1 /*
   2  * Copyright (c) 2014, 2021, Red Hat, Inc. 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 
  27 #include "compiler/oopMap.hpp"
  28 #include "gc/shared/continuationGCSupport.hpp"
  29 #include "gc/shared/gcTraceTime.inline.hpp"
  30 #include "gc/shared/preservedMarks.inline.hpp"
  31 #include "gc/shared/slidingForwarding.inline.hpp"
  32 #include "gc/shared/tlab_globals.hpp"
  33 #include "gc/shared/workerThread.hpp"
  34 #include "gc/shenandoah/heuristics/shenandoahHeuristics.hpp"
  35 #include "gc/shenandoah/shenandoahCollectorPolicy.hpp"
  36 #include "gc/shenandoah/shenandoahConcurrentGC.hpp"
  37 #include "gc/shenandoah/shenandoahCollectionSet.hpp"
  38 #include "gc/shenandoah/shenandoahFreeSet.hpp"
  39 #include "gc/shenandoah/shenandoahFullGC.hpp"
  40 #include "gc/shenandoah/shenandoahPhaseTimings.hpp"
  41 #include "gc/shenandoah/shenandoahMark.inline.hpp"
  42 #include "gc/shenandoah/shenandoahMonitoringSupport.hpp"
  43 #include "gc/shenandoah/shenandoahHeapRegionSet.hpp"
  44 #include "gc/shenandoah/shenandoahHeap.inline.hpp"
  45 #include "gc/shenandoah/shenandoahHeapRegion.inline.hpp"
  46 #include "gc/shenandoah/shenandoahMarkingContext.inline.hpp"
  47 #include "gc/shenandoah/shenandoahMetrics.hpp"
  48 #include "gc/shenandoah/shenandoahOopClosures.inline.hpp"
  49 #include "gc/shenandoah/shenandoahReferenceProcessor.hpp"
  50 #include "gc/shenandoah/shenandoahRootProcessor.inline.hpp"
  51 #include "gc/shenandoah/shenandoahSTWMark.hpp"
  52 #include "gc/shenandoah/shenandoahUtils.hpp"
  53 #include "gc/shenandoah/shenandoahVerifier.hpp"
  54 #include "gc/shenandoah/shenandoahVMOperations.hpp"
  55 #include "gc/shenandoah/shenandoahWorkerPolicy.hpp"
  56 #include "memory/metaspaceUtils.hpp"
  57 #include "memory/universe.hpp"
  58 #include "oops/compressedOops.inline.hpp"
  59 #include "oops/oop.inline.hpp"
  60 #include "runtime/javaThread.hpp"
  61 #include "runtime/orderAccess.hpp"
  62 #include "runtime/vmThread.hpp"
  63 #include "utilities/copy.hpp"
  64 #include "utilities/events.hpp"
  65 #include "utilities/growableArray.hpp"
  66 
  67 ShenandoahFullGC::ShenandoahFullGC() :
  68   _gc_timer(ShenandoahHeap::heap()->gc_timer()),
  69   _preserved_marks(new PreservedMarksSet(true)) {}
  70 
  71 ShenandoahFullGC::~ShenandoahFullGC() {
  72   delete _preserved_marks;
  73 }
  74 
  75 bool ShenandoahFullGC::collect(GCCause::Cause cause) {
  76   vmop_entry_full(cause);
  77   // Always success
  78   return true;
  79 }
  80 
  81 void ShenandoahFullGC::vmop_entry_full(GCCause::Cause cause) {
  82   ShenandoahHeap* const heap = ShenandoahHeap::heap();
  83   TraceCollectorStats tcs(heap->monitoring_support()->full_stw_collection_counters());
  84   ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::full_gc_gross);
  85 
  86   heap->try_inject_alloc_failure();
  87   VM_ShenandoahFullGC op(cause, this);
  88   VMThread::execute(&op);
  89 }
  90 
  91 void ShenandoahFullGC::entry_full(GCCause::Cause cause) {
  92   static const char* msg = "Pause Full";
  93   ShenandoahPausePhase gc_phase(msg, ShenandoahPhaseTimings::full_gc, true /* log_heap_usage */);
  94   EventMark em("%s", msg);
  95 
  96   ShenandoahWorkerScope scope(ShenandoahHeap::heap()->workers(),
  97                               ShenandoahWorkerPolicy::calc_workers_for_fullgc(),
  98                               "full gc");
  99 
 100   op_full(cause);
 101 }
 102 
 103 void ShenandoahFullGC::op_full(GCCause::Cause cause) {
 104   ShenandoahMetricsSnapshot metrics;
 105   metrics.snap_before();
 106 
 107   // Perform full GC
 108   do_it(cause);
 109 
 110   ShenandoahHeap* const heap = ShenandoahHeap::heap();
 111 
 112   metrics.snap_after();
 113 
 114   if (metrics.is_good_progress()) {
 115     heap->notify_gc_progress();
 116   } else {
 117     // Nothing to do. Tell the allocation path that we have failed to make
 118     // progress, and it can finally fail.
 119     heap->notify_gc_no_progress();
 120   }
 121 
 122   // Regardless if progress was made, we record that we completed a "successful" full GC.
 123   heap->heuristics()->record_success_full();
 124   heap->shenandoah_policy()->record_success_full();
 125 }
 126 
 127 void ShenandoahFullGC::do_it(GCCause::Cause gc_cause) {
 128   ShenandoahHeap* heap = ShenandoahHeap::heap();
 129 
 130   if (ShenandoahVerify) {
 131     heap->verifier()->verify_before_fullgc();
 132   }
 133 
 134   if (VerifyBeforeGC) {
 135     Universe::verify();
 136   }
 137 
 138   // Degenerated GC may carry concurrent root flags when upgrading to
 139   // full GC. We need to reset it before mutators resume.
 140   heap->set_concurrent_strong_root_in_progress(false);
 141   heap->set_concurrent_weak_root_in_progress(false);
 142 
 143   heap->set_full_gc_in_progress(true);
 144 
 145   assert(ShenandoahSafepoint::is_at_shenandoah_safepoint(), "must be at a safepoint");
 146   assert(Thread::current()->is_VM_thread(), "Do full GC only while world is stopped");
 147 
 148   {
 149     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_heapdump_pre);
 150     heap->pre_full_gc_dump(_gc_timer);
 151   }
 152 
 153   {
 154     ShenandoahGCPhase prepare_phase(ShenandoahPhaseTimings::full_gc_prepare);
 155     // Full GC is supposed to recover from any GC state:
 156 
 157     // a0. Remember if we have forwarded objects
 158     bool has_forwarded_objects = heap->has_forwarded_objects();
 159 
 160     // a1. Cancel evacuation, if in progress
 161     if (heap->is_evacuation_in_progress()) {
 162       heap->set_evacuation_in_progress(false);
 163     }
 164     assert(!heap->is_evacuation_in_progress(), "sanity");
 165 
 166     // a2. Cancel update-refs, if in progress
 167     if (heap->is_update_refs_in_progress()) {
 168       heap->set_update_refs_in_progress(false);
 169     }
 170     assert(!heap->is_update_refs_in_progress(), "sanity");
 171 
 172     // b. Cancel concurrent mark, if in progress
 173     if (heap->is_concurrent_mark_in_progress()) {
 174       ShenandoahConcurrentGC::cancel();
 175       heap->set_concurrent_mark_in_progress(false);
 176     }
 177     assert(!heap->is_concurrent_mark_in_progress(), "sanity");
 178 
 179     // c. Update roots if this full GC is due to evac-oom, which may carry from-space pointers in roots.
 180     if (has_forwarded_objects) {
 181       update_roots(true /*full_gc*/);
 182     }
 183 
 184     // d. Reset the bitmaps for new marking
 185     heap->reset_mark_bitmap();
 186     assert(heap->marking_context()->is_bitmap_clear(), "sanity");
 187     assert(!heap->marking_context()->is_complete(), "sanity");
 188 
 189     // e. Abandon reference discovery and clear all discovered references.
 190     ShenandoahReferenceProcessor* rp = heap->ref_processor();
 191     rp->abandon_partial_discovery();
 192 
 193     // f. Sync pinned region status from the CP marks
 194     heap->sync_pinned_region_status();
 195 
 196     // The rest of prologue:
 197     _preserved_marks->init(heap->workers()->active_workers());
 198 
 199     assert(heap->has_forwarded_objects() == has_forwarded_objects, "This should not change");
 200   }
 201 
 202   if (UseTLAB) {
 203     heap->gclabs_retire(ResizeTLAB);
 204     heap->tlabs_retire(ResizeTLAB);
 205   }
 206 
 207   OrderAccess::fence();
 208 
 209   phase1_mark_heap();
 210 
 211   // Once marking is done, which may have fixed up forwarded objects, we can drop it.
 212   // Coming out of Full GC, we would not have any forwarded objects.
 213   // This also prevents resolves with fwdptr from kicking in while adjusting pointers in phase3.
 214   heap->set_has_forwarded_objects(false);
 215 
 216   heap->set_full_gc_move_in_progress(true);
 217 
 218   // Setup workers for the rest
 219   OrderAccess::fence();
 220 
 221   // Initialize worker slices
 222   ShenandoahHeapRegionSet** worker_slices = NEW_C_HEAP_ARRAY(ShenandoahHeapRegionSet*, heap->max_workers(), mtGC);
 223   for (uint i = 0; i < heap->max_workers(); i++) {
 224     worker_slices[i] = new ShenandoahHeapRegionSet();
 225   }
 226 
 227   {
 228     // The rest of code performs region moves, where region status is undefined
 229     // until all phases run together.
 230     ShenandoahHeapLocker lock(heap->lock());
 231 
 232     SlidingForwarding::begin();
 233 
 234     phase2_calculate_target_addresses(worker_slices);
 235 
 236     OrderAccess::fence();
 237 
 238     phase3_update_references();
 239 
 240     phase4_compact_objects(worker_slices);
 241 
 242     phase5_epilog();
 243 
 244     SlidingForwarding::end();
 245   }
 246 
 247   // Resize metaspace
 248   MetaspaceGC::compute_new_size();
 249 
 250   // Free worker slices
 251   for (uint i = 0; i < heap->max_workers(); i++) {
 252     delete worker_slices[i];
 253   }
 254   FREE_C_HEAP_ARRAY(ShenandoahHeapRegionSet*, worker_slices);
 255 
 256   heap->set_full_gc_move_in_progress(false);
 257   heap->set_full_gc_in_progress(false);
 258 
 259   if (ShenandoahVerify) {
 260     heap->verifier()->verify_after_fullgc();
 261   }
 262 
 263   if (VerifyAfterGC) {
 264     Universe::verify();
 265   }
 266 
 267   {
 268     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_heapdump_post);
 269     heap->post_full_gc_dump(_gc_timer);
 270   }
 271 }
 272 
 273 class ShenandoahPrepareForMarkClosure: public ShenandoahHeapRegionClosure {
 274 private:
 275   ShenandoahMarkingContext* const _ctx;
 276 
 277 public:
 278   ShenandoahPrepareForMarkClosure() : _ctx(ShenandoahHeap::heap()->marking_context()) {}
 279 
 280   void heap_region_do(ShenandoahHeapRegion *r) {
 281     _ctx->capture_top_at_mark_start(r);
 282     r->clear_live_data();
 283   }
 284 
 285   bool is_thread_safe() { return true; }
 286 };
 287 
 288 void ShenandoahFullGC::phase1_mark_heap() {
 289   GCTraceTime(Info, gc, phases) time("Phase 1: Mark live objects", _gc_timer);
 290   ShenandoahGCPhase mark_phase(ShenandoahPhaseTimings::full_gc_mark);
 291 
 292   ShenandoahHeap* heap = ShenandoahHeap::heap();
 293 
 294   ShenandoahPrepareForMarkClosure cl;
 295   heap->parallel_heap_region_iterate(&cl);
 296 
 297   heap->set_unload_classes(heap->heuristics()->can_unload_classes());
 298 
 299   ShenandoahReferenceProcessor* rp = heap->ref_processor();
 300   // enable ("weak") refs discovery
 301   rp->set_soft_reference_policy(true); // forcefully purge all soft references
 302 
 303   ShenandoahSTWMark mark(true /*full_gc*/);
 304   mark.mark();
 305   heap->parallel_cleaning(true /* full_gc */);
 306 }
 307 
 308 class ShenandoahPrepareForCompactionObjectClosure : public ObjectClosure {
 309 private:
 310   PreservedMarks*          const _preserved_marks;
 311   ShenandoahHeap*          const _heap;
 312   GrowableArray<ShenandoahHeapRegion*>& _empty_regions;
 313   int _empty_regions_pos;
 314   ShenandoahHeapRegion*          _to_region;
 315   ShenandoahHeapRegion*          _from_region;
 316   HeapWord* _compact_point;
 317 
 318 public:
 319   ShenandoahPrepareForCompactionObjectClosure(PreservedMarks* preserved_marks,
 320                                               GrowableArray<ShenandoahHeapRegion*>& empty_regions,
 321                                               ShenandoahHeapRegion* to_region) :
 322     _preserved_marks(preserved_marks),
 323     _heap(ShenandoahHeap::heap()),
 324     _empty_regions(empty_regions),
 325     _empty_regions_pos(0),
 326     _to_region(to_region),
 327     _from_region(nullptr),
 328     _compact_point(to_region->bottom()) {}
 329 
 330   void set_from_region(ShenandoahHeapRegion* from_region) {
 331     _from_region = from_region;
 332   }
 333 
 334   void finish() {
 335     assert(_to_region != nullptr, "should not happen");
 336     _to_region->set_new_top(_compact_point);
 337   }
 338 
 339   bool is_compact_same_region() {
 340     return _from_region == _to_region;
 341   }
 342 
 343   int empty_regions_pos() {
 344     return _empty_regions_pos;
 345   }
 346 
 347   void do_object(oop p) {
 348     assert(_from_region != nullptr, "must set before work");
 349     assert(_heap->complete_marking_context()->is_marked(p), "must be marked");
 350     assert(!_heap->complete_marking_context()->allocated_after_mark_start(p), "must be truly marked");
 351 
 352     size_t obj_size = p->size();
 353     if (_compact_point + obj_size > _to_region->end()) {
 354       finish();
 355 
 356       // Object doesn't fit. Pick next empty region and start compacting there.
 357       ShenandoahHeapRegion* new_to_region;
 358       if (_empty_regions_pos < _empty_regions.length()) {
 359         new_to_region = _empty_regions.at(_empty_regions_pos);
 360         _empty_regions_pos++;
 361       } else {
 362         // Out of empty region? Compact within the same region.
 363         new_to_region = _from_region;
 364       }
 365 
 366       assert(new_to_region != _to_region, "must not reuse same to-region");
 367       assert(new_to_region != nullptr, "must not be null");
 368       _to_region = new_to_region;
 369       _compact_point = _to_region->bottom();
 370     }
 371 
 372     // Object fits into current region, record new location, if object does not move:
 373     assert(_compact_point + obj_size <= _to_region->end(), "must fit");
 374     shenandoah_assert_not_forwarded(nullptr, p);
 375     if (_compact_point != cast_from_oop<HeapWord*>(p)) {
 376       _preserved_marks->push_if_necessary(p, p->mark());
 377       SlidingForwarding::forward_to(p, cast_to_oop(_compact_point));
 378     }
 379     _compact_point += obj_size;
 380   }
 381 };
 382 
 383 class ShenandoahPrepareForCompactionTask : public WorkerTask {
 384 private:
 385   PreservedMarksSet*        const _preserved_marks;
 386   ShenandoahHeap*           const _heap;
 387   ShenandoahHeapRegionSet** const _worker_slices;
 388 
 389 public:
 390   ShenandoahPrepareForCompactionTask(PreservedMarksSet *preserved_marks, ShenandoahHeapRegionSet **worker_slices) :
 391     WorkerTask("Shenandoah Prepare For Compaction"),
 392     _preserved_marks(preserved_marks),
 393     _heap(ShenandoahHeap::heap()), _worker_slices(worker_slices) {
 394   }
 395 
 396   static bool is_candidate_region(ShenandoahHeapRegion* r) {
 397     // Empty region: get it into the slice to defragment the slice itself.
 398     // We could have skipped this without violating correctness, but we really
 399     // want to compact all live regions to the start of the heap, which sometimes
 400     // means moving them into the fully empty regions.
 401     if (r->is_empty()) return true;
 402 
 403     // Can move the region, and this is not the humongous region. Humongous
 404     // moves are special cased here, because their moves are handled separately.
 405     return r->is_stw_move_allowed() && !r->is_humongous();
 406   }
 407 
 408   void work(uint worker_id) override;
 409 private:
 410   template<typename ClosureType>
 411   void prepare_for_compaction(ClosureType& cl,
 412                               GrowableArray<ShenandoahHeapRegion*>& empty_regions,
 413                               ShenandoahHeapRegionSetIterator& it,
 414                               ShenandoahHeapRegion* from_region);
 415 };
 416 
 417 void ShenandoahPrepareForCompactionTask::work(uint worker_id) {
 418   ShenandoahParallelWorkerSession worker_session(worker_id);
 419   ShenandoahHeapRegionSet* slice = _worker_slices[worker_id];
 420   ShenandoahHeapRegionSetIterator it(slice);
 421   ShenandoahHeapRegion* from_region = it.next();
 422   // No work?
 423   if (from_region == nullptr) {
 424     return;
 425   }
 426 
 427   // Sliding compaction. Walk all regions in the slice, and compact them.
 428   // Remember empty regions and reuse them as needed.
 429   ResourceMark rm;
 430 
 431   GrowableArray<ShenandoahHeapRegion*> empty_regions((int)_heap->num_regions());
 432 
 433   ShenandoahPrepareForCompactionObjectClosure cl(_preserved_marks->get(worker_id), empty_regions, from_region);
 434   prepare_for_compaction(cl, empty_regions, it, from_region);
 435 }
 436 
 437 template<typename ClosureType>
 438 void ShenandoahPrepareForCompactionTask::prepare_for_compaction(ClosureType& cl,
 439                                                                 GrowableArray<ShenandoahHeapRegion*>& empty_regions,
 440                                                                 ShenandoahHeapRegionSetIterator& it,
 441                                                                 ShenandoahHeapRegion* from_region) {
 442   while (from_region != nullptr) {
 443     assert(is_candidate_region(from_region), "Sanity");
 444     cl.set_from_region(from_region);
 445     if (from_region->has_live()) {
 446       _heap->marked_object_iterate(from_region, &cl);
 447     }
 448 
 449     // Compacted the region to somewhere else? From-region is empty then.
 450     if (!cl.is_compact_same_region()) {
 451       empty_regions.append(from_region);
 452     }
 453     from_region = it.next();
 454   }
 455   cl.finish();
 456 
 457   // Mark all remaining regions as empty
 458   for (int pos = cl.empty_regions_pos(); pos < empty_regions.length(); ++pos) {
 459     ShenandoahHeapRegion* r = empty_regions.at(pos);
 460     r->set_new_top(r->bottom());
 461   }
 462 }
 463 
 464 void ShenandoahFullGC::calculate_target_humongous_objects() {
 465   ShenandoahHeap* heap = ShenandoahHeap::heap();
 466 
 467   // Compute the new addresses for humongous objects. We need to do this after addresses
 468   // for regular objects are calculated, and we know what regions in heap suffix are
 469   // available for humongous moves.
 470   //
 471   // Scan the heap backwards, because we are compacting humongous regions towards the end.
 472   // Maintain the contiguous compaction window in [to_begin; to_end), so that we can slide
 473   // humongous start there.
 474   //
 475   // The complication is potential non-movable regions during the scan. If such region is
 476   // detected, then sliding restarts towards that non-movable region.
 477 
 478   size_t to_begin = heap->num_regions();
 479   size_t to_end = heap->num_regions();
 480 
 481   for (size_t c = heap->num_regions(); c > 0; c--) {
 482     ShenandoahHeapRegion *r = heap->get_region(c - 1);
 483     if (r->is_humongous_continuation() || (r->new_top() == r->bottom())) {
 484       // To-region candidate: record this, and continue scan
 485       to_begin = r->index();
 486       continue;
 487     }
 488 
 489     if (r->is_humongous_start() && r->is_stw_move_allowed()) {
 490       // From-region candidate: movable humongous region
 491       oop old_obj = cast_to_oop(r->bottom());
 492       size_t words_size = old_obj->size();
 493       size_t num_regions = ShenandoahHeapRegion::required_regions(words_size * HeapWordSize);
 494 
 495       size_t start = to_end - num_regions;
 496 
 497       if (start >= to_begin && start != r->index()) {
 498         // Fits into current window, and the move is non-trivial. Record the move then, and continue scan.
 499         _preserved_marks->get(0)->push_if_necessary(old_obj, old_obj->mark());
 500         SlidingForwarding::forward_to(old_obj, cast_to_oop(heap->get_region(start)->bottom()));
 501         to_end = start;
 502         continue;
 503       }
 504     }
 505 
 506     // Failed to fit. Scan starting from current region.
 507     to_begin = r->index();
 508     to_end = r->index();
 509   }
 510 }
 511 
 512 class ShenandoahEnsureHeapActiveClosure: public ShenandoahHeapRegionClosure {
 513 private:
 514   ShenandoahHeap* const _heap;
 515 
 516 public:
 517   ShenandoahEnsureHeapActiveClosure() : _heap(ShenandoahHeap::heap()) {}
 518   void heap_region_do(ShenandoahHeapRegion* r) {
 519     if (r->is_trash()) {
 520       r->recycle();
 521     }
 522     if (r->is_cset()) {
 523       r->make_regular_bypass();
 524     }
 525     if (r->is_empty_uncommitted()) {
 526       r->make_committed_bypass();
 527     }
 528     assert (r->is_committed(), "only committed regions in heap now, see region " SIZE_FORMAT, r->index());
 529 
 530     // Record current region occupancy: this communicates empty regions are free
 531     // to the rest of Full GC code.
 532     r->set_new_top(r->top());
 533   }
 534 };
 535 
 536 class ShenandoahTrashImmediateGarbageClosure: public ShenandoahHeapRegionClosure {
 537 private:
 538   ShenandoahHeap* const _heap;
 539   ShenandoahMarkingContext* const _ctx;
 540 
 541 public:
 542   ShenandoahTrashImmediateGarbageClosure() :
 543     _heap(ShenandoahHeap::heap()),
 544     _ctx(ShenandoahHeap::heap()->complete_marking_context()) {}
 545 
 546   void heap_region_do(ShenandoahHeapRegion* r) {
 547     if (r->is_humongous_start()) {
 548       oop humongous_obj = cast_to_oop(r->bottom());
 549       if (!_ctx->is_marked(humongous_obj)) {
 550         assert(!r->has_live(),
 551                "Region " SIZE_FORMAT " is not marked, should not have live", r->index());
 552         _heap->trash_humongous_region_at(r);
 553       } else {
 554         assert(r->has_live(),
 555                "Region " SIZE_FORMAT " should have live", r->index());
 556       }
 557     } else if (r->is_humongous_continuation()) {
 558       // If we hit continuation, the non-live humongous starts should have been trashed already
 559       assert(r->humongous_start_region()->has_live(),
 560              "Region " SIZE_FORMAT " should have live", r->index());
 561     } else if (r->is_regular()) {
 562       if (!r->has_live()) {
 563         r->make_trash_immediate();
 564       }
 565     }
 566   }
 567 };
 568 
 569 void ShenandoahFullGC::distribute_slices(ShenandoahHeapRegionSet** worker_slices) {
 570   ShenandoahHeap* heap = ShenandoahHeap::heap();
 571 
 572   uint n_workers = heap->workers()->active_workers();
 573   size_t n_regions = heap->num_regions();
 574 
 575   // What we want to accomplish: have the dense prefix of data, while still balancing
 576   // out the parallel work.
 577   //
 578   // Assuming the amount of work is driven by the live data that needs moving, we can slice
 579   // the entire heap into equal-live-sized prefix slices, and compact into them. So, each
 580   // thread takes all regions in its prefix subset, and then it takes some regions from
 581   // the tail.
 582   //
 583   // Tail region selection becomes interesting.
 584   //
 585   // First, we want to distribute the regions fairly between the workers, and those regions
 586   // might have different amount of live data. So, until we sure no workers need live data,
 587   // we need to only take what the worker needs.
 588   //
 589   // Second, since we slide everything to the left in each slice, the most busy regions
 590   // would be the ones on the left. Which means we want to have all workers have their after-tail
 591   // regions as close to the left as possible.
 592   //
 593   // The easiest way to do this is to distribute after-tail regions in round-robin between
 594   // workers that still need live data.
 595   //
 596   // Consider parallel workers A, B, C, then the target slice layout would be:
 597   //
 598   //  AAAAAAAABBBBBBBBCCCCCCCC|ABCABCABCABCABCABCABCABABABABABABABABABABAAAAA
 599   //
 600   //  (.....dense-prefix.....) (.....................tail...................)
 601   //  [all regions fully live] [left-most regions are fuller that right-most]
 602   //
 603 
 604   // Compute how much live data is there. This would approximate the size of dense prefix
 605   // we target to create.
 606   size_t total_live = 0;
 607   for (size_t idx = 0; idx < n_regions; idx++) {
 608     ShenandoahHeapRegion *r = heap->get_region(idx);
 609     if (ShenandoahPrepareForCompactionTask::is_candidate_region(r)) {
 610       total_live += r->get_live_data_words();
 611     }
 612   }
 613 
 614   // Estimate the size for the dense prefix. Note that we specifically count only the
 615   // "full" regions, so there would be some non-full regions in the slice tail.
 616   size_t live_per_worker = total_live / n_workers;
 617   size_t prefix_regions_per_worker = live_per_worker / ShenandoahHeapRegion::region_size_words();
 618   size_t prefix_regions_total = prefix_regions_per_worker * n_workers;
 619   prefix_regions_total = MIN2(prefix_regions_total, n_regions);
 620   assert(prefix_regions_total <= n_regions, "Sanity");
 621 
 622   // There might be non-candidate regions in the prefix. To compute where the tail actually
 623   // ends up being, we need to account those as well.
 624   size_t prefix_end = prefix_regions_total;
 625   for (size_t idx = 0; idx < prefix_regions_total; idx++) {
 626     ShenandoahHeapRegion *r = heap->get_region(idx);
 627     if (!ShenandoahPrepareForCompactionTask::is_candidate_region(r)) {
 628       prefix_end++;
 629     }
 630   }
 631   prefix_end = MIN2(prefix_end, n_regions);
 632   assert(prefix_end <= n_regions, "Sanity");
 633 
 634   // Distribute prefix regions per worker: each thread definitely gets its own same-sized
 635   // subset of dense prefix.
 636   size_t prefix_idx = 0;
 637 
 638   size_t* live = NEW_C_HEAP_ARRAY(size_t, n_workers, mtGC);
 639 
 640   for (size_t wid = 0; wid < n_workers; wid++) {
 641     ShenandoahHeapRegionSet* slice = worker_slices[wid];
 642 
 643     live[wid] = 0;
 644     size_t regs = 0;
 645 
 646     // Add all prefix regions for this worker
 647     while (prefix_idx < prefix_end && regs < prefix_regions_per_worker) {
 648       ShenandoahHeapRegion *r = heap->get_region(prefix_idx);
 649       if (ShenandoahPrepareForCompactionTask::is_candidate_region(r)) {
 650         slice->add_region(r);
 651         live[wid] += r->get_live_data_words();
 652         regs++;
 653       }
 654       prefix_idx++;
 655     }
 656   }
 657 
 658   // Distribute the tail among workers in round-robin fashion.
 659   size_t wid = n_workers - 1;
 660 
 661   for (size_t tail_idx = prefix_end; tail_idx < n_regions; tail_idx++) {
 662     ShenandoahHeapRegion *r = heap->get_region(tail_idx);
 663     if (ShenandoahPrepareForCompactionTask::is_candidate_region(r)) {
 664       assert(wid < n_workers, "Sanity");
 665 
 666       size_t live_region = r->get_live_data_words();
 667 
 668       // Select next worker that still needs live data.
 669       size_t old_wid = wid;
 670       do {
 671         wid++;
 672         if (wid == n_workers) wid = 0;
 673       } while (live[wid] + live_region >= live_per_worker && old_wid != wid);
 674 
 675       if (old_wid == wid) {
 676         // Circled back to the same worker? This means liveness data was
 677         // miscalculated. Bump the live_per_worker limit so that
 678         // everyone gets a piece of the leftover work.
 679         live_per_worker += ShenandoahHeapRegion::region_size_words();
 680       }
 681 
 682       worker_slices[wid]->add_region(r);
 683       live[wid] += live_region;
 684     }
 685   }
 686 
 687   FREE_C_HEAP_ARRAY(size_t, live);
 688 
 689 #ifdef ASSERT
 690   ResourceBitMap map(n_regions);
 691   for (size_t wid = 0; wid < n_workers; wid++) {
 692     ShenandoahHeapRegionSetIterator it(worker_slices[wid]);
 693     ShenandoahHeapRegion* r = it.next();
 694     while (r != nullptr) {
 695       size_t idx = r->index();
 696       assert(ShenandoahPrepareForCompactionTask::is_candidate_region(r), "Sanity: " SIZE_FORMAT, idx);
 697       assert(!map.at(idx), "No region distributed twice: " SIZE_FORMAT, idx);
 698       map.at_put(idx, true);
 699       r = it.next();
 700     }
 701   }
 702 
 703   for (size_t rid = 0; rid < n_regions; rid++) {
 704     bool is_candidate = ShenandoahPrepareForCompactionTask::is_candidate_region(heap->get_region(rid));
 705     bool is_distributed = map.at(rid);
 706     assert(is_distributed || !is_candidate, "All candidates are distributed: " SIZE_FORMAT, rid);
 707   }
 708 #endif
 709 }
 710 
 711 void ShenandoahFullGC::phase2_calculate_target_addresses(ShenandoahHeapRegionSet** worker_slices) {
 712   GCTraceTime(Info, gc, phases) time("Phase 2: Compute new object addresses", _gc_timer);
 713   ShenandoahGCPhase calculate_address_phase(ShenandoahPhaseTimings::full_gc_calculate_addresses);
 714 
 715   ShenandoahHeap* heap = ShenandoahHeap::heap();
 716 
 717   // About to figure out which regions can be compacted, make sure pinning status
 718   // had been updated in GC prologue.
 719   heap->assert_pinned_region_status();
 720 
 721   {
 722     // Trash the immediately collectible regions before computing addresses
 723     ShenandoahTrashImmediateGarbageClosure tigcl;
 724     heap->heap_region_iterate(&tigcl);
 725 
 726     // Make sure regions are in good state: committed, active, clean.
 727     // This is needed because we are potentially sliding the data through them.
 728     ShenandoahEnsureHeapActiveClosure ecl;
 729     heap->heap_region_iterate(&ecl);
 730   }
 731 
 732   // Compute the new addresses for regular objects
 733   {
 734     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_calculate_addresses_regular);
 735 
 736     distribute_slices(worker_slices);
 737 
 738     ShenandoahPrepareForCompactionTask task(_preserved_marks, worker_slices);
 739     heap->workers()->run_task(&task);
 740   }
 741 
 742   // Compute the new addresses for humongous objects
 743   {
 744     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_calculate_addresses_humong);
 745     calculate_target_humongous_objects();
 746   }
 747 }
 748 
 749 class ShenandoahAdjustPointersClosure : public MetadataVisitingOopIterateClosure {
 750 private:
 751   ShenandoahHeap* const _heap;
 752   ShenandoahMarkingContext* const _ctx;
 753 
 754   template <class T>
 755   inline void do_oop_work(T* p) {
 756     T o = RawAccess<>::oop_load(p);
 757     if (!CompressedOops::is_null(o)) {
 758       oop obj = CompressedOops::decode_not_null(o);
 759       assert(_ctx->is_marked(obj), "must be marked");
 760       if (SlidingForwarding::is_forwarded(obj)) {
 761         oop forw = SlidingForwarding::forwardee(obj);
 762         RawAccess<IS_NOT_NULL>::oop_store(p, forw);
 763       }
 764     }
 765   }
 766 
 767 public:
 768   ShenandoahAdjustPointersClosure() :
 769     _heap(ShenandoahHeap::heap()),
 770     _ctx(ShenandoahHeap::heap()->complete_marking_context()) {}
 771 
 772   void do_oop(oop* p)       { do_oop_work(p); }
 773   void do_oop(narrowOop* p) { do_oop_work(p); }
 774   void do_method(Method* m) {}
 775   void do_nmethod(nmethod* nm) {}
 776 };
 777 
 778 class ShenandoahAdjustPointersObjectClosure : public ObjectClosure {
 779 private:
 780   ShenandoahHeap* const _heap;
 781   ShenandoahAdjustPointersClosure _cl;
 782 
 783 public:
 784   ShenandoahAdjustPointersObjectClosure() :
 785     _heap(ShenandoahHeap::heap()) {
 786   }
 787   void do_object(oop p) {
 788     assert(_heap->complete_marking_context()->is_marked(p), "must be marked");
 789     p->oop_iterate(&_cl);
 790   }
 791 };
 792 
 793 class ShenandoahAdjustPointersTask : public WorkerTask {
 794 private:
 795   ShenandoahHeap*          const _heap;
 796   ShenandoahRegionIterator       _regions;
 797 
 798 public:
 799   ShenandoahAdjustPointersTask() :
 800     WorkerTask("Shenandoah Adjust Pointers"),
 801     _heap(ShenandoahHeap::heap()) {
 802   }
 803 
 804   void work(uint worker_id) {
 805     ShenandoahParallelWorkerSession worker_session(worker_id);
 806     ShenandoahAdjustPointersObjectClosure obj_cl;
 807     ShenandoahHeapRegion* r = _regions.next();
 808     while (r != nullptr) {
 809       if (!r->is_humongous_continuation() && r->has_live()) {
 810         _heap->marked_object_iterate(r, &obj_cl);
 811       }
 812       r = _regions.next();
 813     }
 814   }
 815 };
 816 
 817 class ShenandoahAdjustRootPointersTask : public WorkerTask {
 818 private:
 819   ShenandoahRootAdjuster* _rp;
 820   PreservedMarksSet* _preserved_marks;
 821 public:
 822   ShenandoahAdjustRootPointersTask(ShenandoahRootAdjuster* rp, PreservedMarksSet* preserved_marks) :
 823     WorkerTask("Shenandoah Adjust Root Pointers"),
 824     _rp(rp),
 825     _preserved_marks(preserved_marks) {}
 826 
 827   void work(uint worker_id) {
 828     ShenandoahParallelWorkerSession worker_session(worker_id);
 829     ShenandoahAdjustPointersClosure cl;
 830     _rp->roots_do(worker_id, &cl);
 831     _preserved_marks->get(worker_id)->adjust_during_full_gc();
 832   }
 833 };
 834 
 835 void ShenandoahFullGC::phase3_update_references() {
 836   GCTraceTime(Info, gc, phases) time("Phase 3: Adjust pointers", _gc_timer);
 837   ShenandoahGCPhase adjust_pointer_phase(ShenandoahPhaseTimings::full_gc_adjust_pointers);
 838 
 839   ShenandoahHeap* heap = ShenandoahHeap::heap();
 840 
 841   WorkerThreads* workers = heap->workers();
 842   uint nworkers = workers->active_workers();
 843   {
 844 #if COMPILER2_OR_JVMCI
 845     DerivedPointerTable::clear();
 846 #endif
 847     ShenandoahRootAdjuster rp(nworkers, ShenandoahPhaseTimings::full_gc_adjust_roots);
 848     ShenandoahAdjustRootPointersTask task(&rp, _preserved_marks);
 849     workers->run_task(&task);
 850 #if COMPILER2_OR_JVMCI
 851     DerivedPointerTable::update_pointers();
 852 #endif
 853   }
 854 
 855   ShenandoahAdjustPointersTask adjust_pointers_task;
 856   workers->run_task(&adjust_pointers_task);
 857 }
 858 
 859 class ShenandoahCompactObjectsClosure : public ObjectClosure {
 860 private:
 861   ShenandoahHeap* const _heap;
 862   uint            const _worker_id;
 863 
 864 public:
 865   ShenandoahCompactObjectsClosure(uint worker_id) :
 866     _heap(ShenandoahHeap::heap()), _worker_id(worker_id) {}
 867 
 868   void do_object(oop p) {
 869     assert(_heap->complete_marking_context()->is_marked(p), "must be marked");
 870     size_t size = p->size();
 871     if (SlidingForwarding::is_forwarded(p)) {
 872       HeapWord* compact_from = cast_from_oop<HeapWord*>(p);
 873       HeapWord* compact_to = cast_from_oop<HeapWord*>(SlidingForwarding::forwardee(p));
 874       assert(compact_from != compact_to, "Forwarded object should move");
 875       Copy::aligned_conjoint_words(compact_from, compact_to, size);
 876       oop new_obj = cast_to_oop(compact_to);
 877 
 878       ContinuationGCSupport::relativize_stack_chunk(new_obj);
 879       new_obj->init_mark();
 880     }
 881   }
 882 };
 883 
 884 class ShenandoahCompactObjectsTask : public WorkerTask {
 885 private:
 886   ShenandoahHeap* const _heap;
 887   ShenandoahHeapRegionSet** const _worker_slices;
 888 
 889 public:
 890   ShenandoahCompactObjectsTask(ShenandoahHeapRegionSet** worker_slices) :
 891     WorkerTask("Shenandoah Compact Objects"),
 892     _heap(ShenandoahHeap::heap()),
 893     _worker_slices(worker_slices) {
 894   }
 895 
 896   void work(uint worker_id) {
 897     ShenandoahParallelWorkerSession worker_session(worker_id);
 898     ShenandoahHeapRegionSetIterator slice(_worker_slices[worker_id]);
 899 
 900     ShenandoahCompactObjectsClosure cl(worker_id);
 901     ShenandoahHeapRegion* r = slice.next();
 902     while (r != nullptr) {
 903       assert(!r->is_humongous(), "must not get humongous regions here");
 904       if (r->has_live()) {
 905         _heap->marked_object_iterate(r, &cl);
 906       }
 907       r->set_top(r->new_top());
 908       r = slice.next();
 909     }
 910   }
 911 };
 912 
 913 class ShenandoahPostCompactClosure : public ShenandoahHeapRegionClosure {
 914 private:
 915   ShenandoahHeap* const _heap;
 916   size_t _live;
 917 
 918 public:
 919   ShenandoahPostCompactClosure() : _heap(ShenandoahHeap::heap()), _live(0) {
 920   }
 921 
 922   void heap_region_do(ShenandoahHeapRegion* r) {
 923     assert (!r->is_cset(), "cset regions should have been demoted already");
 924 
 925     // Need to reset the complete-top-at-mark-start pointer here because
 926     // the complete marking bitmap is no longer valid. This ensures
 927     // size-based iteration in marked_object_iterate().
 928     // NOTE: See blurb at ShenandoahMCResetCompleteBitmapTask on why we need to skip
 929     // pinned regions.
 930     if (!r->is_pinned()) {
 931       _heap->complete_marking_context()->reset_top_at_mark_start(r);
 932     }
 933 
 934     size_t live = r->used();
 935 
 936     // Make empty regions that have been allocated into regular
 937     if (r->is_empty() && live > 0) {
 938       r->make_regular_bypass();
 939       if (ZapUnusedHeapArea) {
 940         SpaceMangler::mangle_region(MemRegion(r->top(), r->end()));
 941       }
 942     }
 943 
 944     // Reclaim regular regions that became empty
 945     if (r->is_regular() && live == 0) {
 946       r->make_trash();
 947     }
 948 
 949     // Recycle all trash regions
 950     if (r->is_trash()) {
 951       live = 0;
 952       r->recycle();
 953     }
 954 
 955     r->set_live_data(live);
 956     r->reset_alloc_metadata();
 957     _live += live;
 958   }
 959 
 960   size_t get_live() {
 961     return _live;
 962   }
 963 };
 964 
 965 void ShenandoahFullGC::compact_humongous_objects() {
 966   // Compact humongous regions, based on their fwdptr objects.
 967   //
 968   // This code is serial, because doing the in-slice parallel sliding is tricky. In most cases,
 969   // humongous regions are already compacted, and do not require further moves, which alleviates
 970   // sliding costs. We may consider doing this in parallel in the future.
 971 
 972   ShenandoahHeap* heap = ShenandoahHeap::heap();
 973 
 974   for (size_t c = heap->num_regions(); c > 0; c--) {
 975     ShenandoahHeapRegion* r = heap->get_region(c - 1);
 976     if (r->is_humongous_start()) {
 977       oop old_obj = cast_to_oop(r->bottom());
 978       if (SlidingForwarding::is_not_forwarded(old_obj)) {
 979         // No need to move the object, it stays at the same slot
 980         continue;
 981       }
 982       size_t words_size = old_obj->size();
 983       size_t num_regions = ShenandoahHeapRegion::required_regions(words_size * HeapWordSize);
 984 
 985       size_t old_start = r->index();
 986       size_t old_end   = old_start + num_regions - 1;
 987       size_t new_start = heap->heap_region_index_containing(SlidingForwarding::forwardee(old_obj));
 988       size_t new_end   = new_start + num_regions - 1;
 989       assert(old_start != new_start, "must be real move");
 990       assert(r->is_stw_move_allowed(), "Region " SIZE_FORMAT " should be movable", r->index());
 991 
 992       Copy::aligned_conjoint_words(r->bottom(), heap->get_region(new_start)->bottom(), words_size);
 993       ContinuationGCSupport::relativize_stack_chunk(cast_to_oop<HeapWord*>(r->bottom()));
 994 
 995       oop new_obj = cast_to_oop(heap->get_region(new_start)->bottom());
 996       new_obj->init_mark();
 997 
 998       {
 999         for (size_t c = old_start; c <= old_end; c++) {
1000           ShenandoahHeapRegion* r = heap->get_region(c);
1001           r->make_regular_bypass();
1002           r->set_top(r->bottom());
1003         }
1004 
1005         for (size_t c = new_start; c <= new_end; c++) {
1006           ShenandoahHeapRegion* r = heap->get_region(c);
1007           if (c == new_start) {
1008             r->make_humongous_start_bypass();
1009           } else {
1010             r->make_humongous_cont_bypass();
1011           }
1012 
1013           // Trailing region may be non-full, record the remainder there
1014           size_t remainder = words_size & ShenandoahHeapRegion::region_size_words_mask();
1015           if ((c == new_end) && (remainder != 0)) {
1016             r->set_top(r->bottom() + remainder);
1017           } else {
1018             r->set_top(r->end());
1019           }
1020 
1021           r->reset_alloc_metadata();
1022         }
1023       }
1024     }
1025   }
1026 }
1027 
1028 // This is slightly different to ShHeap::reset_next_mark_bitmap:
1029 // we need to remain able to walk pinned regions.
1030 // Since pinned region do not move and don't get compacted, we will get holes with
1031 // unreachable objects in them (which may have pointers to unloaded Klasses and thus
1032 // cannot be iterated over using oop->size(). The only way to safely iterate over those is using
1033 // a valid marking bitmap and valid TAMS pointer. This class only resets marking
1034 // bitmaps for un-pinned regions, and later we only reset TAMS for unpinned regions.
1035 class ShenandoahMCResetCompleteBitmapTask : public WorkerTask {
1036 private:
1037   ShenandoahRegionIterator _regions;
1038 
1039 public:
1040   ShenandoahMCResetCompleteBitmapTask() :
1041     WorkerTask("Shenandoah Reset Bitmap") {
1042   }
1043 
1044   void work(uint worker_id) {
1045     ShenandoahParallelWorkerSession worker_session(worker_id);
1046     ShenandoahHeapRegion* region = _regions.next();
1047     ShenandoahHeap* heap = ShenandoahHeap::heap();
1048     ShenandoahMarkingContext* const ctx = heap->complete_marking_context();
1049     while (region != nullptr) {
1050       if (heap->is_bitmap_slice_committed(region) && !region->is_pinned() && region->has_live()) {
1051         ctx->clear_bitmap(region);
1052       }
1053       region = _regions.next();
1054     }
1055   }
1056 };
1057 
1058 void ShenandoahFullGC::phase4_compact_objects(ShenandoahHeapRegionSet** worker_slices) {
1059   GCTraceTime(Info, gc, phases) time("Phase 4: Move objects", _gc_timer);
1060   ShenandoahGCPhase compaction_phase(ShenandoahPhaseTimings::full_gc_copy_objects);
1061 
1062   ShenandoahHeap* heap = ShenandoahHeap::heap();
1063 
1064   // Compact regular objects first
1065   {
1066     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_copy_objects_regular);
1067     ShenandoahCompactObjectsTask compact_task(worker_slices);
1068     heap->workers()->run_task(&compact_task);
1069   }
1070 
1071   // Compact humongous objects after regular object moves
1072   {
1073     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_copy_objects_humong);
1074     compact_humongous_objects();
1075   }
1076 }
1077 
1078 void ShenandoahFullGC::phase5_epilog() {
1079   GCTraceTime(Info, gc, phases) time("Phase 5: Full GC epilog", _gc_timer);
1080   ShenandoahHeap* heap = ShenandoahHeap::heap();
1081 
1082   // Reset complete bitmap. We're about to reset the complete-top-at-mark-start pointer
1083   // and must ensure the bitmap is in sync.
1084   {
1085     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_copy_objects_reset_complete);
1086     ShenandoahMCResetCompleteBitmapTask task;
1087     heap->workers()->run_task(&task);
1088   }
1089 
1090   // Bring regions in proper states after the collection, and set heap properties.
1091   {
1092     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_copy_objects_rebuild);
1093     ShenandoahPostCompactClosure post_compact;
1094     heap->heap_region_iterate(&post_compact);
1095     heap->set_used(post_compact.get_live());
1096 
1097     heap->collection_set()->clear();
1098     heap->free_set()->rebuild();
1099     heap->clear_cancelled_gc();
1100   }
1101 
1102   _preserved_marks->restore(heap->workers());
1103   _preserved_marks->reclaim();
1104 }