1 /*
   2  * Copyright (c) 2001, 2011, 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 "classfile/symbolTable.hpp"
  27 #include "classfile/systemDictionary.hpp"
  28 #include "code/codeCache.hpp"
  29 #include "gc_implementation/parallelScavenge/generationSizer.hpp"
  30 #include "gc_implementation/parallelScavenge/parallelScavengeHeap.hpp"
  31 #include "gc_implementation/parallelScavenge/psAdaptiveSizePolicy.hpp"
  32 #include "gc_implementation/parallelScavenge/psMarkSweep.hpp"
  33 #include "gc_implementation/parallelScavenge/psMarkSweepDecorator.hpp"
  34 #include "gc_implementation/parallelScavenge/psOldGen.hpp"
  35 #include "gc_implementation/parallelScavenge/psPermGen.hpp"
  36 #include "gc_implementation/parallelScavenge/psScavenge.hpp"
  37 #include "gc_implementation/parallelScavenge/psYoungGen.hpp"
  38 #include "gc_implementation/shared/isGCActiveMark.hpp"
  39 #include "gc_implementation/shared/spaceDecorator.hpp"
  40 #include "gc_interface/gcCause.hpp"
  41 #include "memory/gcLocker.inline.hpp"
  42 #include "memory/referencePolicy.hpp"
  43 #include "memory/referenceProcessor.hpp"
  44 #include "oops/oop.inline.hpp"
  45 #include "runtime/biasedLocking.hpp"
  46 #include "runtime/fprofiler.hpp"
  47 #include "runtime/safepoint.hpp"
  48 #include "runtime/vmThread.hpp"
  49 #include "services/management.hpp"
  50 #include "services/memoryService.hpp"
  51 #include "utilities/events.hpp"
  52 #include "utilities/stack.inline.hpp"
  53 
  54 elapsedTimer        PSMarkSweep::_accumulated_time;
  55 unsigned int        PSMarkSweep::_total_invocations = 0;
  56 jlong               PSMarkSweep::_time_of_last_gc   = 0;
  57 CollectorCounters*  PSMarkSweep::_counters = NULL;
  58 
  59 void PSMarkSweep::initialize() {
  60   MemRegion mr = Universe::heap()->reserved_region();
  61   _ref_processor = new ReferenceProcessor(mr);     // a vanilla ref proc
  62   _counters = new CollectorCounters("PSMarkSweep", 1);
  63 }
  64 
  65 // This method contains all heap specific policy for invoking mark sweep.
  66 // PSMarkSweep::invoke_no_policy() will only attempt to mark-sweep-compact
  67 // the heap. It will do nothing further. If we need to bail out for policy
  68 // reasons, scavenge before full gc, or any other specialized behavior, it
  69 // needs to be added here.
  70 //
  71 // Note that this method should only be called from the vm_thread while
  72 // at a safepoint!
  73 //
  74 // Note that the all_soft_refs_clear flag in the collector policy
  75 // may be true because this method can be called without intervening
  76 // activity.  For example when the heap space is tight and full measure
  77 // are being taken to free space.
  78 
  79 void PSMarkSweep::invoke(bool maximum_heap_compaction) {
  80   assert(SafepointSynchronize::is_at_safepoint(), "should be at safepoint");
  81   assert(Thread::current() == (Thread*)VMThread::vm_thread(), "should be in vm thread");
  82   assert(!Universe::heap()->is_gc_active(), "not reentrant");
  83 
  84   ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();
  85   GCCause::Cause gc_cause = heap->gc_cause();
  86   PSAdaptiveSizePolicy* policy = heap->size_policy();
  87   IsGCActiveMark mark;
  88 
  89   if (ScavengeBeforeFullGC) {
  90     PSScavenge::invoke_no_policy();
  91   }
  92 
  93   const bool clear_all_soft_refs =
  94     heap->collector_policy()->should_clear_all_soft_refs();
  95 
  96   int count = (maximum_heap_compaction)?1:MarkSweepAlwaysCompactCount;
  97   IntFlagSetting flag_setting(MarkSweepAlwaysCompactCount, count);
  98   PSMarkSweep::invoke_no_policy(clear_all_soft_refs || maximum_heap_compaction);
  99 }
 100 
 101 // This method contains no policy. You should probably
 102 // be calling invoke() instead.
 103 void PSMarkSweep::invoke_no_policy(bool clear_all_softrefs) {
 104   assert(SafepointSynchronize::is_at_safepoint(), "must be at a safepoint");
 105   assert(ref_processor() != NULL, "Sanity");
 106 
 107   if (GC_locker::check_active_before_gc()) {
 108     return;
 109   }
 110 
 111   ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();
 112   GCCause::Cause gc_cause = heap->gc_cause();
 113   assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity");
 114   PSAdaptiveSizePolicy* size_policy = heap->size_policy();
 115 
 116   // The scope of casr should end after code that can change
 117   // CollectorPolicy::_should_clear_all_soft_refs.
 118   ClearedAllSoftRefs casr(clear_all_softrefs, heap->collector_policy());
 119 
 120   PSYoungGen* young_gen = heap->young_gen();
 121   PSOldGen* old_gen = heap->old_gen();
 122   PSPermGen* perm_gen = heap->perm_gen();
 123 
 124   // Increment the invocation count
 125   heap->increment_total_collections(true /* full */);
 126 
 127   // Save information needed to minimize mangling
 128   heap->record_gen_tops_before_GC();
 129 
 130   // We need to track unique mark sweep invocations as well.
 131   _total_invocations++;
 132 
 133   AdaptiveSizePolicyOutput(size_policy, heap->total_collections());
 134 
 135   if (PrintHeapAtGC) {
 136     Universe::print_heap_before_gc();
 137   }
 138 
 139   // Fill in TLABs
 140   heap->accumulate_statistics_all_tlabs();
 141   heap->ensure_parsability(true);  // retire TLABs
 142 
 143   if (VerifyBeforeGC && heap->total_collections() >= VerifyGCStartAt) {
 144     HandleMark hm;  // Discard invalid handles created during verification
 145     gclog_or_tty->print(" VerifyBeforeGC:");
 146     Universe::verify(true);
 147   }
 148 
 149   // Verify object start arrays
 150   if (VerifyObjectStartArray &&
 151       VerifyBeforeGC) {
 152     old_gen->verify_object_start_array();
 153     perm_gen->verify_object_start_array();
 154   }
 155 
 156   heap->pre_full_gc_dump();
 157 
 158   // Filled in below to track the state of the young gen after the collection.
 159   bool eden_empty;
 160   bool survivors_empty;
 161   bool young_gen_empty;
 162 
 163   {
 164     HandleMark hm;
 165     const bool is_system_gc = gc_cause == GCCause::_java_lang_system_gc;
 166     // This is useful for debugging but don't change the output the
 167     // the customer sees.
 168     const char* gc_cause_str = "Full GC";
 169     if (is_system_gc && PrintGCDetails) {
 170       gc_cause_str = "Full GC (System)";
 171     }
 172     gclog_or_tty->date_stamp(PrintGC && PrintGCDateStamps);
 173     TraceCPUTime tcpu(PrintGCDetails, true, gclog_or_tty);
 174     TraceTime t1(gc_cause_str, PrintGC, !PrintGCDetails, gclog_or_tty);
 175     TraceCollectorStats tcs(counters());
 176     TraceMemoryManagerStats tms(true /* Full GC */,gc_cause);
 177 
 178     if (TraceGen1Time) accumulated_time()->start();
 179 
 180     // Let the size policy know we're starting
 181     size_policy->major_collection_begin();
 182 
 183     // When collecting the permanent generation methodOops may be moving,
 184     // so we either have to flush all bcp data or convert it into bci.
 185     CodeCache::gc_prologue();
 186     Threads::gc_prologue();
 187     BiasedLocking::preserve_marks();
 188 
 189     // Capture heap size before collection for printing.
 190     size_t prev_used = heap->used();
 191 
 192     // Capture perm gen size before collection for sizing.
 193     size_t perm_gen_prev_used = perm_gen->used_in_bytes();
 194 
 195     // For PrintGCDetails
 196     size_t old_gen_prev_used = old_gen->used_in_bytes();
 197     size_t young_gen_prev_used = young_gen->used_in_bytes();
 198 
 199     allocate_stacks();
 200 
 201     COMPILER2_PRESENT(DerivedPointerTable::clear());
 202 
 203     ref_processor()->enable_discovery(true /*verify_disabled*/, true /*verify_no_refs*/);
 204     ref_processor()->setup_policy(clear_all_softrefs);
 205 
 206     mark_sweep_phase1(clear_all_softrefs);
 207 
 208     mark_sweep_phase2();
 209 
 210     // Don't add any more derived pointers during phase3
 211     COMPILER2_PRESENT(assert(DerivedPointerTable::is_active(), "Sanity"));
 212     COMPILER2_PRESENT(DerivedPointerTable::set_active(false));
 213 
 214     mark_sweep_phase3();
 215 
 216     mark_sweep_phase4();
 217 
 218     restore_marks();
 219 
 220     deallocate_stacks();
 221 
 222     if (ZapUnusedHeapArea) {
 223       // Do a complete mangle (top to end) because the usage for
 224       // scratch does not maintain a top pointer.
 225       young_gen->to_space()->mangle_unused_area_complete();
 226     }
 227 
 228     eden_empty = young_gen->eden_space()->is_empty();
 229     if (!eden_empty) {
 230       eden_empty = absorb_live_data_from_eden(size_policy, young_gen, old_gen);
 231     }
 232 
 233     // Update heap occupancy information which is used as
 234     // input to soft ref clearing policy at the next gc.
 235     Universe::update_heap_info_at_gc();
 236 
 237     survivors_empty = young_gen->from_space()->is_empty() &&
 238                       young_gen->to_space()->is_empty();
 239     young_gen_empty = eden_empty && survivors_empty;
 240 
 241     BarrierSet* bs = heap->barrier_set();
 242     if (bs->is_a(BarrierSet::ModRef)) {
 243       ModRefBarrierSet* modBS = (ModRefBarrierSet*)bs;
 244       MemRegion old_mr = heap->old_gen()->reserved();
 245       MemRegion perm_mr = heap->perm_gen()->reserved();
 246       assert(perm_mr.end() <= old_mr.start(), "Generations out of order");
 247 
 248       if (young_gen_empty) {
 249         modBS->clear(MemRegion(perm_mr.start(), old_mr.end()));
 250       } else {
 251         modBS->invalidate(MemRegion(perm_mr.start(), old_mr.end()));
 252       }
 253     }
 254 
 255     BiasedLocking::restore_marks();
 256     Threads::gc_epilogue();
 257     CodeCache::gc_epilogue();
 258     JvmtiExport::gc_epilogue();
 259 
 260     COMPILER2_PRESENT(DerivedPointerTable::update_pointers());
 261 
 262     ref_processor()->enqueue_discovered_references(NULL);
 263 
 264     // Update time of last GC
 265     reset_millis_since_last_gc();
 266 
 267     // Let the size policy know we're done
 268     size_policy->major_collection_end(old_gen->used_in_bytes(), gc_cause);
 269 
 270     if (UseAdaptiveSizePolicy) {
 271 
 272       if (PrintAdaptiveSizePolicy) {
 273         gclog_or_tty->print("AdaptiveSizeStart: ");
 274         gclog_or_tty->stamp();
 275         gclog_or_tty->print_cr(" collection: %d ",
 276                        heap->total_collections());
 277         if (Verbose) {
 278           gclog_or_tty->print("old_gen_capacity: %d young_gen_capacity: %d"
 279             " perm_gen_capacity: %d ",
 280             old_gen->capacity_in_bytes(), young_gen->capacity_in_bytes(),
 281             perm_gen->capacity_in_bytes());
 282         }
 283       }
 284 
 285       // Don't check if the size_policy is ready here.  Let
 286       // the size_policy check that internally.
 287       if (UseAdaptiveGenerationSizePolicyAtMajorCollection &&
 288           ((gc_cause != GCCause::_java_lang_system_gc) ||
 289             UseAdaptiveSizePolicyWithSystemGC)) {
 290         // Calculate optimal free space amounts
 291         assert(young_gen->max_size() >
 292           young_gen->from_space()->capacity_in_bytes() +
 293           young_gen->to_space()->capacity_in_bytes(),
 294           "Sizes of space in young gen are out-of-bounds");
 295         size_t max_eden_size = young_gen->max_size() -
 296           young_gen->from_space()->capacity_in_bytes() -
 297           young_gen->to_space()->capacity_in_bytes();
 298         size_policy->compute_generation_free_space(young_gen->used_in_bytes(),
 299                                  young_gen->eden_space()->used_in_bytes(),
 300                                  old_gen->used_in_bytes(),
 301                                  perm_gen->used_in_bytes(),
 302                                  young_gen->eden_space()->capacity_in_bytes(),
 303                                  old_gen->max_gen_size(),
 304                                  max_eden_size,
 305                                  true /* full gc*/,
 306                                  gc_cause,
 307                                  heap->collector_policy());
 308 
 309         heap->resize_old_gen(size_policy->calculated_old_free_size_in_bytes());
 310 
 311         // Don't resize the young generation at an major collection.  A
 312         // desired young generation size may have been calculated but
 313         // resizing the young generation complicates the code because the
 314         // resizing of the old generation may have moved the boundary
 315         // between the young generation and the old generation.  Let the
 316         // young generation resizing happen at the minor collections.
 317       }
 318       if (PrintAdaptiveSizePolicy) {
 319         gclog_or_tty->print_cr("AdaptiveSizeStop: collection: %d ",
 320                        heap->total_collections());
 321       }
 322     }
 323 
 324     if (UsePerfData) {
 325       heap->gc_policy_counters()->update_counters();
 326       heap->gc_policy_counters()->update_old_capacity(
 327         old_gen->capacity_in_bytes());
 328       heap->gc_policy_counters()->update_young_capacity(
 329         young_gen->capacity_in_bytes());
 330     }
 331 
 332     heap->resize_all_tlabs();
 333 
 334     // We collected the perm gen, so we'll resize it here.
 335     perm_gen->compute_new_size(perm_gen_prev_used);
 336 
 337     if (TraceGen1Time) accumulated_time()->stop();
 338 
 339     if (PrintGC) {
 340       if (PrintGCDetails) {
 341         // Don't print a GC timestamp here.  This is after the GC so
 342         // would be confusing.
 343         young_gen->print_used_change(young_gen_prev_used);
 344         old_gen->print_used_change(old_gen_prev_used);
 345       }
 346       heap->print_heap_change(prev_used);
 347       // Do perm gen after heap becase prev_used does
 348       // not include the perm gen (done this way in the other
 349       // collectors).
 350       if (PrintGCDetails) {
 351         perm_gen->print_used_change(perm_gen_prev_used);
 352       }
 353     }
 354 
 355     // Track memory usage and detect low memory
 356     MemoryService::track_memory_usage();
 357     heap->update_counters();
 358   }
 359 
 360   if (VerifyAfterGC && heap->total_collections() >= VerifyGCStartAt) {
 361     HandleMark hm;  // Discard invalid handles created during verification
 362     gclog_or_tty->print(" VerifyAfterGC:");
 363     Universe::verify(false);
 364   }
 365 
 366   // Re-verify object start arrays
 367   if (VerifyObjectStartArray &&
 368       VerifyAfterGC) {
 369     old_gen->verify_object_start_array();
 370     perm_gen->verify_object_start_array();
 371   }
 372 
 373   if (ZapUnusedHeapArea) {
 374     old_gen->object_space()->check_mangled_unused_area_complete();
 375     perm_gen->object_space()->check_mangled_unused_area_complete();
 376   }
 377 
 378   NOT_PRODUCT(ref_processor()->verify_no_references_recorded());
 379 
 380   if (PrintHeapAtGC) {
 381     Universe::print_heap_after_gc();
 382   }
 383 
 384   heap->post_full_gc_dump();
 385 
 386 #ifdef TRACESPINNING
 387   ParallelTaskTerminator::print_termination_counts();
 388 #endif
 389 }
 390 
 391 bool PSMarkSweep::absorb_live_data_from_eden(PSAdaptiveSizePolicy* size_policy,
 392                                              PSYoungGen* young_gen,
 393                                              PSOldGen* old_gen) {
 394   MutableSpace* const eden_space = young_gen->eden_space();
 395   assert(!eden_space->is_empty(), "eden must be non-empty");
 396   assert(young_gen->virtual_space()->alignment() ==
 397          old_gen->virtual_space()->alignment(), "alignments do not match");
 398 
 399   if (!(UseAdaptiveSizePolicy && UseAdaptiveGCBoundary)) {
 400     return false;
 401   }
 402 
 403   // Both generations must be completely committed.
 404   if (young_gen->virtual_space()->uncommitted_size() != 0) {
 405     return false;
 406   }
 407   if (old_gen->virtual_space()->uncommitted_size() != 0) {
 408     return false;
 409   }
 410 
 411   // Figure out how much to take from eden.  Include the average amount promoted
 412   // in the total; otherwise the next young gen GC will simply bail out to a
 413   // full GC.
 414   const size_t alignment = old_gen->virtual_space()->alignment();
 415   const size_t eden_used = eden_space->used_in_bytes();
 416   const size_t promoted = (size_t)size_policy->avg_promoted()->padded_average();
 417   const size_t absorb_size = align_size_up(eden_used + promoted, alignment);
 418   const size_t eden_capacity = eden_space->capacity_in_bytes();
 419 
 420   if (absorb_size >= eden_capacity) {
 421     return false; // Must leave some space in eden.
 422   }
 423 
 424   const size_t new_young_size = young_gen->capacity_in_bytes() - absorb_size;
 425   if (new_young_size < young_gen->min_gen_size()) {
 426     return false; // Respect young gen minimum size.
 427   }
 428 
 429   if (TraceAdaptiveGCBoundary && Verbose) {
 430     gclog_or_tty->print(" absorbing " SIZE_FORMAT "K:  "
 431                         "eden " SIZE_FORMAT "K->" SIZE_FORMAT "K "
 432                         "from " SIZE_FORMAT "K, to " SIZE_FORMAT "K "
 433                         "young_gen " SIZE_FORMAT "K->" SIZE_FORMAT "K ",
 434                         absorb_size / K,
 435                         eden_capacity / K, (eden_capacity - absorb_size) / K,
 436                         young_gen->from_space()->used_in_bytes() / K,
 437                         young_gen->to_space()->used_in_bytes() / K,
 438                         young_gen->capacity_in_bytes() / K, new_young_size / K);
 439   }
 440 
 441   // Fill the unused part of the old gen.
 442   MutableSpace* const old_space = old_gen->object_space();
 443   HeapWord* const unused_start = old_space->top();
 444   size_t const unused_words = pointer_delta(old_space->end(), unused_start);
 445 
 446   if (unused_words > 0) {
 447     if (unused_words < CollectedHeap::min_fill_size()) {
 448       return false;  // If the old gen cannot be filled, must give up.
 449     }
 450     CollectedHeap::fill_with_objects(unused_start, unused_words);
 451   }
 452 
 453   // Take the live data from eden and set both top and end in the old gen to
 454   // eden top.  (Need to set end because reset_after_change() mangles the region
 455   // from end to virtual_space->high() in debug builds).
 456   HeapWord* const new_top = eden_space->top();
 457   old_gen->virtual_space()->expand_into(young_gen->virtual_space(),
 458                                         absorb_size);
 459   young_gen->reset_after_change();
 460   old_space->set_top(new_top);
 461   old_space->set_end(new_top);
 462   old_gen->reset_after_change();
 463 
 464   // Update the object start array for the filler object and the data from eden.
 465   ObjectStartArray* const start_array = old_gen->start_array();
 466   for (HeapWord* p = unused_start; p < new_top; p += oop(p)->size()) {
 467     start_array->allocate_block(p);
 468   }
 469 
 470   // Could update the promoted average here, but it is not typically updated at
 471   // full GCs and the value to use is unclear.  Something like
 472   //
 473   // cur_promoted_avg + absorb_size / number_of_scavenges_since_last_full_gc.
 474 
 475   size_policy->set_bytes_absorbed_from_eden(absorb_size);
 476   return true;
 477 }
 478 
 479 void PSMarkSweep::allocate_stacks() {
 480   ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();
 481   assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity");
 482 
 483   PSYoungGen* young_gen = heap->young_gen();
 484 
 485   MutableSpace* to_space = young_gen->to_space();
 486   _preserved_marks = (PreservedMark*)to_space->top();
 487   _preserved_count = 0;
 488 
 489   // We want to calculate the size in bytes first.
 490   _preserved_count_max  = pointer_delta(to_space->end(), to_space->top(), sizeof(jbyte));
 491   // Now divide by the size of a PreservedMark
 492   _preserved_count_max /= sizeof(PreservedMark);
 493 }
 494 
 495 
 496 void PSMarkSweep::deallocate_stacks() {
 497   _preserved_mark_stack.clear(true);
 498   _preserved_oop_stack.clear(true);
 499   _marking_stack.clear();
 500   _objarray_stack.clear(true);
 501   _revisit_klass_stack.clear(true);
 502   _revisit_mdo_stack.clear(true);
 503 }
 504 
 505 void PSMarkSweep::mark_sweep_phase1(bool clear_all_softrefs) {
 506   // Recursively traverse all live objects and mark them
 507   EventMark m("1 mark object");
 508   TraceTime tm("phase 1", PrintGCDetails && Verbose, true, gclog_or_tty);
 509   trace(" 1");
 510 
 511   ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();
 512   assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity");
 513 
 514   // General strong roots.
 515   {
 516     ParallelScavengeHeap::ParStrongRootsScope psrs;
 517     Universe::oops_do(mark_and_push_closure());
 518     JNIHandles::oops_do(mark_and_push_closure());   // Global (strong) JNI handles
 519     CodeBlobToOopClosure each_active_code_blob(mark_and_push_closure(), /*do_marking=*/ true);
 520     Threads::oops_do(mark_and_push_closure(), &each_active_code_blob);
 521     ObjectSynchronizer::oops_do(mark_and_push_closure());
 522     FlatProfiler::oops_do(mark_and_push_closure());
 523     Management::oops_do(mark_and_push_closure());
 524     JvmtiExport::oops_do(mark_and_push_closure());
 525     SystemDictionary::always_strong_oops_do(mark_and_push_closure());
 526     // Do not treat nmethods as strong roots for mark/sweep, since we can unload them.
 527     //CodeCache::scavenge_root_nmethods_do(CodeBlobToOopClosure(mark_and_push_closure()));
 528   }
 529 
 530   // Flush marking stack.
 531   follow_stack();
 532 
 533   // Process reference objects found during marking
 534   {
 535     ref_processor()->setup_policy(clear_all_softrefs);
 536     ref_processor()->process_discovered_references(
 537       is_alive_closure(), mark_and_push_closure(), follow_stack_closure(), NULL);
 538   }
 539 
 540   // Follow system dictionary roots and unload classes
 541   bool purged_class = SystemDictionary::do_unloading(is_alive_closure());
 542 
 543   // Follow code cache roots
 544   CodeCache::do_unloading(is_alive_closure(), mark_and_push_closure(),
 545                           purged_class);
 546   follow_stack(); // Flush marking stack
 547 
 548   // Update subklass/sibling/implementor links of live klasses
 549   follow_weak_klass_links();
 550   assert(_marking_stack.is_empty(), "just drained");
 551 
 552   // Visit memoized mdo's and clear unmarked weak refs
 553   follow_mdo_weak_refs();
 554   assert(_marking_stack.is_empty(), "just drained");
 555 
 556   // Visit interned string tables and delete unmarked oops
 557   StringTable::unlink(is_alive_closure());
 558   // Clean up unreferenced symbols in symbol table.
 559   SymbolTable::unlink();
 560 
 561   assert(_marking_stack.is_empty(), "stack should be empty by now");
 562 }
 563 
 564 
 565 void PSMarkSweep::mark_sweep_phase2() {
 566   EventMark m("2 compute new addresses");
 567   TraceTime tm("phase 2", PrintGCDetails && Verbose, true, gclog_or_tty);
 568   trace("2");
 569 
 570   // Now all live objects are marked, compute the new object addresses.
 571 
 572   // It is imperative that we traverse perm_gen LAST. If dead space is
 573   // allowed a range of dead object may get overwritten by a dead int
 574   // array. If perm_gen is not traversed last a klassOop may get
 575   // overwritten. This is fine since it is dead, but if the class has dead
 576   // instances we have to skip them, and in order to find their size we
 577   // need the klassOop!
 578   //
 579   // It is not required that we traverse spaces in the same order in
 580   // phase2, phase3 and phase4, but the ValidateMarkSweep live oops
 581   // tracking expects us to do so. See comment under phase4.
 582 
 583   ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();
 584   assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity");
 585 
 586   PSOldGen* old_gen = heap->old_gen();
 587   PSPermGen* perm_gen = heap->perm_gen();
 588 
 589   // Begin compacting into the old gen
 590   PSMarkSweepDecorator::set_destination_decorator_tenured();
 591 
 592   // This will also compact the young gen spaces.
 593   old_gen->precompact();
 594 
 595   // Compact the perm gen into the perm gen
 596   PSMarkSweepDecorator::set_destination_decorator_perm_gen();
 597 
 598   perm_gen->precompact();
 599 }
 600 
 601 // This should be moved to the shared markSweep code!
 602 class PSAlwaysTrueClosure: public BoolObjectClosure {
 603 public:
 604   void do_object(oop p) { ShouldNotReachHere(); }
 605   bool do_object_b(oop p) { return true; }
 606 };
 607 static PSAlwaysTrueClosure always_true;
 608 
 609 void PSMarkSweep::mark_sweep_phase3() {
 610   // Adjust the pointers to reflect the new locations
 611   EventMark m("3 adjust pointers");
 612   TraceTime tm("phase 3", PrintGCDetails && Verbose, true, gclog_or_tty);
 613   trace("3");
 614 
 615   ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();
 616   assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity");
 617 
 618   PSYoungGen* young_gen = heap->young_gen();
 619   PSOldGen* old_gen = heap->old_gen();
 620   PSPermGen* perm_gen = heap->perm_gen();
 621 
 622   // General strong roots.
 623   Universe::oops_do(adjust_root_pointer_closure());
 624   JNIHandles::oops_do(adjust_root_pointer_closure());   // Global (strong) JNI handles
 625   Threads::oops_do(adjust_root_pointer_closure(), NULL);
 626   ObjectSynchronizer::oops_do(adjust_root_pointer_closure());
 627   FlatProfiler::oops_do(adjust_root_pointer_closure());
 628   Management::oops_do(adjust_root_pointer_closure());
 629   JvmtiExport::oops_do(adjust_root_pointer_closure());
 630   // SO_AllClasses
 631   SystemDictionary::oops_do(adjust_root_pointer_closure());
 632   //CodeCache::scavenge_root_nmethods_oops_do(adjust_root_pointer_closure());
 633 
 634   // Now adjust pointers in remaining weak roots.  (All of which should
 635   // have been cleared if they pointed to non-surviving objects.)
 636   // Global (weak) JNI handles
 637   JNIHandles::weak_oops_do(&always_true, adjust_root_pointer_closure());
 638 
 639   CodeCache::oops_do(adjust_pointer_closure());
 640   StringTable::oops_do(adjust_root_pointer_closure());
 641   ref_processor()->weak_oops_do(adjust_root_pointer_closure());
 642   PSScavenge::reference_processor()->weak_oops_do(adjust_root_pointer_closure());
 643 
 644   adjust_marks();
 645 
 646   young_gen->adjust_pointers();
 647   old_gen->adjust_pointers();
 648   perm_gen->adjust_pointers();
 649 }
 650 
 651 void PSMarkSweep::mark_sweep_phase4() {
 652   EventMark m("4 compact heap");
 653   TraceTime tm("phase 4", PrintGCDetails && Verbose, true, gclog_or_tty);
 654   trace("4");
 655 
 656   // All pointers are now adjusted, move objects accordingly
 657 
 658   // It is imperative that we traverse perm_gen first in phase4. All
 659   // classes must be allocated earlier than their instances, and traversing
 660   // perm_gen first makes sure that all klassOops have moved to their new
 661   // location before any instance does a dispatch through it's klass!
 662   ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();
 663   assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity");
 664 
 665   PSYoungGen* young_gen = heap->young_gen();
 666   PSOldGen* old_gen = heap->old_gen();
 667   PSPermGen* perm_gen = heap->perm_gen();
 668 
 669   perm_gen->compact();
 670   old_gen->compact();
 671   young_gen->compact();
 672 }
 673 
 674 jlong PSMarkSweep::millis_since_last_gc() {
 675   // os::javaTimeMillis() does not guarantee monotonicity.
 676   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
 677   jlong ret_val = now - _time_of_last_gc;
 678   // XXX See note in genCollectedHeap::millis_since_last_gc().
 679   if (ret_val < 0) {
 680     NOT_PRODUCT(warning("time warp: "INT64_FORMAT, ret_val);)
 681     return 0;
 682   }
 683   return ret_val;
 684 }
 685 
 686 void PSMarkSweep::reset_millis_since_last_gc() {
 687   // os::javaTimeMillis() does not guarantee monotonicity.
 688   _time_of_last_gc = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
 689 }