue(_full);

   }

   GenGCPrologueClosure(bool full) : _full(full) {};

 };

 void GenCollectedHeap::gc_prologue(bool full) {

   assert(InlineCacheBuffer::is_empty(), "should have cleaned up ICBuffer");

   always_do_update_barrier = false;

   // Fill TLAB's and such

   CollectedHeap::accumulate_statistics_all_tlabs();

   ensure_parsability(true);   // retire TLABs

   // Walk generations

   GenGCPrologueClosure blk(full);

   generation_iterate(&blk, false);  // not old-to-young.

 };

 class GenGCEpilogueClosure: public GenCollectedHeap::GenClosure {

  private:

   bool _full;

  public:

   void do_generation(Generation* gen) {

     gen->gc_epilogue(_full);

   }

   GenGCEpilogueClosure(bool full) : _full(full) {};

 };

 void GenCollectedHeap::gc_epilogue(bool full) {

 #ifdef COMPILER2

   assert(DerivedPointerTable::is_empty(), "derived pointer present");

   size_t actual_gap = pointer_delta((HeapWord*) (max_uintx-3), *(end_addr()));

   guarantee(actual_gap > (size_t)FastAllocateSizeLimit, "inline allocation wraps");

 #endif /* COMPILER2 */

   resize_all_tlabs();

   GenGCEpilogueClosure blk(full);

   generation_iterate(&blk, false);  // not old-to-young.

   if (!CleanChunkPoolAsync) {

     Chunk::clean_chunk_pool();

   }

   MetaspaceCounters::update_performance_counters();

   CompressedClassSpaceCounters::update_performance_counters();

   always_do_update_barrier = UseConcMarkSweepGC;

 };

 #ifndef PRODUCT

 class GenGCSaveTopsBeforeGCClosure: public GenCollectedHeap::GenClosure {

  private:

  public:

   void do_generation(Generation* gen) {

     gen->record_spaces_top();

   }

 };

 void GenCollectedHeap::record_gen_tops_before_GC() {

   if (ZapUnusedHeapArea) {

     GenGCSaveTopsBeforeGCClosure blk;

     generation_iterate(&blk, false);  // not old-to-young.

   }

 }

 #endif  // not PRODUCT

 class GenEnsureParsabilityClosure: public GenCollectedHeap::GenClosure {

  public:

   void do_generation(Generation* gen) {

     gen->ensure_parsability();

   }

 };

 void GenCollectedHeap::ensure_parsability(bool retire_tlabs) {

   CollectedHeap::ensure_parsability(retire_tlabs);

   GenEnsureParsabilityClosure ep_cl;

   generation_iterate(&ep_cl, false);

 }

 oop GenCollectedHeap::handle_failed_promotion(Generation* old_gen,

                                               oop obj,

                                               size_t obj_size) {

   guarantee(old_gen->level() == 1, "We only get here with an old generation");

   assert(obj_size == (size_t)obj->size(), "bad obj_size passed in");

   HeapWord* result = NULL;

   result = old_gen->expand_and_allocate(obj_size, false);

   if (result != NULL) {

     Copy::aligned_disjoint_words((HeapWord*)obj, result, obj_size);

   }

   return oop(result);

 }

 class GenTimeOfLastGCClosure: public GenCollectedHeap::GenClosure {

   jlong _time;   // in ms

   jlong _now;    // in ms

  public:

   GenTimeOfLastGCClosure(jlong now) : _time(now), _now(now) { }

   jlong time() { return _time; }

   void do_generation(Generation* gen) {

     _time = MIN2(_time, gen->time_of_last_gc(_now));

   }

 };

 jlong GenCollectedHeap::millis_since_last_gc() {

   // We need a monotonically non-deccreasing time in ms but

   // os::javaTimeMillis() does not guarantee monotonicity.

   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;

   GenTimeOfLastGCClosure tolgc_cl(now);

   // iterate over generations getting the oldest

   // time that a generation was collected

   generation_iterate(&tolgc_cl, false);

   // javaTimeNanos() is guaranteed to be monotonically non-decreasing

   // provided the underlying platform provides such a time source

   // (and it is bug free). So we still have to guard against getting

   // back a time later than 'now'.

   jlong retVal = now - tolgc_cl.time();

   if (retVal < 0) {

     NOT_PRODUCT(warning("time warp: "INT64_FORMAT, (int64_t) retVal);)

     return 0;

   }

   return retVal;

 }