1 /*
   2  * Copyright (c) 2000, 2017, 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 #include "precompiled.hpp"
  26 #include "aot/aotLoader.hpp"
  27 #include "classfile/symbolTable.hpp"
  28 #include "classfile/stringTable.hpp"
  29 #include "classfile/systemDictionary.hpp"
  30 #include "classfile/vmSymbols.hpp"
  31 #include "code/codeCache.hpp"
  32 #include "code/icBuffer.hpp"
  33 #include "gc/shared/collectedHeap.inline.hpp"
  34 #include "gc/shared/collectorCounters.hpp"
  35 #include "gc/shared/gcId.hpp"
  36 #include "gc/shared/gcLocker.inline.hpp"
  37 #include "gc/shared/gcTrace.hpp"
  38 #include "gc/shared/gcTraceTime.inline.hpp"
  39 #include "gc/shared/genCollectedHeap.hpp"
  40 #include "gc/shared/genOopClosures.inline.hpp"
  41 #include "gc/shared/generationSpec.hpp"
  42 #include "gc/shared/space.hpp"
  43 #include "gc/shared/strongRootsScope.hpp"
  44 #include "gc/shared/vmGCOperations.hpp"
  45 #include "gc/shared/weakProcessor.hpp"
  46 #include "gc/shared/workgroup.hpp"
  47 #include "memory/filemap.hpp"
  48 #include "memory/resourceArea.hpp"
  49 #include "oops/oop.inline.hpp"
  50 #include "runtime/biasedLocking.hpp"
  51 #include "runtime/handles.hpp"
  52 #include "runtime/handles.inline.hpp"
  53 #include "runtime/java.hpp"
  54 #include "runtime/vmThread.hpp"
  55 #include "services/management.hpp"
  56 #include "services/memoryService.hpp"
  57 #include "utilities/debug.hpp"
  58 #include "utilities/formatBuffer.hpp"
  59 #include "utilities/macros.hpp"
  60 #include "utilities/stack.inline.hpp"
  61 #include "utilities/vmError.hpp"
  62 
  63 GenCollectedHeap::GenCollectedHeap(GenCollectorPolicy *policy) :
  64   CollectedHeap(),
  65   _rem_set(NULL),
  66   _gen_policy(policy),
  67   _process_strong_tasks(new SubTasksDone(GCH_PS_NumElements)),
  68   _full_collections_completed(0)
  69 {
  70   assert(policy != NULL, "Sanity check");
  71 }
  72 
  73 jint GenCollectedHeap::initialize() {
  74   CollectedHeap::pre_initialize();
  75 
  76   // While there are no constraints in the GC code that HeapWordSize
  77   // be any particular value, there are multiple other areas in the
  78   // system which believe this to be true (e.g. oop->object_size in some
  79   // cases incorrectly returns the size in wordSize units rather than
  80   // HeapWordSize).
  81   guarantee(HeapWordSize == wordSize, "HeapWordSize must equal wordSize");
  82 
  83   // Allocate space for the heap.
  84 
  85   char* heap_address;
  86   ReservedSpace heap_rs;
  87 
  88   size_t heap_alignment = collector_policy()->heap_alignment();
  89 
  90   heap_address = allocate(heap_alignment, &heap_rs);
  91 
  92   if (!heap_rs.is_reserved()) {
  93     vm_shutdown_during_initialization(
  94       "Could not reserve enough space for object heap");
  95     return JNI_ENOMEM;
  96   }
  97 
  98   initialize_reserved_region((HeapWord*)heap_rs.base(), (HeapWord*)(heap_rs.base() + heap_rs.size()));
  99 
 100   _rem_set = collector_policy()->create_rem_set(reserved_region());
 101   set_barrier_set(rem_set()->bs());
 102 
 103   ReservedSpace young_rs = heap_rs.first_part(gen_policy()->young_gen_spec()->max_size(), false, false);
 104   _young_gen = gen_policy()->young_gen_spec()->init(young_rs, rem_set());
 105   heap_rs = heap_rs.last_part(gen_policy()->young_gen_spec()->max_size());
 106 
 107   ReservedSpace old_rs = heap_rs.first_part(gen_policy()->old_gen_spec()->max_size(), false, false);
 108   _old_gen = gen_policy()->old_gen_spec()->init(old_rs, rem_set());
 109   clear_incremental_collection_failed();
 110 
 111   return JNI_OK;
 112 }
 113 
 114 char* GenCollectedHeap::allocate(size_t alignment,
 115                                  ReservedSpace* heap_rs){
 116   // Now figure out the total size.
 117   const size_t pageSize = UseLargePages ? os::large_page_size() : os::vm_page_size();
 118   assert(alignment % pageSize == 0, "Must be");
 119 
 120   GenerationSpec* young_spec = gen_policy()->young_gen_spec();
 121   GenerationSpec* old_spec = gen_policy()->old_gen_spec();
 122 
 123   // Check for overflow.
 124   size_t total_reserved = young_spec->max_size() + old_spec->max_size();
 125   if (total_reserved < young_spec->max_size()) {
 126     vm_exit_during_initialization("The size of the object heap + VM data exceeds "
 127                                   "the maximum representable size");
 128   }
 129   assert(total_reserved % alignment == 0,
 130          "Gen size; total_reserved=" SIZE_FORMAT ", alignment="
 131          SIZE_FORMAT, total_reserved, alignment);
 132 
 133   *heap_rs = Universe::reserve_heap(total_reserved, alignment);
 134 
 135   os::trace_page_sizes("Heap",
 136                        collector_policy()->min_heap_byte_size(),
 137                        total_reserved,
 138                        alignment,
 139                        heap_rs->base(),
 140                        heap_rs->size());
 141 
 142   return heap_rs->base();
 143 }
 144 
 145 void GenCollectedHeap::post_initialize() {
 146   ref_processing_init();
 147   check_gen_kinds();
 148   DefNewGeneration* def_new_gen = (DefNewGeneration*)_young_gen;
 149 
 150   _gen_policy->initialize_size_policy(def_new_gen->eden()->capacity(),
 151                                       _old_gen->capacity(),
 152                                       def_new_gen->from()->capacity());
 153   _gen_policy->initialize_gc_policy_counters();
 154 }
 155 
 156 void GenCollectedHeap::ref_processing_init() {
 157   _young_gen->ref_processor_init();
 158   _old_gen->ref_processor_init();
 159 }
 160 
 161 size_t GenCollectedHeap::capacity() const {
 162   return _young_gen->capacity() + _old_gen->capacity();
 163 }
 164 
 165 size_t GenCollectedHeap::used() const {
 166   return _young_gen->used() + _old_gen->used();
 167 }
 168 
 169 void GenCollectedHeap::save_used_regions() {
 170   _old_gen->save_used_region();
 171   _young_gen->save_used_region();
 172 }
 173 
 174 size_t GenCollectedHeap::max_capacity() const {
 175   return _young_gen->max_capacity() + _old_gen->max_capacity();
 176 }
 177 
 178 // Update the _full_collections_completed counter
 179 // at the end of a stop-world full GC.
 180 unsigned int GenCollectedHeap::update_full_collections_completed() {
 181   MonitorLockerEx ml(FullGCCount_lock, Mutex::_no_safepoint_check_flag);
 182   assert(_full_collections_completed <= _total_full_collections,
 183          "Can't complete more collections than were started");
 184   _full_collections_completed = _total_full_collections;
 185   ml.notify_all();
 186   return _full_collections_completed;
 187 }
 188 
 189 // Update the _full_collections_completed counter, as appropriate,
 190 // at the end of a concurrent GC cycle. Note the conditional update
 191 // below to allow this method to be called by a concurrent collector
 192 // without synchronizing in any manner with the VM thread (which
 193 // may already have initiated a STW full collection "concurrently").
 194 unsigned int GenCollectedHeap::update_full_collections_completed(unsigned int count) {
 195   MonitorLockerEx ml(FullGCCount_lock, Mutex::_no_safepoint_check_flag);
 196   assert((_full_collections_completed <= _total_full_collections) &&
 197          (count <= _total_full_collections),
 198          "Can't complete more collections than were started");
 199   if (count > _full_collections_completed) {
 200     _full_collections_completed = count;
 201     ml.notify_all();
 202   }
 203   return _full_collections_completed;
 204 }
 205 
 206 
 207 #ifndef PRODUCT
 208 // Override of memory state checking method in CollectedHeap:
 209 // Some collectors (CMS for example) can't have badHeapWordVal written
 210 // in the first two words of an object. (For instance , in the case of
 211 // CMS these words hold state used to synchronize between certain
 212 // (concurrent) GC steps and direct allocating mutators.)
 213 // The skip_header_HeapWords() method below, allows us to skip
 214 // over the requisite number of HeapWord's. Note that (for
 215 // generational collectors) this means that those many words are
 216 // skipped in each object, irrespective of the generation in which
 217 // that object lives. The resultant loss of precision seems to be
 218 // harmless and the pain of avoiding that imprecision appears somewhat
 219 // higher than we are prepared to pay for such rudimentary debugging
 220 // support.
 221 void GenCollectedHeap::check_for_non_bad_heap_word_value(HeapWord* addr,
 222                                                          size_t size) {
 223   if (CheckMemoryInitialization && ZapUnusedHeapArea) {
 224     // We are asked to check a size in HeapWords,
 225     // but the memory is mangled in juint words.
 226     juint* start = (juint*) (addr + skip_header_HeapWords());
 227     juint* end   = (juint*) (addr + size);
 228     for (juint* slot = start; slot < end; slot += 1) {
 229       assert(*slot == badHeapWordVal,
 230              "Found non badHeapWordValue in pre-allocation check");
 231     }
 232   }
 233 }
 234 #endif
 235 
 236 HeapWord* GenCollectedHeap::attempt_allocation(size_t size,
 237                                                bool is_tlab,
 238                                                bool first_only) {
 239   HeapWord* res = NULL;
 240 
 241   if (_young_gen->should_allocate(size, is_tlab)) {
 242     res = _young_gen->allocate(size, is_tlab);
 243     if (res != NULL || first_only) {
 244       return res;
 245     }
 246   }
 247 
 248   if (_old_gen->should_allocate(size, is_tlab)) {
 249     res = _old_gen->allocate(size, is_tlab);
 250   }
 251 
 252   return res;
 253 }
 254 
 255 HeapWord* GenCollectedHeap::mem_allocate(size_t size,
 256                                          bool* gc_overhead_limit_was_exceeded) {
 257   return gen_policy()->mem_allocate_work(size,
 258                                          false /* is_tlab */,
 259                                          gc_overhead_limit_was_exceeded);
 260 }
 261 
 262 bool GenCollectedHeap::must_clear_all_soft_refs() {
 263   return _gc_cause == GCCause::_metadata_GC_clear_soft_refs ||
 264          _gc_cause == GCCause::_wb_full_gc;
 265 }
 266 
 267 void GenCollectedHeap::collect_generation(Generation* gen, bool full, size_t size,
 268                                           bool is_tlab, bool run_verification, bool clear_soft_refs,
 269                                           bool restore_marks_for_biased_locking) {
 270   FormatBuffer<> title("Collect gen: %s", gen->short_name());
 271   GCTraceTime(Trace, gc, phases) t1(title);
 272   TraceCollectorStats tcs(gen->counters());
 273   TraceMemoryManagerStats tmms(gen->kind(),gc_cause());
 274 
 275   gen->stat_record()->invocations++;
 276   gen->stat_record()->accumulated_time.start();
 277 
 278   // Must be done anew before each collection because
 279   // a previous collection will do mangling and will
 280   // change top of some spaces.
 281   record_gen_tops_before_GC();
 282 
 283   log_trace(gc)("%s invoke=%d size=" SIZE_FORMAT, heap()->is_young_gen(gen) ? "Young" : "Old", gen->stat_record()->invocations, size * HeapWordSize);
 284 
 285   if (run_verification && VerifyBeforeGC) {
 286     HandleMark hm;  // Discard invalid handles created during verification
 287     Universe::verify("Before GC");
 288   }
 289   COMPILER2_PRESENT(DerivedPointerTable::clear());
 290 
 291   if (restore_marks_for_biased_locking) {
 292     // We perform this mark word preservation work lazily
 293     // because it's only at this point that we know whether we
 294     // absolutely have to do it; we want to avoid doing it for
 295     // scavenge-only collections where it's unnecessary
 296     BiasedLocking::preserve_marks();
 297   }
 298 
 299   // Do collection work
 300   {
 301     // Note on ref discovery: For what appear to be historical reasons,
 302     // GCH enables and disabled (by enqueing) refs discovery.
 303     // In the future this should be moved into the generation's
 304     // collect method so that ref discovery and enqueueing concerns
 305     // are local to a generation. The collect method could return
 306     // an appropriate indication in the case that notification on
 307     // the ref lock was needed. This will make the treatment of
 308     // weak refs more uniform (and indeed remove such concerns
 309     // from GCH). XXX
 310 
 311     HandleMark hm;  // Discard invalid handles created during gc
 312     save_marks();   // save marks for all gens
 313     // We want to discover references, but not process them yet.
 314     // This mode is disabled in process_discovered_references if the
 315     // generation does some collection work, or in
 316     // enqueue_discovered_references if the generation returns
 317     // without doing any work.
 318     ReferenceProcessor* rp = gen->ref_processor();
 319     // If the discovery of ("weak") refs in this generation is
 320     // atomic wrt other collectors in this configuration, we
 321     // are guaranteed to have empty discovered ref lists.
 322     if (rp->discovery_is_atomic()) {
 323       rp->enable_discovery();
 324       rp->setup_policy(clear_soft_refs);
 325     } else {
 326       // collect() below will enable discovery as appropriate
 327     }
 328     gen->collect(full, clear_soft_refs, size, is_tlab);
 329     if (!rp->enqueuing_is_done()) {
 330       ReferenceProcessorPhaseTimes pt(NULL, rp->num_q());
 331       rp->enqueue_discovered_references(NULL, &pt);
 332       pt.print_enqueue_phase();
 333     } else {
 334       rp->set_enqueuing_is_done(false);
 335     }
 336     rp->verify_no_references_recorded();
 337   }
 338 
 339   COMPILER2_PRESENT(DerivedPointerTable::update_pointers());
 340 
 341   gen->stat_record()->accumulated_time.stop();
 342 
 343   update_gc_stats(gen, full);
 344 
 345   if (run_verification && VerifyAfterGC) {
 346     HandleMark hm;  // Discard invalid handles created during verification
 347     Universe::verify("After GC");
 348   }
 349 }
 350 
 351 void GenCollectedHeap::do_collection(bool           full,
 352                                      bool           clear_all_soft_refs,
 353                                      size_t         size,
 354                                      bool           is_tlab,
 355                                      GenerationType max_generation) {
 356   ResourceMark rm;
 357   DEBUG_ONLY(Thread* my_thread = Thread::current();)
 358 
 359   assert(SafepointSynchronize::is_at_safepoint(), "should be at safepoint");
 360   assert(my_thread->is_VM_thread() ||
 361          my_thread->is_ConcurrentGC_thread(),
 362          "incorrect thread type capability");
 363   assert(Heap_lock->is_locked(),
 364          "the requesting thread should have the Heap_lock");
 365   guarantee(!is_gc_active(), "collection is not reentrant");
 366 
 367   if (GCLocker::check_active_before_gc()) {
 368     return; // GC is disabled (e.g. JNI GetXXXCritical operation)
 369   }
 370 
 371   GCIdMarkAndRestore gc_id_mark;
 372 
 373   const bool do_clear_all_soft_refs = clear_all_soft_refs ||
 374                           collector_policy()->should_clear_all_soft_refs();
 375 
 376   ClearedAllSoftRefs casr(do_clear_all_soft_refs, collector_policy());
 377 
 378   const size_t metadata_prev_used = MetaspaceAux::used_bytes();
 379 
 380   print_heap_before_gc();
 381 
 382   {
 383     FlagSetting fl(_is_gc_active, true);
 384 
 385     bool complete = full && (max_generation == OldGen);
 386     bool old_collects_young = complete && !ScavengeBeforeFullGC;
 387     bool do_young_collection = !old_collects_young && _young_gen->should_collect(full, size, is_tlab);
 388 
 389     FormatBuffer<> gc_string("%s", "Pause ");
 390     if (do_young_collection) {
 391       gc_string.append("Young");
 392     } else {
 393       gc_string.append("Full");
 394     }
 395 
 396     GCTraceCPUTime tcpu;
 397     GCTraceTime(Info, gc) t(gc_string, NULL, gc_cause(), true);
 398 
 399     gc_prologue(complete);
 400     increment_total_collections(complete);
 401 
 402     size_t young_prev_used = _young_gen->used();
 403     size_t old_prev_used = _old_gen->used();
 404 
 405     bool run_verification = total_collections() >= VerifyGCStartAt;
 406 
 407     bool prepared_for_verification = false;
 408     bool collected_old = false;
 409 
 410     if (do_young_collection) {
 411       if (run_verification && VerifyGCLevel <= 0 && VerifyBeforeGC) {
 412         prepare_for_verify();
 413         prepared_for_verification = true;
 414       }
 415 
 416       collect_generation(_young_gen,
 417                          full,
 418                          size,
 419                          is_tlab,
 420                          run_verification && VerifyGCLevel <= 0,
 421                          do_clear_all_soft_refs,
 422                          false);
 423 
 424       if (size > 0 && (!is_tlab || _young_gen->supports_tlab_allocation()) &&
 425           size * HeapWordSize <= _young_gen->unsafe_max_alloc_nogc()) {
 426         // Allocation request was met by young GC.
 427         size = 0;
 428       }
 429     }
 430 
 431     bool must_restore_marks_for_biased_locking = false;
 432 
 433     if (max_generation == OldGen && _old_gen->should_collect(full, size, is_tlab)) {
 434       if (!complete) {
 435         // The full_collections increment was missed above.
 436         increment_total_full_collections();
 437       }
 438 
 439       if (!prepared_for_verification && run_verification &&
 440           VerifyGCLevel <= 1 && VerifyBeforeGC) {
 441         prepare_for_verify();
 442       }
 443 
 444       if (do_young_collection) {
 445         // We did a young GC. Need a new GC id for the old GC.
 446         GCIdMarkAndRestore gc_id_mark;
 447         GCTraceTime(Info, gc) t("Pause Full", NULL, gc_cause(), true);
 448         collect_generation(_old_gen, full, size, is_tlab, run_verification && VerifyGCLevel <= 1, do_clear_all_soft_refs, true);
 449       } else {
 450         // No young GC done. Use the same GC id as was set up earlier in this method.
 451         collect_generation(_old_gen, full, size, is_tlab, run_verification && VerifyGCLevel <= 1, do_clear_all_soft_refs, true);
 452       }
 453 
 454       must_restore_marks_for_biased_locking = true;
 455       collected_old = true;
 456     }
 457 
 458     // Update "complete" boolean wrt what actually transpired --
 459     // for instance, a promotion failure could have led to
 460     // a whole heap collection.
 461     complete = complete || collected_old;
 462 
 463     print_heap_change(young_prev_used, old_prev_used);
 464     MetaspaceAux::print_metaspace_change(metadata_prev_used);
 465 
 466     // Adjust generation sizes.
 467     if (collected_old) {
 468       _old_gen->compute_new_size();
 469     }
 470     _young_gen->compute_new_size();
 471 
 472     if (complete) {
 473       // Delete metaspaces for unloaded class loaders and clean up loader_data graph
 474       ClassLoaderDataGraph::purge();
 475       MetaspaceAux::verify_metrics();
 476       // Resize the metaspace capacity after full collections
 477       MetaspaceGC::compute_new_size();
 478       update_full_collections_completed();
 479     }
 480 
 481     // Track memory usage and detect low memory after GC finishes
 482     MemoryService::track_memory_usage();
 483 
 484     gc_epilogue(complete);
 485 
 486     if (must_restore_marks_for_biased_locking) {
 487       BiasedLocking::restore_marks();
 488     }
 489   }
 490 
 491   print_heap_after_gc();
 492 
 493 #ifdef TRACESPINNING
 494   ParallelTaskTerminator::print_termination_counts();
 495 #endif
 496 }
 497 
 498 void GenCollectedHeap::register_nmethod(nmethod* nm) {
 499   CodeCache::register_scavenge_root_nmethod(nm);
 500 }
 501 
 502 void GenCollectedHeap::verify_nmethod(nmethod* nm) {
 503   CodeCache::verify_scavenge_root_nmethod(nm);
 504 }
 505 
 506 HeapWord* GenCollectedHeap::satisfy_failed_allocation(size_t size, bool is_tlab) {
 507   return gen_policy()->satisfy_failed_allocation(size, is_tlab);
 508 }
 509 
 510 #ifdef ASSERT
 511 class AssertNonScavengableClosure: public OopClosure {
 512 public:
 513   virtual void do_oop(oop* p) {
 514     assert(!GenCollectedHeap::heap()->is_in_partial_collection(*p),
 515       "Referent should not be scavengable.");  }
 516   virtual void do_oop(narrowOop* p) { ShouldNotReachHere(); }
 517 };
 518 static AssertNonScavengableClosure assert_is_non_scavengable_closure;
 519 #endif
 520 
 521 void GenCollectedHeap::process_roots(StrongRootsScope* scope,
 522                                      ScanningOption so,
 523                                      OopClosure* strong_roots,
 524                                      OopClosure* weak_roots,
 525                                      CLDClosure* strong_cld_closure,
 526                                      CLDClosure* weak_cld_closure,
 527                                      CodeBlobToOopClosure* code_roots) {
 528   // General roots.
 529   assert(Threads::thread_claim_parity() != 0, "must have called prologue code");
 530   assert(code_roots != NULL, "code root closure should always be set");
 531   // _n_termination for _process_strong_tasks should be set up stream
 532   // in a method not running in a GC worker.  Otherwise the GC worker
 533   // could be trying to change the termination condition while the task
 534   // is executing in another GC worker.
 535 
 536   if (!_process_strong_tasks->is_task_claimed(GCH_PS_ClassLoaderDataGraph_oops_do)) {
 537     ClassLoaderDataGraph::roots_cld_do(strong_cld_closure, weak_cld_closure);
 538   }
 539 
 540   // Only process code roots from thread stacks if we aren't visiting the entire CodeCache anyway
 541   CodeBlobToOopClosure* roots_from_code_p = (so & SO_AllCodeCache) ? NULL : code_roots;
 542 
 543   bool is_par = scope->n_threads() > 1;
 544   Threads::possibly_parallel_oops_do(is_par, strong_roots, roots_from_code_p);
 545 
 546   if (!_process_strong_tasks->is_task_claimed(GCH_PS_Universe_oops_do)) {
 547     Universe::oops_do(strong_roots);
 548   }
 549   // Global (strong) JNI handles
 550   if (!_process_strong_tasks->is_task_claimed(GCH_PS_JNIHandles_oops_do)) {
 551     JNIHandles::oops_do(strong_roots);
 552   }
 553 
 554   if (!_process_strong_tasks->is_task_claimed(GCH_PS_ObjectSynchronizer_oops_do)) {
 555     ObjectSynchronizer::oops_do(strong_roots);
 556   }
 557   if (!_process_strong_tasks->is_task_claimed(GCH_PS_Management_oops_do)) {
 558     Management::oops_do(strong_roots);
 559   }
 560   if (!_process_strong_tasks->is_task_claimed(GCH_PS_jvmti_oops_do)) {
 561     JvmtiExport::oops_do(strong_roots);
 562   }
 563   if (UseAOT && !_process_strong_tasks->is_task_claimed(GCH_PS_aot_oops_do)) {
 564     AOTLoader::oops_do(strong_roots);
 565   }
 566 
 567   if (!_process_strong_tasks->is_task_claimed(GCH_PS_SystemDictionary_oops_do)) {
 568     SystemDictionary::roots_oops_do(strong_roots, weak_roots);
 569   }
 570 
 571   if (!_process_strong_tasks->is_task_claimed(GCH_PS_CodeCache_oops_do)) {
 572     if (so & SO_ScavengeCodeCache) {
 573       assert(code_roots != NULL, "must supply closure for code cache");
 574 
 575       // We only visit parts of the CodeCache when scavenging.
 576       CodeCache::scavenge_root_nmethods_do(code_roots);
 577     }
 578     if (so & SO_AllCodeCache) {
 579       assert(code_roots != NULL, "must supply closure for code cache");
 580 
 581       // CMSCollector uses this to do intermediate-strength collections.
 582       // We scan the entire code cache, since CodeCache::do_unloading is not called.
 583       CodeCache::blobs_do(code_roots);
 584     }
 585     // Verify that the code cache contents are not subject to
 586     // movement by a scavenging collection.
 587     DEBUG_ONLY(CodeBlobToOopClosure assert_code_is_non_scavengable(&assert_is_non_scavengable_closure, !CodeBlobToOopClosure::FixRelocations));
 588     DEBUG_ONLY(CodeCache::asserted_non_scavengable_nmethods_do(&assert_code_is_non_scavengable));
 589   }
 590 }
 591 
 592 void GenCollectedHeap::process_string_table_roots(StrongRootsScope* scope,
 593                                                   OopClosure* root_closure) {
 594   assert(root_closure != NULL, "Must be set");
 595   // All threads execute the following. A specific chunk of buckets
 596   // from the StringTable are the individual tasks.
 597   if (scope->n_threads() > 1) {
 598     StringTable::possibly_parallel_oops_do(root_closure);
 599   } else {
 600     StringTable::oops_do(root_closure);
 601   }
 602 }
 603 
 604 void GenCollectedHeap::young_process_roots(StrongRootsScope* scope,
 605                                            OopsInGenClosure* root_closure,
 606                                            OopsInGenClosure* old_gen_closure,
 607                                            CLDClosure* cld_closure) {
 608   MarkingCodeBlobClosure mark_code_closure(root_closure, CodeBlobToOopClosure::FixRelocations);
 609 
 610   process_roots(scope, SO_ScavengeCodeCache, root_closure, root_closure,
 611                 cld_closure, cld_closure, &mark_code_closure);
 612   process_string_table_roots(scope, root_closure);
 613 
 614   if (!_process_strong_tasks->is_task_claimed(GCH_PS_younger_gens)) {
 615     root_closure->reset_generation();
 616   }
 617 
 618   // When collection is parallel, all threads get to cooperate to do
 619   // old generation scanning.
 620   old_gen_closure->set_generation(_old_gen);
 621   rem_set()->younger_refs_iterate(_old_gen, old_gen_closure, scope->n_threads());
 622   old_gen_closure->reset_generation();
 623 
 624   _process_strong_tasks->all_tasks_completed(scope->n_threads());
 625 }
 626 
 627 void GenCollectedHeap::full_process_roots(StrongRootsScope* scope,
 628                                           bool is_adjust_phase,
 629                                           ScanningOption so,
 630                                           bool only_strong_roots,
 631                                           OopsInGenClosure* root_closure,
 632                                           CLDClosure* cld_closure) {
 633   MarkingCodeBlobClosure mark_code_closure(root_closure, is_adjust_phase);
 634   OopsInGenClosure* weak_roots = only_strong_roots ? NULL : root_closure;
 635   CLDClosure* weak_cld_closure = only_strong_roots ? NULL : cld_closure;
 636 
 637   process_roots(scope, so, root_closure, weak_roots, cld_closure, weak_cld_closure, &mark_code_closure);
 638   if (is_adjust_phase) {
 639     // We never treat the string table as roots during marking
 640     // for the full gc, so we only need to process it during
 641     // the adjust phase.
 642     process_string_table_roots(scope, root_closure);
 643   }
 644 
 645   _process_strong_tasks->all_tasks_completed(scope->n_threads());
 646 }
 647 
 648 void GenCollectedHeap::gen_process_weak_roots(OopClosure* root_closure) {
 649   WeakProcessor::oops_do(root_closure);
 650   _young_gen->ref_processor()->weak_oops_do(root_closure);
 651   _old_gen->ref_processor()->weak_oops_do(root_closure);
 652 }
 653 
 654 #define GCH_SINCE_SAVE_MARKS_ITERATE_DEFN(OopClosureType, nv_suffix)    \
 655 void GenCollectedHeap::                                                 \
 656 oop_since_save_marks_iterate(GenerationType gen,                        \
 657                              OopClosureType* cur,                       \
 658                              OopClosureType* older) {                   \
 659   if (gen == YoungGen) {                              \
 660     _young_gen->oop_since_save_marks_iterate##nv_suffix(cur);           \
 661     _old_gen->oop_since_save_marks_iterate##nv_suffix(older);           \
 662   } else {                                                              \
 663     _old_gen->oop_since_save_marks_iterate##nv_suffix(cur);             \
 664   }                                                                     \
 665 }
 666 
 667 ALL_SINCE_SAVE_MARKS_CLOSURES(GCH_SINCE_SAVE_MARKS_ITERATE_DEFN)
 668 
 669 #undef GCH_SINCE_SAVE_MARKS_ITERATE_DEFN
 670 
 671 bool GenCollectedHeap::no_allocs_since_save_marks() {
 672   return _young_gen->no_allocs_since_save_marks() &&
 673          _old_gen->no_allocs_since_save_marks();
 674 }
 675 
 676 bool GenCollectedHeap::supports_inline_contig_alloc() const {
 677   return _young_gen->supports_inline_contig_alloc();
 678 }
 679 
 680 HeapWord* volatile* GenCollectedHeap::top_addr() const {
 681   return _young_gen->top_addr();
 682 }
 683 
 684 HeapWord** GenCollectedHeap::end_addr() const {
 685   return _young_gen->end_addr();
 686 }
 687 
 688 // public collection interfaces
 689 
 690 void GenCollectedHeap::collect(GCCause::Cause cause) {
 691   if (cause == GCCause::_wb_young_gc) {
 692     // Young collection for the WhiteBox API.
 693     collect(cause, YoungGen);
 694   } else {
 695 #ifdef ASSERT
 696   if (cause == GCCause::_scavenge_alot) {
 697     // Young collection only.
 698     collect(cause, YoungGen);
 699   } else {
 700     // Stop-the-world full collection.
 701     collect(cause, OldGen);
 702   }
 703 #else
 704     // Stop-the-world full collection.
 705     collect(cause, OldGen);
 706 #endif
 707   }
 708 }
 709 
 710 void GenCollectedHeap::collect(GCCause::Cause cause, GenerationType max_generation) {
 711   // The caller doesn't have the Heap_lock
 712   assert(!Heap_lock->owned_by_self(), "this thread should not own the Heap_lock");
 713   MutexLocker ml(Heap_lock);
 714   collect_locked(cause, max_generation);
 715 }
 716 
 717 void GenCollectedHeap::collect_locked(GCCause::Cause cause) {
 718   // The caller has the Heap_lock
 719   assert(Heap_lock->owned_by_self(), "this thread should own the Heap_lock");
 720   collect_locked(cause, OldGen);
 721 }
 722 
 723 // this is the private collection interface
 724 // The Heap_lock is expected to be held on entry.
 725 
 726 void GenCollectedHeap::collect_locked(GCCause::Cause cause, GenerationType max_generation) {
 727   // Read the GC count while holding the Heap_lock
 728   unsigned int gc_count_before      = total_collections();
 729   unsigned int full_gc_count_before = total_full_collections();
 730   {
 731     MutexUnlocker mu(Heap_lock);  // give up heap lock, execute gets it back
 732     VM_GenCollectFull op(gc_count_before, full_gc_count_before,
 733                          cause, max_generation);
 734     VMThread::execute(&op);
 735   }
 736 }
 737 
 738 void GenCollectedHeap::do_full_collection(bool clear_all_soft_refs) {
 739    do_full_collection(clear_all_soft_refs, OldGen);
 740 }
 741 
 742 void GenCollectedHeap::do_full_collection(bool clear_all_soft_refs,
 743                                           GenerationType last_generation) {
 744   GenerationType local_last_generation;
 745   if (!incremental_collection_will_fail(false /* don't consult_young */) &&
 746       gc_cause() == GCCause::_gc_locker) {
 747     local_last_generation = YoungGen;
 748   } else {
 749     local_last_generation = last_generation;
 750   }
 751 
 752   do_collection(true,                   // full
 753                 clear_all_soft_refs,    // clear_all_soft_refs
 754                 0,                      // size
 755                 false,                  // is_tlab
 756                 local_last_generation); // last_generation
 757   // Hack XXX FIX ME !!!
 758   // A scavenge may not have been attempted, or may have
 759   // been attempted and failed, because the old gen was too full
 760   if (local_last_generation == YoungGen && gc_cause() == GCCause::_gc_locker &&
 761       incremental_collection_will_fail(false /* don't consult_young */)) {
 762     log_debug(gc, jni)("GC locker: Trying a full collection because scavenge failed");
 763     // This time allow the old gen to be collected as well
 764     do_collection(true,                // full
 765                   clear_all_soft_refs, // clear_all_soft_refs
 766                   0,                   // size
 767                   false,               // is_tlab
 768                   OldGen);             // last_generation
 769   }
 770 }
 771 
 772 bool GenCollectedHeap::is_in_young(oop p) {
 773   bool result = ((HeapWord*)p) < _old_gen->reserved().start();
 774   assert(result == _young_gen->is_in_reserved(p),
 775          "incorrect test - result=%d, p=" INTPTR_FORMAT, result, p2i((void*)p));
 776   return result;
 777 }
 778 
 779 // Returns "TRUE" iff "p" points into the committed areas of the heap.
 780 bool GenCollectedHeap::is_in(const void* p) const {
 781   return _young_gen->is_in(p) || _old_gen->is_in(p);
 782 }
 783 
 784 #ifdef ASSERT
 785 // Don't implement this by using is_in_young().  This method is used
 786 // in some cases to check that is_in_young() is correct.
 787 bool GenCollectedHeap::is_in_partial_collection(const void* p) {
 788   assert(is_in_reserved(p) || p == NULL,
 789     "Does not work if address is non-null and outside of the heap");
 790   return p < _young_gen->reserved().end() && p != NULL;
 791 }
 792 #endif
 793 
 794 void GenCollectedHeap::oop_iterate_no_header(OopClosure* cl) {
 795   NoHeaderExtendedOopClosure no_header_cl(cl);
 796   oop_iterate(&no_header_cl);
 797 }
 798 
 799 void GenCollectedHeap::oop_iterate(ExtendedOopClosure* cl) {
 800   _young_gen->oop_iterate(cl);
 801   _old_gen->oop_iterate(cl);
 802 }
 803 
 804 void GenCollectedHeap::object_iterate(ObjectClosure* cl) {
 805   _young_gen->object_iterate(cl);
 806   _old_gen->object_iterate(cl);
 807 }
 808 
 809 void GenCollectedHeap::safe_object_iterate(ObjectClosure* cl) {
 810   _young_gen->safe_object_iterate(cl);
 811   _old_gen->safe_object_iterate(cl);
 812 }
 813 
 814 Space* GenCollectedHeap::space_containing(const void* addr) const {
 815   Space* res = _young_gen->space_containing(addr);
 816   if (res != NULL) {
 817     return res;
 818   }
 819   res = _old_gen->space_containing(addr);
 820   assert(res != NULL, "Could not find containing space");
 821   return res;
 822 }
 823 
 824 HeapWord* GenCollectedHeap::block_start(const void* addr) const {
 825   assert(is_in_reserved(addr), "block_start of address outside of heap");
 826   if (_young_gen->is_in_reserved(addr)) {
 827     assert(_young_gen->is_in(addr), "addr should be in allocated part of generation");
 828     return _young_gen->block_start(addr);
 829   }
 830 
 831   assert(_old_gen->is_in_reserved(addr), "Some generation should contain the address");
 832   assert(_old_gen->is_in(addr), "addr should be in allocated part of generation");
 833   return _old_gen->block_start(addr);
 834 }
 835 
 836 size_t GenCollectedHeap::block_size(const HeapWord* addr) const {
 837   assert(is_in_reserved(addr), "block_size of address outside of heap");
 838   if (_young_gen->is_in_reserved(addr)) {
 839     assert(_young_gen->is_in(addr), "addr should be in allocated part of generation");
 840     return _young_gen->block_size(addr);
 841   }
 842 
 843   assert(_old_gen->is_in_reserved(addr), "Some generation should contain the address");
 844   assert(_old_gen->is_in(addr), "addr should be in allocated part of generation");
 845   return _old_gen->block_size(addr);
 846 }
 847 
 848 bool GenCollectedHeap::block_is_obj(const HeapWord* addr) const {
 849   assert(is_in_reserved(addr), "block_is_obj of address outside of heap");
 850   assert(block_start(addr) == addr, "addr must be a block start");
 851   if (_young_gen->is_in_reserved(addr)) {
 852     return _young_gen->block_is_obj(addr);
 853   }
 854 
 855   assert(_old_gen->is_in_reserved(addr), "Some generation should contain the address");
 856   return _old_gen->block_is_obj(addr);
 857 }
 858 
 859 bool GenCollectedHeap::supports_tlab_allocation() const {
 860   assert(!_old_gen->supports_tlab_allocation(), "Old gen supports TLAB allocation?!");
 861   return _young_gen->supports_tlab_allocation();
 862 }
 863 
 864 size_t GenCollectedHeap::tlab_capacity(Thread* thr) const {
 865   assert(!_old_gen->supports_tlab_allocation(), "Old gen supports TLAB allocation?!");
 866   if (_young_gen->supports_tlab_allocation()) {
 867     return _young_gen->tlab_capacity();
 868   }
 869   return 0;
 870 }
 871 
 872 size_t GenCollectedHeap::tlab_used(Thread* thr) const {
 873   assert(!_old_gen->supports_tlab_allocation(), "Old gen supports TLAB allocation?!");
 874   if (_young_gen->supports_tlab_allocation()) {
 875     return _young_gen->tlab_used();
 876   }
 877   return 0;
 878 }
 879 
 880 size_t GenCollectedHeap::unsafe_max_tlab_alloc(Thread* thr) const {
 881   assert(!_old_gen->supports_tlab_allocation(), "Old gen supports TLAB allocation?!");
 882   if (_young_gen->supports_tlab_allocation()) {
 883     return _young_gen->unsafe_max_tlab_alloc();
 884   }
 885   return 0;
 886 }
 887 
 888 HeapWord* GenCollectedHeap::allocate_new_tlab(size_t size) {
 889   bool gc_overhead_limit_was_exceeded;
 890   return gen_policy()->mem_allocate_work(size /* size */,
 891                                          true /* is_tlab */,
 892                                          &gc_overhead_limit_was_exceeded);
 893 }
 894 
 895 // Requires "*prev_ptr" to be non-NULL.  Deletes and a block of minimal size
 896 // from the list headed by "*prev_ptr".
 897 static ScratchBlock *removeSmallestScratch(ScratchBlock **prev_ptr) {
 898   bool first = true;
 899   size_t min_size = 0;   // "first" makes this conceptually infinite.
 900   ScratchBlock **smallest_ptr, *smallest;
 901   ScratchBlock  *cur = *prev_ptr;
 902   while (cur) {
 903     assert(*prev_ptr == cur, "just checking");
 904     if (first || cur->num_words < min_size) {
 905       smallest_ptr = prev_ptr;
 906       smallest     = cur;
 907       min_size     = smallest->num_words;
 908       first        = false;
 909     }
 910     prev_ptr = &cur->next;
 911     cur     =  cur->next;
 912   }
 913   smallest      = *smallest_ptr;
 914   *smallest_ptr = smallest->next;
 915   return smallest;
 916 }
 917 
 918 // Sort the scratch block list headed by res into decreasing size order,
 919 // and set "res" to the result.
 920 static void sort_scratch_list(ScratchBlock*& list) {
 921   ScratchBlock* sorted = NULL;
 922   ScratchBlock* unsorted = list;
 923   while (unsorted) {
 924     ScratchBlock *smallest = removeSmallestScratch(&unsorted);
 925     smallest->next  = sorted;
 926     sorted          = smallest;
 927   }
 928   list = sorted;
 929 }
 930 
 931 ScratchBlock* GenCollectedHeap::gather_scratch(Generation* requestor,
 932                                                size_t max_alloc_words) {
 933   ScratchBlock* res = NULL;
 934   _young_gen->contribute_scratch(res, requestor, max_alloc_words);
 935   _old_gen->contribute_scratch(res, requestor, max_alloc_words);
 936   sort_scratch_list(res);
 937   return res;
 938 }
 939 
 940 void GenCollectedHeap::release_scratch() {
 941   _young_gen->reset_scratch();
 942   _old_gen->reset_scratch();
 943 }
 944 
 945 class GenPrepareForVerifyClosure: public GenCollectedHeap::GenClosure {
 946   void do_generation(Generation* gen) {
 947     gen->prepare_for_verify();
 948   }
 949 };
 950 
 951 void GenCollectedHeap::prepare_for_verify() {
 952   ensure_parsability(false);        // no need to retire TLABs
 953   GenPrepareForVerifyClosure blk;
 954   generation_iterate(&blk, false);
 955 }
 956 
 957 void GenCollectedHeap::generation_iterate(GenClosure* cl,
 958                                           bool old_to_young) {
 959   if (old_to_young) {
 960     cl->do_generation(_old_gen);
 961     cl->do_generation(_young_gen);
 962   } else {
 963     cl->do_generation(_young_gen);
 964     cl->do_generation(_old_gen);
 965   }
 966 }
 967 
 968 bool GenCollectedHeap::is_maximal_no_gc() const {
 969   return _young_gen->is_maximal_no_gc() && _old_gen->is_maximal_no_gc();
 970 }
 971 
 972 void GenCollectedHeap::save_marks() {
 973   _young_gen->save_marks();
 974   _old_gen->save_marks();
 975 }
 976 
 977 GenCollectedHeap* GenCollectedHeap::heap() {
 978   CollectedHeap* heap = Universe::heap();
 979   assert(heap != NULL, "Uninitialized access to GenCollectedHeap::heap()");
 980   assert(heap->kind() == CollectedHeap::SerialHeap ||
 981          heap->kind() == CollectedHeap::CMSHeap, "Not a GenCollectedHeap");
 982   return (GenCollectedHeap*) heap;
 983 }
 984 
 985 void GenCollectedHeap::prepare_for_compaction() {
 986   // Start by compacting into same gen.
 987   CompactPoint cp(_old_gen);
 988   _old_gen->prepare_for_compaction(&cp);
 989   _young_gen->prepare_for_compaction(&cp);
 990 }
 991 
 992 void GenCollectedHeap::verify(VerifyOption option /* ignored */) {
 993   log_debug(gc, verify)("%s", _old_gen->name());
 994   _old_gen->verify();
 995 
 996   log_debug(gc, verify)("%s", _old_gen->name());
 997   _young_gen->verify();
 998 
 999   log_debug(gc, verify)("RemSet");
1000   rem_set()->verify();
1001 }
1002 
1003 void GenCollectedHeap::print_on(outputStream* st) const {
1004   _young_gen->print_on(st);
1005   _old_gen->print_on(st);
1006   MetaspaceAux::print_on(st);
1007 }
1008 
1009 void GenCollectedHeap::gc_threads_do(ThreadClosure* tc) const {
1010 }
1011 
1012 void GenCollectedHeap::print_gc_threads_on(outputStream* st) const {
1013 }
1014 
1015 void GenCollectedHeap::print_tracing_info() const {
1016   if (log_is_enabled(Debug, gc, heap, exit)) {
1017     LogStreamHandle(Debug, gc, heap, exit) lsh;
1018     _young_gen->print_summary_info_on(&lsh);
1019     _old_gen->print_summary_info_on(&lsh);
1020   }
1021 }
1022 
1023 void GenCollectedHeap::print_heap_change(size_t young_prev_used, size_t old_prev_used) const {
1024   log_info(gc, heap)("%s: " SIZE_FORMAT "K->" SIZE_FORMAT "K("  SIZE_FORMAT "K)",
1025                      _young_gen->short_name(), young_prev_used / K, _young_gen->used() /K, _young_gen->capacity() /K);
1026   log_info(gc, heap)("%s: " SIZE_FORMAT "K->" SIZE_FORMAT "K("  SIZE_FORMAT "K)",
1027                      _old_gen->short_name(), old_prev_used / K, _old_gen->used() /K, _old_gen->capacity() /K);
1028 }
1029 
1030 class GenGCPrologueClosure: public GenCollectedHeap::GenClosure {
1031  private:
1032   bool _full;
1033  public:
1034   void do_generation(Generation* gen) {
1035     gen->gc_prologue(_full);
1036   }
1037   GenGCPrologueClosure(bool full) : _full(full) {};
1038 };
1039 
1040 void GenCollectedHeap::gc_prologue(bool full) {
1041   assert(InlineCacheBuffer::is_empty(), "should have cleaned up ICBuffer");
1042 
1043   // Fill TLAB's and such
1044   CollectedHeap::accumulate_statistics_all_tlabs();
1045   ensure_parsability(true);   // retire TLABs
1046 
1047   // Walk generations
1048   GenGCPrologueClosure blk(full);
1049   generation_iterate(&blk, false);  // not old-to-young.
1050 };
1051 
1052 class GenGCEpilogueClosure: public GenCollectedHeap::GenClosure {
1053  private:
1054   bool _full;
1055  public:
1056   void do_generation(Generation* gen) {
1057     gen->gc_epilogue(_full);
1058   }
1059   GenGCEpilogueClosure(bool full) : _full(full) {};
1060 };
1061 
1062 void GenCollectedHeap::gc_epilogue(bool full) {
1063 #if COMPILER2_OR_JVMCI
1064   assert(DerivedPointerTable::is_empty(), "derived pointer present");
1065   size_t actual_gap = pointer_delta((HeapWord*) (max_uintx-3), *(end_addr()));
1066   guarantee(is_client_compilation_mode_vm() || actual_gap > (size_t)FastAllocateSizeLimit, "inline allocation wraps");
1067 #endif // COMPILER2_OR_JVMCI
1068 
1069   resize_all_tlabs();
1070 
1071   GenGCEpilogueClosure blk(full);
1072   generation_iterate(&blk, false);  // not old-to-young.
1073 
1074   if (!CleanChunkPoolAsync) {
1075     Chunk::clean_chunk_pool();
1076   }
1077 
1078   MetaspaceCounters::update_performance_counters();
1079   CompressedClassSpaceCounters::update_performance_counters();
1080 };
1081 
1082 #ifndef PRODUCT
1083 class GenGCSaveTopsBeforeGCClosure: public GenCollectedHeap::GenClosure {
1084  private:
1085  public:
1086   void do_generation(Generation* gen) {
1087     gen->record_spaces_top();
1088   }
1089 };
1090 
1091 void GenCollectedHeap::record_gen_tops_before_GC() {
1092   if (ZapUnusedHeapArea) {
1093     GenGCSaveTopsBeforeGCClosure blk;
1094     generation_iterate(&blk, false);  // not old-to-young.
1095   }
1096 }
1097 #endif  // not PRODUCT
1098 
1099 class GenEnsureParsabilityClosure: public GenCollectedHeap::GenClosure {
1100  public:
1101   void do_generation(Generation* gen) {
1102     gen->ensure_parsability();
1103   }
1104 };
1105 
1106 void GenCollectedHeap::ensure_parsability(bool retire_tlabs) {
1107   CollectedHeap::ensure_parsability(retire_tlabs);
1108   GenEnsureParsabilityClosure ep_cl;
1109   generation_iterate(&ep_cl, false);
1110 }
1111 
1112 oop GenCollectedHeap::handle_failed_promotion(Generation* old_gen,
1113                                               oop obj,
1114                                               size_t obj_size) {
1115   guarantee(old_gen == _old_gen, "We only get here with an old generation");
1116   assert(obj_size == (size_t)obj->size(), "bad obj_size passed in");
1117   HeapWord* result = NULL;
1118 
1119   result = old_gen->expand_and_allocate(obj_size, false);
1120 
1121   if (result != NULL) {
1122     Copy::aligned_disjoint_words((HeapWord*)obj, result, obj_size);
1123   }
1124   return oop(result);
1125 }
1126 
1127 class GenTimeOfLastGCClosure: public GenCollectedHeap::GenClosure {
1128   jlong _time;   // in ms
1129   jlong _now;    // in ms
1130 
1131  public:
1132   GenTimeOfLastGCClosure(jlong now) : _time(now), _now(now) { }
1133 
1134   jlong time() { return _time; }
1135 
1136   void do_generation(Generation* gen) {
1137     _time = MIN2(_time, gen->time_of_last_gc(_now));
1138   }
1139 };
1140 
1141 jlong GenCollectedHeap::millis_since_last_gc() {
1142   // javaTimeNanos() is guaranteed to be monotonically non-decreasing
1143   // provided the underlying platform provides such a time source
1144   // (and it is bug free). So we still have to guard against getting
1145   // back a time later than 'now'.
1146   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
1147   GenTimeOfLastGCClosure tolgc_cl(now);
1148   // iterate over generations getting the oldest
1149   // time that a generation was collected
1150   generation_iterate(&tolgc_cl, false);
1151 
1152   jlong retVal = now - tolgc_cl.time();
1153   if (retVal < 0) {
1154     log_warning(gc)("millis_since_last_gc() would return : " JLONG_FORMAT
1155        ". returning zero instead.", retVal);
1156     return 0;
1157   }
1158   return retVal;
1159 }