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/gcTraceTime.inline.hpp"
  29 #include "gc/shared/preservedMarks.inline.hpp"
  30 #include "gc/shared/slidingForwarding.inline.hpp"
  31 #include "gc/shared/tlab_globals.hpp"
  32 #include "gc/shenandoah/heuristics/shenandoahHeuristics.hpp"
  33 #include "gc/shenandoah/shenandoahConcurrentGC.hpp"
  34 #include "gc/shenandoah/shenandoahCollectionSet.hpp"
  35 #include "gc/shenandoah/shenandoahFreeSet.hpp"
  36 #include "gc/shenandoah/shenandoahFullGC.hpp"
  37 #include "gc/shenandoah/shenandoahPhaseTimings.hpp"
  38 #include "gc/shenandoah/shenandoahMark.inline.hpp"
  39 #include "gc/shenandoah/shenandoahMonitoringSupport.hpp"
  40 #include "gc/shenandoah/shenandoahHeapRegionSet.hpp"
  41 #include "gc/shenandoah/shenandoahHeap.inline.hpp"
  42 #include "gc/shenandoah/shenandoahHeapRegion.inline.hpp"
  43 #include "gc/shenandoah/shenandoahMarkingContext.inline.hpp"
  44 #include "gc/shenandoah/shenandoahMetrics.hpp"
  45 #include "gc/shenandoah/shenandoahOopClosures.inline.hpp"
  46 #include "gc/shenandoah/shenandoahReferenceProcessor.hpp"
  47 #include "gc/shenandoah/shenandoahRootProcessor.inline.hpp"
  48 #include "gc/shenandoah/shenandoahSTWMark.hpp"
  49 #include "gc/shenandoah/shenandoahUtils.hpp"
  50 #include "gc/shenandoah/shenandoahVerifier.hpp"
  51 #include "gc/shenandoah/shenandoahVMOperations.hpp"
  52 #include "gc/shenandoah/shenandoahWorkerPolicy.hpp"
  53 #include "memory/metaspaceUtils.hpp"
  54 #include "memory/universe.hpp"
  55 #include "oops/compressedOops.inline.hpp"
  56 #include "oops/oop.inline.hpp"
  57 #include "runtime/biasedLocking.hpp"
  58 #include "runtime/orderAccess.hpp"
  59 #include "runtime/thread.hpp"
  60 #include "runtime/vmThread.hpp"
  61 #include "utilities/copy.hpp"
  62 #include "utilities/events.hpp"
  63 #include "utilities/growableArray.hpp"
  64 #include "gc/shared/workgroup.hpp"
  65 
  66 ShenandoahFullGC::ShenandoahFullGC() :
  67   _gc_timer(ShenandoahHeap::heap()->gc_timer()),
  68   _preserved_marks(new PreservedMarksSet(true)) {}
  69 
  70 ShenandoahFullGC::~ShenandoahFullGC() {
  71   delete _preserved_marks;
  72 }
  73 
  74 bool ShenandoahFullGC::collect(GCCause::Cause cause) {
  75   vmop_entry_full(cause);
  76   // Always success
  77   return true;
  78 }
  79 
  80 void ShenandoahFullGC::vmop_entry_full(GCCause::Cause cause) {
  81   ShenandoahHeap* const heap = ShenandoahHeap::heap();
  82   TraceCollectorStats tcs(heap->monitoring_support()->full_stw_collection_counters());
  83   ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::full_gc_gross);
  84 
  85   heap->try_inject_alloc_failure();
  86   VM_ShenandoahFullGC op(cause, this);
  87   VMThread::execute(&op);
  88 }
  89 
  90 void ShenandoahFullGC::entry_full(GCCause::Cause cause) {
  91   static const char* msg = "Pause Full";
  92   ShenandoahPausePhase gc_phase(msg, ShenandoahPhaseTimings::full_gc, true /* log_heap_usage */);
  93   EventMark em("%s", msg);
  94 
  95   ShenandoahWorkerScope scope(ShenandoahHeap::heap()->workers(),
  96                               ShenandoahWorkerPolicy::calc_workers_for_fullgc(),
  97                               "full gc");
  98 
  99   op_full(cause);
 100 }
 101 
 102 void ShenandoahFullGC::op_full(GCCause::Cause cause) {
 103   ShenandoahMetricsSnapshot metrics;
 104   metrics.snap_before();
 105 
 106   // Perform full GC
 107   do_it(cause);
 108 
 109   metrics.snap_after();
 110 
 111   if (metrics.is_good_progress()) {
 112     ShenandoahHeap::heap()->notify_gc_progress();
 113   } else {
 114     // Nothing to do. Tell the allocation path that we have failed to make
 115     // progress, and it can finally fail.
 116     ShenandoahHeap::heap()->notify_gc_no_progress();
 117   }
 118 }
 119 
 120 void ShenandoahFullGC::do_it(GCCause::Cause gc_cause) {
 121   ShenandoahHeap* heap = ShenandoahHeap::heap();
 122 
 123   if (ShenandoahVerify) {
 124     heap->verifier()->verify_before_fullgc();
 125   }
 126 
 127   if (VerifyBeforeGC) {
 128     Universe::verify();
 129   }
 130 
 131   // Degenerated GC may carry concurrent root flags when upgrading to
 132   // full GC. We need to reset it before mutators resume.
 133   heap->set_concurrent_strong_root_in_progress(false);
 134   heap->set_concurrent_weak_root_in_progress(false);
 135 
 136   heap->set_full_gc_in_progress(true);
 137 
 138   assert(ShenandoahSafepoint::is_at_shenandoah_safepoint(), "must be at a safepoint");
 139   assert(Thread::current()->is_VM_thread(), "Do full GC only while world is stopped");
 140 
 141   {
 142     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_heapdump_pre);
 143     heap->pre_full_gc_dump(_gc_timer);
 144   }
 145 
 146   {
 147     ShenandoahGCPhase prepare_phase(ShenandoahPhaseTimings::full_gc_prepare);
 148     // Full GC is supposed to recover from any GC state:
 149 
 150     // a0. Remember if we have forwarded objects
 151     bool has_forwarded_objects = heap->has_forwarded_objects();
 152 
 153     // a1. Cancel evacuation, if in progress
 154     if (heap->is_evacuation_in_progress()) {
 155       heap->set_evacuation_in_progress(false);
 156     }
 157     assert(!heap->is_evacuation_in_progress(), "sanity");
 158 
 159     // a2. Cancel update-refs, if in progress
 160     if (heap->is_update_refs_in_progress()) {
 161       heap->set_update_refs_in_progress(false);
 162     }
 163     assert(!heap->is_update_refs_in_progress(), "sanity");
 164 
 165     // b. Cancel concurrent mark, if in progress
 166     if (heap->is_concurrent_mark_in_progress()) {
 167       ShenandoahConcurrentGC::cancel();
 168       heap->set_concurrent_mark_in_progress(false);
 169     }
 170     assert(!heap->is_concurrent_mark_in_progress(), "sanity");
 171 
 172     // c. Update roots if this full GC is due to evac-oom, which may carry from-space pointers in roots.
 173     if (has_forwarded_objects) {
 174       update_roots(true /*full_gc*/);
 175     }
 176 
 177     // d. Reset the bitmaps for new marking
 178     heap->reset_mark_bitmap();
 179     assert(heap->marking_context()->is_bitmap_clear(), "sanity");
 180     assert(!heap->marking_context()->is_complete(), "sanity");
 181 
 182     // e. Abandon reference discovery and clear all discovered references.
 183     ShenandoahReferenceProcessor* rp = heap->ref_processor();
 184     rp->abandon_partial_discovery();
 185 
 186     // f. Sync pinned region status from the CP marks
 187     heap->sync_pinned_region_status();
 188 
 189     // The rest of prologue:
 190     BiasedLocking::preserve_marks();
 191     _preserved_marks->init(heap->workers()->active_workers());
 192 
 193     assert(heap->has_forwarded_objects() == has_forwarded_objects, "This should not change");
 194   }
 195 
 196   if (UseTLAB) {
 197     heap->gclabs_retire(ResizeTLAB);
 198     heap->tlabs_retire(ResizeTLAB);
 199   }
 200 
 201   OrderAccess::fence();
 202 
 203   phase1_mark_heap();
 204 
 205   // Once marking is done, which may have fixed up forwarded objects, we can drop it.
 206   // Coming out of Full GC, we would not have any forwarded objects.
 207   // This also prevents resolves with fwdptr from kicking in while adjusting pointers in phase3.
 208   heap->set_has_forwarded_objects(false);
 209 
 210   heap->set_full_gc_move_in_progress(true);
 211 
 212   // Setup workers for the rest
 213   OrderAccess::fence();
 214 
 215   // Initialize worker slices
 216   ShenandoahHeapRegionSet** worker_slices = NEW_C_HEAP_ARRAY(ShenandoahHeapRegionSet*, heap->max_workers(), mtGC);
 217   for (uint i = 0; i < heap->max_workers(); i++) {
 218     worker_slices[i] = new ShenandoahHeapRegionSet();
 219   }
 220 
 221   {
 222     // The rest of code performs region moves, where region status is undefined
 223     // until all phases run together.
 224     ShenandoahHeapLocker lock(heap->lock());
 225 
 226     SlidingForwarding::begin();
 227 
 228     phase2_calculate_target_addresses(worker_slices);
 229 
 230     OrderAccess::fence();
 231 
 232     phase3_update_references();
 233 
 234     phase4_compact_objects(worker_slices);
 235   }
 236 
 237   {
 238     // Epilogue
 239     _preserved_marks->restore(heap->workers());
 240     BiasedLocking::restore_marks();
 241     _preserved_marks->reclaim();
 242     SlidingForwarding::end();
 243   }
 244 
 245   // Resize metaspace
 246   MetaspaceGC::compute_new_size();
 247 
 248   // Free worker slices
 249   for (uint i = 0; i < heap->max_workers(); i++) {
 250     delete worker_slices[i];
 251   }
 252   FREE_C_HEAP_ARRAY(ShenandoahHeapRegionSet*, worker_slices);
 253 
 254   heap->set_full_gc_move_in_progress(false);
 255   heap->set_full_gc_in_progress(false);
 256 
 257   if (ShenandoahVerify) {
 258     heap->verifier()->verify_after_fullgc();
 259   }
 260 
 261   if (VerifyAfterGC) {
 262     Universe::verify();
 263   }
 264 
 265   {
 266     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_heapdump_post);
 267     heap->post_full_gc_dump(_gc_timer);
 268   }
 269 }
 270 
 271 class ShenandoahPrepareForMarkClosure: public ShenandoahHeapRegionClosure {
 272 private:
 273   ShenandoahMarkingContext* const _ctx;
 274 
 275 public:
 276   ShenandoahPrepareForMarkClosure() : _ctx(ShenandoahHeap::heap()->marking_context()) {}
 277 
 278   void heap_region_do(ShenandoahHeapRegion *r) {
 279     _ctx->capture_top_at_mark_start(r);
 280     r->clear_live_data();
 281   }
 282 };
 283 
 284 void ShenandoahFullGC::phase1_mark_heap() {
 285   GCTraceTime(Info, gc, phases) time("Phase 1: Mark live objects", _gc_timer);
 286   ShenandoahGCPhase mark_phase(ShenandoahPhaseTimings::full_gc_mark);
 287 
 288   ShenandoahHeap* heap = ShenandoahHeap::heap();
 289 
 290   ShenandoahPrepareForMarkClosure cl;
 291   heap->heap_region_iterate(&cl);
 292 
 293   heap->set_unload_classes(heap->heuristics()->can_unload_classes());
 294 
 295   ShenandoahReferenceProcessor* rp = heap->ref_processor();
 296   // enable ("weak") refs discovery
 297   rp->set_soft_reference_policy(true); // forcefully purge all soft references
 298 
 299   ShenandoahSTWMark mark(true /*full_gc*/);
 300   mark.mark();
 301   heap->parallel_cleaning(true /* full_gc */);
 302 }
 303 
 304 template <bool ALT_FWD>
 305 class ShenandoahPrepareForCompactionObjectClosure : public ObjectClosure {
 306 private:
 307   PreservedMarks*          const _preserved_marks;
 308   ShenandoahHeap*          const _heap;
 309   GrowableArray<ShenandoahHeapRegion*>& _empty_regions;
 310   int _empty_regions_pos;
 311   ShenandoahHeapRegion*          _to_region;
 312   ShenandoahHeapRegion*          _from_region;
 313   HeapWord* _compact_point;
 314 
 315 public:
 316   ShenandoahPrepareForCompactionObjectClosure(PreservedMarks* preserved_marks,
 317                                               GrowableArray<ShenandoahHeapRegion*>& empty_regions,
 318                                               ShenandoahHeapRegion* to_region) :
 319     _preserved_marks(preserved_marks),
 320     _heap(ShenandoahHeap::heap()),
 321     _empty_regions(empty_regions),
 322     _empty_regions_pos(0),
 323     _to_region(to_region),
 324     _from_region(NULL),
 325     _compact_point(to_region->bottom()) {}
 326 
 327   void set_from_region(ShenandoahHeapRegion* from_region) {
 328     _from_region = from_region;
 329   }
 330 
 331   void finish_region() {
 332     assert(_to_region != NULL, "should not happen");
 333     _to_region->set_new_top(_compact_point);
 334   }
 335 
 336   bool is_compact_same_region() {
 337     return _from_region == _to_region;
 338   }
 339 
 340   int empty_regions_pos() {
 341     return _empty_regions_pos;
 342   }
 343 
 344   void do_object(oop p) {
 345     assert(_from_region != NULL, "must set before work");
 346     assert(_heap->complete_marking_context()->is_marked(p), "must be marked");
 347     assert(!_heap->complete_marking_context()->allocated_after_mark_start(p), "must be truly marked");
 348 
 349     size_t obj_size = p->size();
 350     if (_compact_point + obj_size > _to_region->end()) {
 351       finish_region();
 352 
 353       // Object doesn't fit. Pick next empty region and start compacting there.
 354       ShenandoahHeapRegion* new_to_region;
 355       if (_empty_regions_pos < _empty_regions.length()) {
 356         new_to_region = _empty_regions.at(_empty_regions_pos);
 357         _empty_regions_pos++;
 358       } else {
 359         // Out of empty region? Compact within the same region.
 360         new_to_region = _from_region;
 361       }
 362 
 363       assert(new_to_region != _to_region, "must not reuse same to-region");
 364       assert(new_to_region != NULL, "must not be NULL");
 365       _to_region = new_to_region;
 366       _compact_point = _to_region->bottom();
 367     }
 368 
 369     // Object fits into current region, record new location:
 370     assert(_compact_point + obj_size <= _to_region->end(), "must fit");
 371     shenandoah_assert_not_forwarded(NULL, p);
 372     _preserved_marks->push_if_necessary(p, p->mark());
 373     SlidingForwarding::forward_to<ALT_FWD>(p, cast_to_oop(_compact_point));
 374     _compact_point += obj_size;
 375   }
 376 };
 377 
 378 class ShenandoahPrepareForCompactionTask : public AbstractGangTask {
 379 private:
 380   PreservedMarksSet*        const _preserved_marks;
 381   ShenandoahHeap*           const _heap;
 382   ShenandoahHeapRegionSet** const _worker_slices;
 383 
 384 public:
 385   ShenandoahPrepareForCompactionTask(PreservedMarksSet *preserved_marks, ShenandoahHeapRegionSet **worker_slices) :
 386     AbstractGangTask("Shenandoah Prepare For Compaction"),
 387     _preserved_marks(preserved_marks),
 388     _heap(ShenandoahHeap::heap()), _worker_slices(worker_slices) {
 389   }
 390 
 391   static bool is_candidate_region(ShenandoahHeapRegion* r) {
 392     // Empty region: get it into the slice to defragment the slice itself.
 393     // We could have skipped this without violating correctness, but we really
 394     // want to compact all live regions to the start of the heap, which sometimes
 395     // means moving them into the fully empty regions.
 396     if (r->is_empty()) return true;
 397 
 398     // Can move the region, and this is not the humongous region. Humongous
 399     // moves are special cased here, because their moves are handled separately.
 400     return r->is_stw_move_allowed() && !r->is_humongous();
 401   }
 402 
 403   void work(uint worker_id) {
 404     if (UseAltGCForwarding) {
 405       work_impl<true>(worker_id);
 406     } else {
 407       work_impl<false>(worker_id);
 408     }
 409   }
 410 
 411 private:
 412   template <bool ALT_FWD>
 413   void work_impl(uint worker_id) {
 414     ShenandoahParallelWorkerSession worker_session(worker_id);
 415     ShenandoahHeapRegionSet* slice = _worker_slices[worker_id];
 416     ShenandoahHeapRegionSetIterator it(slice);
 417     ShenandoahHeapRegion* from_region = it.next();
 418     // No work?
 419     if (from_region == NULL) {
 420        return;
 421     }
 422 
 423     // Sliding compaction. Walk all regions in the slice, and compact them.
 424     // Remember empty regions and reuse them as needed.
 425     ResourceMark rm;
 426 
 427     GrowableArray<ShenandoahHeapRegion*> empty_regions((int)_heap->num_regions());
 428 
 429     ShenandoahPrepareForCompactionObjectClosure<ALT_FWD> cl(_preserved_marks->get(worker_id), empty_regions, from_region);
 430 
 431     while (from_region != NULL) {
 432       assert(is_candidate_region(from_region), "Sanity");
 433 
 434       cl.set_from_region(from_region);
 435       if (from_region->has_live()) {
 436         _heap->marked_object_iterate(from_region, &cl);
 437       }
 438 
 439       // Compacted the region to somewhere else? From-region is empty then.
 440       if (!cl.is_compact_same_region()) {
 441         empty_regions.append(from_region);
 442       }
 443       from_region = it.next();
 444     }
 445     cl.finish_region();
 446 
 447     // Mark all remaining regions as empty
 448     for (int pos = cl.empty_regions_pos(); pos < empty_regions.length(); ++pos) {
 449       ShenandoahHeapRegion* r = empty_regions.at(pos);
 450       r->set_new_top(r->bottom());
 451     }
 452   }
 453 };
 454 
 455 template <bool ALT_FWD>
 456 void ShenandoahFullGC::calculate_target_humongous_objects_impl() {
 457   ShenandoahHeap* heap = ShenandoahHeap::heap();
 458 
 459   // Compute the new addresses for humongous objects. We need to do this after addresses
 460   // for regular objects are calculated, and we know what regions in heap suffix are
 461   // available for humongous moves.
 462   //
 463   // Scan the heap backwards, because we are compacting humongous regions towards the end.
 464   // Maintain the contiguous compaction window in [to_begin; to_end), so that we can slide
 465   // humongous start there.
 466   //
 467   // The complication is potential non-movable regions during the scan. If such region is
 468   // detected, then sliding restarts towards that non-movable region.
 469 
 470   size_t to_begin = heap->num_regions();
 471   size_t to_end = heap->num_regions();
 472 
 473   for (size_t c = heap->num_regions(); c > 0; c--) {
 474     ShenandoahHeapRegion *r = heap->get_region(c - 1);
 475     if (r->is_humongous_continuation() || (r->new_top() == r->bottom())) {
 476       // To-region candidate: record this, and continue scan
 477       to_begin = r->index();
 478       continue;
 479     }
 480 
 481     if (r->is_humongous_start() && r->is_stw_move_allowed()) {
 482       // From-region candidate: movable humongous region
 483       oop old_obj = cast_to_oop(r->bottom());
 484       size_t words_size = old_obj->size();
 485       size_t num_regions = ShenandoahHeapRegion::required_regions(words_size * HeapWordSize);
 486 
 487       size_t start = to_end - num_regions;
 488 
 489       if (start >= to_begin && start != r->index()) {
 490         // Fits into current window, and the move is non-trivial. Record the move then, and continue scan.
 491         _preserved_marks->get(0)->push_if_necessary(old_obj, old_obj->mark());
 492         SlidingForwarding::forward_to<ALT_FWD>(old_obj, cast_to_oop(heap->get_region(start)->bottom()));
 493         to_end = start;
 494         continue;
 495       }
 496     }
 497 
 498     // Failed to fit. Scan starting from current region.
 499     to_begin = r->index();
 500     to_end = r->index();
 501   }
 502 }
 503 
 504 void ShenandoahFullGC::calculate_target_humongous_objects() {
 505   if (UseAltGCForwarding) {
 506     calculate_target_humongous_objects_impl<true>();
 507   } else {
 508     calculate_target_humongous_objects_impl<false>();
 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 != NULL) {
 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 template <bool ALT_FWD>
 750 class ShenandoahAdjustPointersClosure : public MetadataVisitingOopIterateClosure {
 751 private:
 752   ShenandoahHeap* const _heap;
 753   ShenandoahMarkingContext* const _ctx;
 754 
 755   template <class T>
 756   inline void do_oop_work(T* p) {
 757     T o = RawAccess<>::oop_load(p);
 758     if (!CompressedOops::is_null(o)) {
 759       oop obj = CompressedOops::decode_not_null(o);
 760       assert(_ctx->is_marked(obj), "must be marked");
 761       if (SlidingForwarding::is_forwarded(obj)) {
 762         oop forw = SlidingForwarding::forwardee<ALT_FWD>(obj);
 763         RawAccess<IS_NOT_NULL>::oop_store(p, forw);
 764       }
 765     }
 766   }
 767 
 768 public:
 769   ShenandoahAdjustPointersClosure() :
 770     _heap(ShenandoahHeap::heap()),
 771     _ctx(ShenandoahHeap::heap()->complete_marking_context()) {}
 772 
 773   void do_oop(oop* p)       { do_oop_work(p); }
 774   void do_oop(narrowOop* p) { do_oop_work(p); }
 775 };
 776 
 777 template <bool ALT_FWD>
 778 class ShenandoahAdjustPointersObjectClosure : public ObjectClosure {
 779 private:
 780   ShenandoahHeap* const _heap;
 781   ShenandoahAdjustPointersClosure<ALT_FWD> _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 AbstractGangTask {
 794 private:
 795   ShenandoahHeap*          const _heap;
 796   ShenandoahRegionIterator       _regions;
 797 
 798 public:
 799   ShenandoahAdjustPointersTask() :
 800     AbstractGangTask("Shenandoah Adjust Pointers"),
 801     _heap(ShenandoahHeap::heap()) {
 802   }
 803 
 804 private:
 805   template <bool ALT_FWD>
 806   void work_impl(uint worker_id) {
 807     ShenandoahParallelWorkerSession worker_session(worker_id);
 808     ShenandoahAdjustPointersObjectClosure<ALT_FWD> obj_cl;
 809     ShenandoahHeapRegion* r = _regions.next();
 810     while (r != NULL) {
 811       if (!r->is_humongous_continuation() && r->has_live()) {
 812         _heap->marked_object_iterate(r, &obj_cl);
 813       }
 814       r = _regions.next();
 815     }
 816   }
 817 
 818 public:
 819   void work(uint worker_id) {
 820     if (UseAltGCForwarding) {
 821       work_impl<true>(worker_id);
 822     } else {
 823       work_impl<false>(worker_id);
 824     }
 825   }
 826 };
 827 
 828 class ShenandoahAdjustRootPointersTask : public AbstractGangTask {
 829 private:
 830   ShenandoahRootAdjuster* _rp;
 831   PreservedMarksSet* _preserved_marks;
 832 
 833 public:
 834   ShenandoahAdjustRootPointersTask(ShenandoahRootAdjuster* rp, PreservedMarksSet* preserved_marks) :
 835     AbstractGangTask("Shenandoah Adjust Root Pointers"),
 836     _rp(rp),
 837     _preserved_marks(preserved_marks) {}
 838 
 839 private:
 840   template <bool ALT_FWD>
 841   void work_impl(uint worker_id) {
 842     ShenandoahParallelWorkerSession worker_session(worker_id);
 843     ShenandoahAdjustPointersClosure<ALT_FWD> cl;
 844     _rp->roots_do(worker_id, &cl);
 845     _preserved_marks->get(worker_id)->adjust_during_full_gc();
 846   }
 847 
 848 public:
 849   void work(uint worker_id) {
 850     if (UseAltGCForwarding) {
 851       work_impl<true>(worker_id);
 852     } else {
 853       work_impl<false>(worker_id);
 854     }
 855   }
 856 };
 857 
 858 void ShenandoahFullGC::phase3_update_references() {
 859   GCTraceTime(Info, gc, phases) time("Phase 3: Adjust pointers", _gc_timer);
 860   ShenandoahGCPhase adjust_pointer_phase(ShenandoahPhaseTimings::full_gc_adjust_pointers);
 861 
 862   ShenandoahHeap* heap = ShenandoahHeap::heap();
 863 
 864   WorkGang* workers = heap->workers();
 865   uint nworkers = workers->active_workers();
 866   {
 867 #if COMPILER2_OR_JVMCI
 868     DerivedPointerTable::clear();
 869 #endif
 870     ShenandoahRootAdjuster rp(nworkers, ShenandoahPhaseTimings::full_gc_adjust_roots);
 871     ShenandoahAdjustRootPointersTask task(&rp, _preserved_marks);
 872     workers->run_task(&task);
 873 #if COMPILER2_OR_JVMCI
 874     DerivedPointerTable::update_pointers();
 875 #endif
 876   }
 877 
 878   ShenandoahAdjustPointersTask adjust_pointers_task;
 879   workers->run_task(&adjust_pointers_task);
 880 }
 881 
 882 template <bool ALT_FWD>
 883 class ShenandoahCompactObjectsClosure : public ObjectClosure {
 884 private:
 885   ShenandoahHeap* const _heap;
 886   uint            const _worker_id;
 887 
 888 public:
 889   ShenandoahCompactObjectsClosure(uint worker_id) :
 890     _heap(ShenandoahHeap::heap()), _worker_id(worker_id) {}
 891 
 892   void do_object(oop p) {
 893     assert(_heap->complete_marking_context()->is_marked(p), "must be marked");
 894     size_t size = (size_t)p->size();
 895     if (SlidingForwarding::is_forwarded(p)) {
 896       HeapWord* compact_from = cast_from_oop<HeapWord*>(p);
 897       HeapWord* compact_to = cast_from_oop<HeapWord*>(SlidingForwarding::forwardee<ALT_FWD>(p));
 898       Copy::aligned_conjoint_words(compact_from, compact_to, size);
 899       oop new_obj = cast_to_oop(compact_to);
 900       new_obj->init_mark();
 901     }
 902   }
 903 };
 904 
 905 class ShenandoahCompactObjectsTask : public AbstractGangTask {
 906 private:
 907   ShenandoahHeap* const _heap;
 908   ShenandoahHeapRegionSet** const _worker_slices;
 909 
 910 public:
 911   ShenandoahCompactObjectsTask(ShenandoahHeapRegionSet** worker_slices) :
 912     AbstractGangTask("Shenandoah Compact Objects"),
 913     _heap(ShenandoahHeap::heap()),
 914     _worker_slices(worker_slices) {
 915   }
 916 
 917 private:
 918   template <bool ALT_FWD>
 919   void work_impl(uint worker_id) {
 920     ShenandoahParallelWorkerSession worker_session(worker_id);
 921     ShenandoahHeapRegionSetIterator slice(_worker_slices[worker_id]);
 922 
 923     ShenandoahCompactObjectsClosure<ALT_FWD> cl(worker_id);
 924     ShenandoahHeapRegion* r = slice.next();
 925     while (r != NULL) {
 926       assert(!r->is_humongous(), "must not get humongous regions here");
 927       if (r->has_live()) {
 928         _heap->marked_object_iterate(r, &cl);
 929       }
 930       r->set_top(r->new_top());
 931       r = slice.next();
 932     }
 933   }
 934 
 935 public:
 936   void work(uint worker_id) {
 937     if (UseAltGCForwarding) {
 938       work_impl<true>(worker_id);
 939     } else {
 940       work_impl<false>(worker_id);
 941     }
 942   }
 943 };
 944 
 945 class ShenandoahPostCompactClosure : public ShenandoahHeapRegionClosure {
 946 private:
 947   ShenandoahHeap* const _heap;
 948   size_t _live;
 949 
 950 public:
 951   ShenandoahPostCompactClosure() : _heap(ShenandoahHeap::heap()), _live(0) {
 952     _heap->free_set()->clear();
 953   }
 954 
 955   void heap_region_do(ShenandoahHeapRegion* r) {
 956     assert (!r->is_cset(), "cset regions should have been demoted already");
 957 
 958     // Need to reset the complete-top-at-mark-start pointer here because
 959     // the complete marking bitmap is no longer valid. This ensures
 960     // size-based iteration in marked_object_iterate().
 961     // NOTE: See blurb at ShenandoahMCResetCompleteBitmapTask on why we need to skip
 962     // pinned regions.
 963     if (!r->is_pinned()) {
 964       _heap->complete_marking_context()->reset_top_at_mark_start(r);
 965     }
 966 
 967     size_t live = r->used();
 968 
 969     // Make empty regions that have been allocated into regular
 970     if (r->is_empty() && live > 0) {
 971       r->make_regular_bypass();
 972       if (ZapUnusedHeapArea) {
 973         SpaceMangler::mangle_region(MemRegion(r->top(), r->end()));
 974       }
 975     }
 976 
 977     // Reclaim regular regions that became empty
 978     if (r->is_regular() && live == 0) {
 979       r->make_trash();
 980     }
 981 
 982     // Recycle all trash regions
 983     if (r->is_trash()) {
 984       live = 0;
 985       r->recycle();
 986     }
 987 
 988     r->set_live_data(live);
 989     r->reset_alloc_metadata();
 990     _live += live;
 991   }
 992 
 993   size_t get_live() {
 994     return _live;
 995   }
 996 };
 997 
 998 template <bool ALT_FWD>
 999 void ShenandoahFullGC::compact_humongous_objects_impl() {
1000   // Compact humongous regions, based on their fwdptr objects.
1001   //
1002   // This code is serial, because doing the in-slice parallel sliding is tricky. In most cases,
1003   // humongous regions are already compacted, and do not require further moves, which alleviates
1004   // sliding costs. We may consider doing this in parallel in future.
1005 
1006   ShenandoahHeap* heap = ShenandoahHeap::heap();
1007 
1008   for (size_t c = heap->num_regions(); c > 0; c--) {
1009     ShenandoahHeapRegion* r = heap->get_region(c - 1);
1010     if (r->is_humongous_start()) {
1011       oop old_obj = cast_to_oop(r->bottom());
1012       if (SlidingForwarding::is_not_forwarded(old_obj)) {
1013         // No need to move the object, it stays at the same slot
1014         continue;
1015       }
1016       size_t words_size = old_obj->size();
1017       size_t num_regions = ShenandoahHeapRegion::required_regions(words_size * HeapWordSize);
1018 
1019       size_t old_start = r->index();
1020       size_t old_end   = old_start + num_regions - 1;
1021       size_t new_start = heap->heap_region_index_containing(SlidingForwarding::forwardee<ALT_FWD>(old_obj));
1022       size_t new_end   = new_start + num_regions - 1;
1023       assert(old_start != new_start, "must be real move");
1024       assert(r->is_stw_move_allowed(), "Region " SIZE_FORMAT " should be movable", r->index());
1025 
1026       Copy::aligned_conjoint_words(heap->get_region(old_start)->bottom(),
1027                                    heap->get_region(new_start)->bottom(),
1028                                    words_size);
1029 
1030       oop new_obj = cast_to_oop(heap->get_region(new_start)->bottom());
1031       new_obj->init_mark();
1032 
1033       {
1034         for (size_t c = old_start; c <= old_end; c++) {
1035           ShenandoahHeapRegion* r = heap->get_region(c);
1036           r->make_regular_bypass();
1037           r->set_top(r->bottom());
1038         }
1039 
1040         for (size_t c = new_start; c <= new_end; c++) {
1041           ShenandoahHeapRegion* r = heap->get_region(c);
1042           if (c == new_start) {
1043             r->make_humongous_start_bypass();
1044           } else {
1045             r->make_humongous_cont_bypass();
1046           }
1047 
1048           // Trailing region may be non-full, record the remainder there
1049           size_t remainder = words_size & ShenandoahHeapRegion::region_size_words_mask();
1050           if ((c == new_end) && (remainder != 0)) {
1051             r->set_top(r->bottom() + remainder);
1052           } else {
1053             r->set_top(r->end());
1054           }
1055 
1056           r->reset_alloc_metadata();
1057         }
1058       }
1059     }
1060   }
1061 }
1062 
1063 void ShenandoahFullGC::compact_humongous_objects() {
1064   if (UseAltGCForwarding) {
1065     compact_humongous_objects_impl<true>();
1066   } else {
1067     compact_humongous_objects_impl<false>();
1068   }
1069 }
1070 
1071 // This is slightly different to ShHeap::reset_next_mark_bitmap:
1072 // we need to remain able to walk pinned regions.
1073 // Since pinned region do not move and don't get compacted, we will get holes with
1074 // unreachable objects in them (which may have pointers to unloaded Klasses and thus
1075 // cannot be iterated over using oop->size(). The only way to safely iterate over those is using
1076 // a valid marking bitmap and valid TAMS pointer. This class only resets marking
1077 // bitmaps for un-pinned regions, and later we only reset TAMS for unpinned regions.
1078 class ShenandoahMCResetCompleteBitmapTask : public AbstractGangTask {
1079 private:
1080   ShenandoahRegionIterator _regions;
1081 
1082 public:
1083   ShenandoahMCResetCompleteBitmapTask() :
1084     AbstractGangTask("Shenandoah Reset Bitmap") {
1085   }
1086 
1087   void work(uint worker_id) {
1088     ShenandoahParallelWorkerSession worker_session(worker_id);
1089     ShenandoahHeapRegion* region = _regions.next();
1090     ShenandoahHeap* heap = ShenandoahHeap::heap();
1091     ShenandoahMarkingContext* const ctx = heap->complete_marking_context();
1092     while (region != NULL) {
1093       if (heap->is_bitmap_slice_committed(region) && !region->is_pinned() && region->has_live()) {
1094         ctx->clear_bitmap(region);
1095       }
1096       region = _regions.next();
1097     }
1098   }
1099 };
1100 
1101 void ShenandoahFullGC::phase4_compact_objects(ShenandoahHeapRegionSet** worker_slices) {
1102   GCTraceTime(Info, gc, phases) time("Phase 4: Move objects", _gc_timer);
1103   ShenandoahGCPhase compaction_phase(ShenandoahPhaseTimings::full_gc_copy_objects);
1104 
1105   ShenandoahHeap* heap = ShenandoahHeap::heap();
1106 
1107   // Compact regular objects first
1108   {
1109     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_copy_objects_regular);
1110     ShenandoahCompactObjectsTask compact_task(worker_slices);
1111     heap->workers()->run_task(&compact_task);
1112   }
1113 
1114   // Compact humongous objects after regular object moves
1115   {
1116     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_copy_objects_humong);
1117     compact_humongous_objects();
1118   }
1119 
1120   // Reset complete bitmap. We're about to reset the complete-top-at-mark-start pointer
1121   // and must ensure the bitmap is in sync.
1122   {
1123     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_copy_objects_reset_complete);
1124     ShenandoahMCResetCompleteBitmapTask task;
1125     heap->workers()->run_task(&task);
1126   }
1127 
1128   // Bring regions in proper states after the collection, and set heap properties.
1129   {
1130     ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_copy_objects_rebuild);
1131 
1132     ShenandoahPostCompactClosure post_compact;
1133     heap->heap_region_iterate(&post_compact);
1134     heap->set_used(post_compact.get_live());
1135 
1136     heap->collection_set()->clear();
1137     heap->free_set()->rebuild();
1138   }
1139 
1140   heap->clear_cancelled_gc();
1141 }