1 /*
2 * Copyright (c) 2014, 2021, Red Hat, Inc. All rights reserved.
3 * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
4 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
5 *
6 * This code is free software; you can redistribute it and/or modify it
7 * under the terms of the GNU General Public License version 2 only, as
8 * published by the Free Software Foundation.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 *
24 */
25
26 #include "precompiled.hpp"
27
28 #include "compiler/oopMap.hpp"
29 #include "gc/shared/continuationGCSupport.hpp"
30 #include "gc/shared/fullGCForwarding.inline.hpp"
31 #include "gc/shared/gcTraceTime.inline.hpp"
32 #include "gc/shared/preservedMarks.inline.hpp"
33 #include "gc/shared/tlab_globals.hpp"
34 #include "gc/shared/workerThread.hpp"
35 #include "gc/shenandoah/heuristics/shenandoahHeuristics.hpp"
36 #include "gc/shenandoah/shenandoahClosures.inline.hpp"
37 #include "gc/shenandoah/shenandoahCollectorPolicy.hpp"
38 #include "gc/shenandoah/shenandoahConcurrentGC.hpp"
39 #include "gc/shenandoah/shenandoahCollectionSet.hpp"
40 #include "gc/shenandoah/shenandoahCollectorPolicy.hpp"
41 #include "gc/shenandoah/shenandoahFreeSet.hpp"
42 #include "gc/shenandoah/shenandoahFullGC.hpp"
43 #include "gc/shenandoah/shenandoahGenerationalFullGC.hpp"
44 #include "gc/shenandoah/shenandoahGlobalGeneration.hpp"
45 #include "gc/shenandoah/shenandoahPhaseTimings.hpp"
46 #include "gc/shenandoah/shenandoahMark.inline.hpp"
47 #include "gc/shenandoah/shenandoahMonitoringSupport.hpp"
48 #include "gc/shenandoah/shenandoahHeapRegionClosures.hpp"
49 #include "gc/shenandoah/shenandoahHeapRegionSet.hpp"
50 #include "gc/shenandoah/shenandoahHeap.inline.hpp"
51 #include "gc/shenandoah/shenandoahHeapRegion.inline.hpp"
52 #include "gc/shenandoah/shenandoahMarkingContext.inline.hpp"
53 #include "gc/shenandoah/shenandoahMetrics.hpp"
54 #include "gc/shenandoah/shenandoahReferenceProcessor.hpp"
55 #include "gc/shenandoah/shenandoahRootProcessor.inline.hpp"
56 #include "gc/shenandoah/shenandoahSTWMark.hpp"
57 #include "gc/shenandoah/shenandoahUtils.hpp"
58 #include "gc/shenandoah/shenandoahVerifier.hpp"
59 #include "gc/shenandoah/shenandoahVMOperations.hpp"
60 #include "gc/shenandoah/shenandoahWorkerPolicy.hpp"
61 #include "memory/metaspaceUtils.hpp"
62 #include "memory/universe.hpp"
63 #include "oops/compressedOops.inline.hpp"
64 #include "oops/oop.inline.hpp"
65 #include "runtime/orderAccess.hpp"
66 #include "runtime/vmThread.hpp"
67 #include "utilities/copy.hpp"
68 #include "utilities/events.hpp"
69 #include "utilities/growableArray.hpp"
70
71 ShenandoahFullGC::ShenandoahFullGC() :
72 _gc_timer(ShenandoahHeap::heap()->gc_timer()),
73 _preserved_marks(new PreservedMarksSet(true)) {}
74
75 ShenandoahFullGC::~ShenandoahFullGC() {
76 delete _preserved_marks;
77 }
78
79 bool ShenandoahFullGC::collect(GCCause::Cause cause) {
80 vmop_entry_full(cause);
81 // Always success
82 return true;
83 }
84
85 void ShenandoahFullGC::vmop_entry_full(GCCause::Cause cause) {
86 ShenandoahHeap* const heap = ShenandoahHeap::heap();
87 TraceCollectorStats tcs(heap->monitoring_support()->full_stw_collection_counters());
88 ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::full_gc_gross);
89
90 heap->try_inject_alloc_failure();
91 VM_ShenandoahFullGC op(cause, this);
92 VMThread::execute(&op);
93 }
94
95 void ShenandoahFullGC::entry_full(GCCause::Cause cause) {
96 static const char* msg = "Pause Full";
97 ShenandoahPausePhase gc_phase(msg, ShenandoahPhaseTimings::full_gc, true /* log_heap_usage */);
98 EventMark em("%s", msg);
99
100 ShenandoahWorkerScope scope(ShenandoahHeap::heap()->workers(),
101 ShenandoahWorkerPolicy::calc_workers_for_fullgc(),
102 "full gc");
103
104 op_full(cause);
105 }
106
107 void ShenandoahFullGC::op_full(GCCause::Cause cause) {
108 ShenandoahMetricsSnapshot metrics;
109 metrics.snap_before();
110
111 // Perform full GC
112 do_it(cause);
113
114 ShenandoahHeap* const heap = ShenandoahHeap::heap();
115
116 if (heap->mode()->is_generational()) {
117 ShenandoahGenerationalFullGC::handle_completion(heap);
118 }
119
120 metrics.snap_after();
121
122 if (metrics.is_good_progress()) {
123 heap->notify_gc_progress();
124 } else {
125 // Nothing to do. Tell the allocation path that we have failed to make
126 // progress, and it can finally fail.
127 heap->notify_gc_no_progress();
128 }
129
130 // Regardless if progress was made, we record that we completed a "successful" full GC.
131 heap->global_generation()->heuristics()->record_success_full();
132 heap->shenandoah_policy()->record_success_full();
133 }
134
135 void ShenandoahFullGC::do_it(GCCause::Cause gc_cause) {
136 ShenandoahHeap* heap = ShenandoahHeap::heap();
137
138 if (heap->mode()->is_generational()) {
139 ShenandoahGenerationalFullGC::prepare();
140 }
141
142 if (ShenandoahVerify) {
143 heap->verifier()->verify_before_fullgc();
144 }
145
146 if (VerifyBeforeGC) {
147 Universe::verify();
148 }
149
150 // Degenerated GC may carry concurrent root flags when upgrading to
151 // full GC. We need to reset it before mutators resume.
152 heap->set_concurrent_strong_root_in_progress(false);
153 heap->set_concurrent_weak_root_in_progress(false);
154
155 heap->set_full_gc_in_progress(true);
156
157 assert(ShenandoahSafepoint::is_at_shenandoah_safepoint(), "must be at a safepoint");
158 assert(Thread::current()->is_VM_thread(), "Do full GC only while world is stopped");
159
160 {
161 ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_heapdump_pre);
162 heap->pre_full_gc_dump(_gc_timer);
163 }
164
165 {
166 ShenandoahGCPhase prepare_phase(ShenandoahPhaseTimings::full_gc_prepare);
167 // Full GC is supposed to recover from any GC state:
168
169 // a0. Remember if we have forwarded objects
170 bool has_forwarded_objects = heap->has_forwarded_objects();
171
172 // a1. Cancel evacuation, if in progress
173 if (heap->is_evacuation_in_progress()) {
174 heap->set_evacuation_in_progress(false);
175 }
176 assert(!heap->is_evacuation_in_progress(), "sanity");
177
178 // a2. Cancel update-refs, if in progress
179 if (heap->is_update_refs_in_progress()) {
180 heap->set_update_refs_in_progress(false);
181 }
182 assert(!heap->is_update_refs_in_progress(), "sanity");
183
184 // b. Cancel all concurrent marks, if in progress
185 if (heap->is_concurrent_mark_in_progress()) {
186 heap->cancel_concurrent_mark();
187 }
188 assert(!heap->is_concurrent_mark_in_progress(), "sanity");
189
190 // c. Update roots if this full GC is due to evac-oom, which may carry from-space pointers in roots.
191 if (has_forwarded_objects) {
192 update_roots(true /*full_gc*/);
193 }
194
195 // d. Reset the bitmaps for new marking
196 heap->global_generation()->reset_mark_bitmap();
197 assert(heap->marking_context()->is_bitmap_clear(), "sanity");
198 assert(!heap->global_generation()->is_mark_complete(), "sanity");
199
200 // e. Abandon reference discovery and clear all discovered references.
201 ShenandoahReferenceProcessor* rp = heap->global_generation()->ref_processor();
202 rp->abandon_partial_discovery();
203
204 // f. Sync pinned region status from the CP marks
205 heap->sync_pinned_region_status();
206
207 if (heap->mode()->is_generational()) {
208 ShenandoahGenerationalFullGC::restore_top_before_promote(heap);
209 }
210
211 // The rest of prologue:
212 _preserved_marks->init(heap->workers()->active_workers());
213
214 assert(heap->has_forwarded_objects() == has_forwarded_objects, "This should not change");
215 }
216
217 if (UseTLAB) {
218 // Note: PLABs are also retired with GCLABs in generational mode.
219 heap->gclabs_retire(ResizeTLAB);
220 heap->tlabs_retire(ResizeTLAB);
221 }
222
223 OrderAccess::fence();
224
225 phase1_mark_heap();
226
227 // Once marking is done, which may have fixed up forwarded objects, we can drop it.
228 // Coming out of Full GC, we would not have any forwarded objects.
229 // This also prevents resolves with fwdptr from kicking in while adjusting pointers in phase3.
230 heap->set_has_forwarded_objects(false);
231
232 heap->set_full_gc_move_in_progress(true);
233
234 // Setup workers for the rest
235 OrderAccess::fence();
236
237 // Initialize worker slices
238 ShenandoahHeapRegionSet** worker_slices = NEW_C_HEAP_ARRAY(ShenandoahHeapRegionSet*, heap->max_workers(), mtGC);
239 for (uint i = 0; i < heap->max_workers(); i++) {
240 worker_slices[i] = new ShenandoahHeapRegionSet();
241 }
242
243 {
244 // The rest of code performs region moves, where region status is undefined
245 // until all phases run together.
246 ShenandoahHeapLocker lock(heap->lock());
247
248 phase2_calculate_target_addresses(worker_slices);
249
250 OrderAccess::fence();
251
252 phase3_update_references();
253
254 phase4_compact_objects(worker_slices);
255
256 phase5_epilog();
257 }
258
259 // Resize metaspace
260 MetaspaceGC::compute_new_size();
261
262 // Free worker slices
263 for (uint i = 0; i < heap->max_workers(); i++) {
264 delete worker_slices[i];
265 }
266 FREE_C_HEAP_ARRAY(ShenandoahHeapRegionSet*, worker_slices);
267
268 heap->set_full_gc_move_in_progress(false);
269 heap->set_full_gc_in_progress(false);
270
271 if (ShenandoahVerify) {
272 heap->verifier()->verify_after_fullgc();
273 }
274
275 if (VerifyAfterGC) {
276 Universe::verify();
277 }
278
279 {
280 ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_heapdump_post);
281 heap->post_full_gc_dump(_gc_timer);
282 }
283 }
284
285 class ShenandoahPrepareForMarkClosure: public ShenandoahHeapRegionClosure {
286 private:
287 ShenandoahMarkingContext* const _ctx;
288
289 public:
290 ShenandoahPrepareForMarkClosure() : _ctx(ShenandoahHeap::heap()->marking_context()) {}
291
292 void heap_region_do(ShenandoahHeapRegion *r) override {
293 _ctx->capture_top_at_mark_start(r);
294 r->clear_live_data();
295 }
296
297 bool is_thread_safe() override { return true; }
298 };
299
300 void ShenandoahFullGC::phase1_mark_heap() {
301 GCTraceTime(Info, gc, phases) time("Phase 1: Mark live objects", _gc_timer);
302 ShenandoahGCPhase mark_phase(ShenandoahPhaseTimings::full_gc_mark);
303
304 ShenandoahHeap* heap = ShenandoahHeap::heap();
305
306 ShenandoahPrepareForMarkClosure prepare_for_mark;
307 ShenandoahExcludeRegionClosure<FREE> cl(&prepare_for_mark);
308 heap->parallel_heap_region_iterate(&cl);
309
310 heap->set_unload_classes(heap->global_generation()->heuristics()->can_unload_classes());
311
312 ShenandoahReferenceProcessor* rp = heap->global_generation()->ref_processor();
313 // enable ("weak") refs discovery
314 rp->set_soft_reference_policy(true); // forcefully purge all soft references
315
316 ShenandoahSTWMark mark(heap->global_generation(), true /*full_gc*/);
317 mark.mark();
318 heap->parallel_cleaning(true /* full_gc */);
319
320 if (ShenandoahHeap::heap()->mode()->is_generational()) {
321 ShenandoahGenerationalFullGC::log_live_in_old(heap);
322 }
323 }
324
325 class ShenandoahPrepareForCompactionObjectClosure : public ObjectClosure {
326 private:
327 PreservedMarks* const _preserved_marks;
328 ShenandoahHeap* const _heap;
329 GrowableArray<ShenandoahHeapRegion*>& _empty_regions;
330 int _empty_regions_pos;
331 ShenandoahHeapRegion* _to_region;
332 ShenandoahHeapRegion* _from_region;
333 HeapWord* _compact_point;
334
335 public:
336 ShenandoahPrepareForCompactionObjectClosure(PreservedMarks* preserved_marks,
337 GrowableArray<ShenandoahHeapRegion*>& empty_regions,
338 ShenandoahHeapRegion* to_region) :
339 _preserved_marks(preserved_marks),
340 _heap(ShenandoahHeap::heap()),
341 _empty_regions(empty_regions),
342 _empty_regions_pos(0),
343 _to_region(to_region),
344 _from_region(nullptr),
345 _compact_point(to_region->bottom()) {}
346
347 void set_from_region(ShenandoahHeapRegion* from_region) {
348 _from_region = from_region;
349 }
350
351 void finish() {
352 assert(_to_region != nullptr, "should not happen");
353 _to_region->set_new_top(_compact_point);
354 }
355
356 bool is_compact_same_region() {
357 return _from_region == _to_region;
358 }
359
360 int empty_regions_pos() {
361 return _empty_regions_pos;
362 }
363
364 void do_object(oop p) {
365 assert(_from_region != nullptr, "must set before work");
366 assert(_heap->complete_marking_context()->is_marked(p), "must be marked");
367 assert(!_heap->complete_marking_context()->allocated_after_mark_start(p), "must be truly marked");
368
369 size_t obj_size = p->size();
370 if (_compact_point + obj_size > _to_region->end()) {
371 finish();
372
373 // Object doesn't fit. Pick next empty region and start compacting there.
374 ShenandoahHeapRegion* new_to_region;
375 if (_empty_regions_pos < _empty_regions.length()) {
376 new_to_region = _empty_regions.at(_empty_regions_pos);
377 _empty_regions_pos++;
378 } else {
379 // Out of empty region? Compact within the same region.
380 new_to_region = _from_region;
381 }
382
383 assert(new_to_region != _to_region, "must not reuse same to-region");
384 assert(new_to_region != nullptr, "must not be null");
385 _to_region = new_to_region;
386 _compact_point = _to_region->bottom();
387 }
388
389 // Object fits into current region, record new location, if object does not move:
390 assert(_compact_point + obj_size <= _to_region->end(), "must fit");
391 shenandoah_assert_not_forwarded(nullptr, p);
392 if (_compact_point != cast_from_oop<HeapWord*>(p)) {
393 _preserved_marks->push_if_necessary(p, p->mark());
394 FullGCForwarding::forward_to(p, cast_to_oop(_compact_point));
395 }
396 _compact_point += obj_size;
397 }
398 };
399
400 class ShenandoahPrepareForCompactionTask : public WorkerTask {
401 private:
402 PreservedMarksSet* const _preserved_marks;
403 ShenandoahHeap* const _heap;
404 ShenandoahHeapRegionSet** const _worker_slices;
405
406 public:
407 ShenandoahPrepareForCompactionTask(PreservedMarksSet *preserved_marks, ShenandoahHeapRegionSet **worker_slices) :
408 WorkerTask("Shenandoah Prepare For Compaction"),
409 _preserved_marks(preserved_marks),
410 _heap(ShenandoahHeap::heap()), _worker_slices(worker_slices) {
411 }
412
413 static bool is_candidate_region(ShenandoahHeapRegion* r) {
414 // Empty region: get it into the slice to defragment the slice itself.
415 // We could have skipped this without violating correctness, but we really
416 // want to compact all live regions to the start of the heap, which sometimes
417 // means moving them into the fully empty regions.
418 if (r->is_empty()) return true;
419
420 // Can move the region, and this is not the humongous region. Humongous
421 // moves are special cased here, because their moves are handled separately.
422 return r->is_stw_move_allowed() && !r->is_humongous();
423 }
424
425 void work(uint worker_id) override;
426 private:
427 template<typename ClosureType>
428 void prepare_for_compaction(ClosureType& cl,
429 GrowableArray<ShenandoahHeapRegion*>& empty_regions,
430 ShenandoahHeapRegionSetIterator& it,
431 ShenandoahHeapRegion* from_region);
432 };
433
434 void ShenandoahPrepareForCompactionTask::work(uint worker_id) {
435 ShenandoahParallelWorkerSession worker_session(worker_id);
436 ShenandoahHeapRegionSet* slice = _worker_slices[worker_id];
437 ShenandoahHeapRegionSetIterator it(slice);
438 ShenandoahHeapRegion* from_region = it.next();
439 // No work?
440 if (from_region == nullptr) {
441 return;
442 }
443
444 // Sliding compaction. Walk all regions in the slice, and compact them.
445 // Remember empty regions and reuse them as needed.
446 ResourceMark rm;
447
448 GrowableArray<ShenandoahHeapRegion*> empty_regions((int)_heap->num_regions());
449
450 if (_heap->mode()->is_generational()) {
451 ShenandoahPrepareForGenerationalCompactionObjectClosure cl(_preserved_marks->get(worker_id),
452 empty_regions, from_region, worker_id);
453 prepare_for_compaction(cl, empty_regions, it, from_region);
454 } else {
455 ShenandoahPrepareForCompactionObjectClosure cl(_preserved_marks->get(worker_id), empty_regions, from_region);
456 prepare_for_compaction(cl, empty_regions, it, from_region);
457 }
458 }
459
460 template<typename ClosureType>
461 void ShenandoahPrepareForCompactionTask::prepare_for_compaction(ClosureType& cl,
462 GrowableArray<ShenandoahHeapRegion*>& empty_regions,
463 ShenandoahHeapRegionSetIterator& it,
464 ShenandoahHeapRegion* from_region) {
465 while (from_region != nullptr) {
466 assert(is_candidate_region(from_region), "Sanity");
467 cl.set_from_region(from_region);
468 if (from_region->has_live()) {
469 _heap->marked_object_iterate(from_region, &cl);
470 }
471
472 // Compacted the region to somewhere else? From-region is empty then.
473 if (!cl.is_compact_same_region()) {
474 empty_regions.append(from_region);
475 }
476 from_region = it.next();
477 }
478 cl.finish();
479
480 // Mark all remaining regions as empty
481 for (int pos = cl.empty_regions_pos(); pos < empty_regions.length(); ++pos) {
482 ShenandoahHeapRegion* r = empty_regions.at(pos);
483 r->set_new_top(r->bottom());
484 }
485 }
486
487 void ShenandoahFullGC::calculate_target_humongous_objects() {
488 ShenandoahHeap* heap = ShenandoahHeap::heap();
489
490 // Compute the new addresses for humongous objects. We need to do this after addresses
491 // for regular objects are calculated, and we know what regions in heap suffix are
492 // available for humongous moves.
493 //
494 // Scan the heap backwards, because we are compacting humongous regions towards the end.
495 // Maintain the contiguous compaction window in [to_begin; to_end), so that we can slide
496 // humongous start there.
497 //
498 // The complication is potential non-movable regions during the scan. If such region is
499 // detected, then sliding restarts towards that non-movable region.
500
501 size_t to_begin = heap->num_regions();
502 size_t to_end = heap->num_regions();
503
504 log_debug(gc)("Full GC calculating target humongous objects from end " SIZE_FORMAT, to_end);
505 for (size_t c = heap->num_regions(); c > 0; c--) {
506 ShenandoahHeapRegion *r = heap->get_region(c - 1);
507 if (r->is_humongous_continuation() || (r->new_top() == r->bottom())) {
508 // To-region candidate: record this, and continue scan
509 to_begin = r->index();
510 continue;
511 }
512
513 if (r->is_humongous_start() && r->is_stw_move_allowed()) {
514 // From-region candidate: movable humongous region
515 oop old_obj = cast_to_oop(r->bottom());
516 size_t words_size = old_obj->size();
517 size_t num_regions = ShenandoahHeapRegion::required_regions(words_size * HeapWordSize);
518
519 size_t start = to_end - num_regions;
520
521 if (start >= to_begin && start != r->index()) {
522 // Fits into current window, and the move is non-trivial. Record the move then, and continue scan.
523 _preserved_marks->get(0)->push_if_necessary(old_obj, old_obj->mark());
524 FullGCForwarding::forward_to(old_obj, cast_to_oop(heap->get_region(start)->bottom()));
525 to_end = start;
526 continue;
527 }
528 }
529
530 // Failed to fit. Scan starting from current region.
531 to_begin = r->index();
532 to_end = r->index();
533 }
534 }
535
536 class ShenandoahEnsureHeapActiveClosure: public ShenandoahHeapRegionClosure {
537 private:
538 ShenandoahHeap* const _heap;
539
540 public:
541 ShenandoahEnsureHeapActiveClosure() : _heap(ShenandoahHeap::heap()) {}
542 void heap_region_do(ShenandoahHeapRegion* r) {
543 if (r->is_trash()) {
544 r->recycle();
545 }
546 if (r->is_cset()) {
547 // Leave affiliation unchanged
548 r->make_regular_bypass();
549 }
550 if (r->is_empty_uncommitted()) {
551 r->make_committed_bypass();
552 }
553 assert (r->is_committed(), "only committed regions in heap now, see region " SIZE_FORMAT, r->index());
554
555 // Record current region occupancy: this communicates empty regions are free
556 // to the rest of Full GC code.
557 r->set_new_top(r->top());
558 }
559 };
560
561 class ShenandoahTrashImmediateGarbageClosure: public ShenandoahHeapRegionClosure {
562 private:
563 ShenandoahHeap* const _heap;
564 ShenandoahMarkingContext* const _ctx;
565
566 public:
567 ShenandoahTrashImmediateGarbageClosure() :
568 _heap(ShenandoahHeap::heap()),
569 _ctx(ShenandoahHeap::heap()->complete_marking_context()) {}
570
571 void heap_region_do(ShenandoahHeapRegion* r) override {
572 if (r->is_humongous_start()) {
573 oop humongous_obj = cast_to_oop(r->bottom());
574 if (!_ctx->is_marked(humongous_obj)) {
575 assert(!r->has_live(), "Region " SIZE_FORMAT " is not marked, should not have live", r->index());
576 _heap->trash_humongous_region_at(r);
577 } else {
578 assert(r->has_live(), "Region " SIZE_FORMAT " should have live", r->index());
579 }
580 } else if (r->is_humongous_continuation()) {
581 // If we hit continuation, the non-live humongous starts should have been trashed already
582 assert(r->humongous_start_region()->has_live(), "Region " SIZE_FORMAT " should have live", r->index());
583 } else if (r->is_regular()) {
584 if (!r->has_live()) {
585 r->make_trash_immediate();
586 }
587 }
588 }
589 };
590
591 void ShenandoahFullGC::distribute_slices(ShenandoahHeapRegionSet** worker_slices) {
592 ShenandoahHeap* heap = ShenandoahHeap::heap();
593
594 uint n_workers = heap->workers()->active_workers();
595 size_t n_regions = heap->num_regions();
596
597 // What we want to accomplish: have the dense prefix of data, while still balancing
598 // out the parallel work.
599 //
600 // Assuming the amount of work is driven by the live data that needs moving, we can slice
601 // the entire heap into equal-live-sized prefix slices, and compact into them. So, each
602 // thread takes all regions in its prefix subset, and then it takes some regions from
603 // the tail.
604 //
605 // Tail region selection becomes interesting.
606 //
607 // First, we want to distribute the regions fairly between the workers, and those regions
608 // might have different amount of live data. So, until we sure no workers need live data,
609 // we need to only take what the worker needs.
610 //
611 // Second, since we slide everything to the left in each slice, the most busy regions
612 // would be the ones on the left. Which means we want to have all workers have their after-tail
613 // regions as close to the left as possible.
614 //
615 // The easiest way to do this is to distribute after-tail regions in round-robin between
616 // workers that still need live data.
617 //
618 // Consider parallel workers A, B, C, then the target slice layout would be:
619 //
620 // AAAAAAAABBBBBBBBCCCCCCCC|ABCABCABCABCABCABCABCABABABABABABABABABABAAAAA
621 //
622 // (.....dense-prefix.....) (.....................tail...................)
623 // [all regions fully live] [left-most regions are fuller that right-most]
624 //
625
626 // Compute how much live data is there. This would approximate the size of dense prefix
627 // we target to create.
628 size_t total_live = 0;
629 for (size_t idx = 0; idx < n_regions; idx++) {
630 ShenandoahHeapRegion *r = heap->get_region(idx);
631 if (ShenandoahPrepareForCompactionTask::is_candidate_region(r)) {
632 total_live += r->get_live_data_words();
633 }
634 }
635
636 // Estimate the size for the dense prefix. Note that we specifically count only the
637 // "full" regions, so there would be some non-full regions in the slice tail.
638 size_t live_per_worker = total_live / n_workers;
639 size_t prefix_regions_per_worker = live_per_worker / ShenandoahHeapRegion::region_size_words();
640 size_t prefix_regions_total = prefix_regions_per_worker * n_workers;
641 prefix_regions_total = MIN2(prefix_regions_total, n_regions);
642 assert(prefix_regions_total <= n_regions, "Sanity");
643
644 // There might be non-candidate regions in the prefix. To compute where the tail actually
645 // ends up being, we need to account those as well.
646 size_t prefix_end = prefix_regions_total;
647 for (size_t idx = 0; idx < prefix_regions_total; idx++) {
648 ShenandoahHeapRegion *r = heap->get_region(idx);
649 if (!ShenandoahPrepareForCompactionTask::is_candidate_region(r)) {
650 prefix_end++;
651 }
652 }
653 prefix_end = MIN2(prefix_end, n_regions);
654 assert(prefix_end <= n_regions, "Sanity");
655
656 // Distribute prefix regions per worker: each thread definitely gets its own same-sized
657 // subset of dense prefix.
658 size_t prefix_idx = 0;
659
660 size_t* live = NEW_C_HEAP_ARRAY(size_t, n_workers, mtGC);
661
662 for (size_t wid = 0; wid < n_workers; wid++) {
663 ShenandoahHeapRegionSet* slice = worker_slices[wid];
664
665 live[wid] = 0;
666 size_t regs = 0;
667
668 // Add all prefix regions for this worker
669 while (prefix_idx < prefix_end && regs < prefix_regions_per_worker) {
670 ShenandoahHeapRegion *r = heap->get_region(prefix_idx);
671 if (ShenandoahPrepareForCompactionTask::is_candidate_region(r)) {
672 slice->add_region(r);
673 live[wid] += r->get_live_data_words();
674 regs++;
675 }
676 prefix_idx++;
677 }
678 }
679
680 // Distribute the tail among workers in round-robin fashion.
681 size_t wid = n_workers - 1;
682
683 for (size_t tail_idx = prefix_end; tail_idx < n_regions; tail_idx++) {
684 ShenandoahHeapRegion *r = heap->get_region(tail_idx);
685 if (ShenandoahPrepareForCompactionTask::is_candidate_region(r)) {
686 assert(wid < n_workers, "Sanity");
687
688 size_t live_region = r->get_live_data_words();
689
690 // Select next worker that still needs live data.
691 size_t old_wid = wid;
692 do {
693 wid++;
694 if (wid == n_workers) wid = 0;
695 } while (live[wid] + live_region >= live_per_worker && old_wid != wid);
696
697 if (old_wid == wid) {
698 // Circled back to the same worker? This means liveness data was
699 // miscalculated. Bump the live_per_worker limit so that
700 // everyone gets a piece of the leftover work.
701 live_per_worker += ShenandoahHeapRegion::region_size_words();
702 }
703
704 worker_slices[wid]->add_region(r);
705 live[wid] += live_region;
706 }
707 }
708
709 FREE_C_HEAP_ARRAY(size_t, live);
710
711 #ifdef ASSERT
712 ResourceBitMap map(n_regions);
713 for (size_t wid = 0; wid < n_workers; wid++) {
714 ShenandoahHeapRegionSetIterator it(worker_slices[wid]);
715 ShenandoahHeapRegion* r = it.next();
716 while (r != nullptr) {
717 size_t idx = r->index();
718 assert(ShenandoahPrepareForCompactionTask::is_candidate_region(r), "Sanity: " SIZE_FORMAT, idx);
719 assert(!map.at(idx), "No region distributed twice: " SIZE_FORMAT, idx);
720 map.at_put(idx, true);
721 r = it.next();
722 }
723 }
724
725 for (size_t rid = 0; rid < n_regions; rid++) {
726 bool is_candidate = ShenandoahPrepareForCompactionTask::is_candidate_region(heap->get_region(rid));
727 bool is_distributed = map.at(rid);
728 assert(is_distributed || !is_candidate, "All candidates are distributed: " SIZE_FORMAT, rid);
729 }
730 #endif
731 }
732
733 void ShenandoahFullGC::phase2_calculate_target_addresses(ShenandoahHeapRegionSet** worker_slices) {
734 GCTraceTime(Info, gc, phases) time("Phase 2: Compute new object addresses", _gc_timer);
735 ShenandoahGCPhase calculate_address_phase(ShenandoahPhaseTimings::full_gc_calculate_addresses);
736
737 ShenandoahHeap* heap = ShenandoahHeap::heap();
738
739 // About to figure out which regions can be compacted, make sure pinning status
740 // had been updated in GC prologue.
741 heap->assert_pinned_region_status();
742
743 {
744 // Trash the immediately collectible regions before computing addresses
745 ShenandoahTrashImmediateGarbageClosure trash_immediate_garbage;
746 ShenandoahExcludeRegionClosure<FREE> cl(&trash_immediate_garbage);
747 heap->heap_region_iterate(&cl);
748
749 // Make sure regions are in good state: committed, active, clean.
750 // This is needed because we are potentially sliding the data through them.
751 ShenandoahEnsureHeapActiveClosure ecl;
752 heap->heap_region_iterate(&ecl);
753 }
754
755 // Compute the new addresses for regular objects
756 {
757 ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_calculate_addresses_regular);
758
759 distribute_slices(worker_slices);
760
761 ShenandoahPrepareForCompactionTask task(_preserved_marks, worker_slices);
762 heap->workers()->run_task(&task);
763 }
764
765 // Compute the new addresses for humongous objects
766 {
767 ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_calculate_addresses_humong);
768 calculate_target_humongous_objects();
769 }
770 }
771
772 class ShenandoahAdjustPointersClosure : public MetadataVisitingOopIterateClosure {
773 private:
774 ShenandoahHeap* const _heap;
775 ShenandoahMarkingContext* const _ctx;
776
777 template <class T>
778 inline void do_oop_work(T* p) {
779 T o = RawAccess<>::oop_load(p);
780 if (!CompressedOops::is_null(o)) {
781 oop obj = CompressedOops::decode_not_null(o);
782 assert(_ctx->is_marked(obj), "must be marked");
783 if (FullGCForwarding::is_forwarded(obj)) {
784 oop forw = FullGCForwarding::forwardee(obj);
785 RawAccess<IS_NOT_NULL>::oop_store(p, forw);
786 }
787 }
788 }
789
790 public:
791 ShenandoahAdjustPointersClosure() :
792 _heap(ShenandoahHeap::heap()),
793 _ctx(ShenandoahHeap::heap()->complete_marking_context()) {}
794
795 void do_oop(oop* p) { do_oop_work(p); }
796 void do_oop(narrowOop* p) { do_oop_work(p); }
797 void do_method(Method* m) {}
798 void do_nmethod(nmethod* nm) {}
799 };
800
801 class ShenandoahAdjustPointersObjectClosure : public ObjectClosure {
802 private:
803 ShenandoahHeap* const _heap;
804 ShenandoahAdjustPointersClosure _cl;
805
806 public:
807 ShenandoahAdjustPointersObjectClosure() :
808 _heap(ShenandoahHeap::heap()) {
809 }
810 void do_object(oop p) {
811 assert(_heap->complete_marking_context()->is_marked(p), "must be marked");
812 p->oop_iterate(&_cl);
813 }
814 };
815
816 class ShenandoahAdjustPointersTask : public WorkerTask {
817 private:
818 ShenandoahHeap* const _heap;
819 ShenandoahRegionIterator _regions;
820
821 public:
822 ShenandoahAdjustPointersTask() :
823 WorkerTask("Shenandoah Adjust Pointers"),
824 _heap(ShenandoahHeap::heap()) {
825 }
826
827 void work(uint worker_id) {
828 ShenandoahParallelWorkerSession worker_session(worker_id);
829 ShenandoahAdjustPointersObjectClosure obj_cl;
830 ShenandoahHeapRegion* r = _regions.next();
831 while (r != nullptr) {
832 if (!r->is_humongous_continuation() && r->has_live()) {
833 _heap->marked_object_iterate(r, &obj_cl);
834 }
835 if (_heap->mode()->is_generational()) {
836 ShenandoahGenerationalFullGC::maybe_coalesce_and_fill_region(r);
837 }
838 r = _regions.next();
839 }
840 }
841 };
842
843 class ShenandoahAdjustRootPointersTask : public WorkerTask {
844 private:
845 ShenandoahRootAdjuster* _rp;
846 PreservedMarksSet* _preserved_marks;
847 public:
848 ShenandoahAdjustRootPointersTask(ShenandoahRootAdjuster* rp, PreservedMarksSet* preserved_marks) :
849 WorkerTask("Shenandoah Adjust Root Pointers"),
850 _rp(rp),
851 _preserved_marks(preserved_marks) {}
852
853 void work(uint worker_id) {
854 ShenandoahParallelWorkerSession worker_session(worker_id);
855 ShenandoahAdjustPointersClosure cl;
856 _rp->roots_do(worker_id, &cl);
857 _preserved_marks->get(worker_id)->adjust_during_full_gc();
858 }
859 };
860
861 void ShenandoahFullGC::phase3_update_references() {
862 GCTraceTime(Info, gc, phases) time("Phase 3: Adjust pointers", _gc_timer);
863 ShenandoahGCPhase adjust_pointer_phase(ShenandoahPhaseTimings::full_gc_adjust_pointers);
864
865 ShenandoahHeap* heap = ShenandoahHeap::heap();
866
867 WorkerThreads* workers = heap->workers();
868 uint nworkers = workers->active_workers();
869 {
870 #if COMPILER2_OR_JVMCI
871 DerivedPointerTable::clear();
872 #endif
873 ShenandoahRootAdjuster rp(nworkers, ShenandoahPhaseTimings::full_gc_adjust_roots);
874 ShenandoahAdjustRootPointersTask task(&rp, _preserved_marks);
875 workers->run_task(&task);
876 #if COMPILER2_OR_JVMCI
877 DerivedPointerTable::update_pointers();
878 #endif
879 }
880
881 ShenandoahAdjustPointersTask adjust_pointers_task;
882 workers->run_task(&adjust_pointers_task);
883 }
884
885 class ShenandoahCompactObjectsClosure : public ObjectClosure {
886 private:
887 ShenandoahHeap* const _heap;
888 uint const _worker_id;
889
890 public:
891 ShenandoahCompactObjectsClosure(uint worker_id) :
892 _heap(ShenandoahHeap::heap()), _worker_id(worker_id) {}
893
894 void do_object(oop p) {
895 assert(_heap->complete_marking_context()->is_marked(p), "must be marked");
896 size_t size = p->size();
897 if (FullGCForwarding::is_forwarded(p)) {
898 HeapWord* compact_from = cast_from_oop<HeapWord*>(p);
899 HeapWord* compact_to = cast_from_oop<HeapWord*>(FullGCForwarding::forwardee(p));
900 assert(compact_from != compact_to, "Forwarded object should move");
901 Copy::aligned_conjoint_words(compact_from, compact_to, size);
902 oop new_obj = cast_to_oop(compact_to);
903
904 ContinuationGCSupport::relativize_stack_chunk(new_obj);
905 new_obj->init_mark();
906 }
907 }
908 };
909
910 class ShenandoahCompactObjectsTask : public WorkerTask {
911 private:
912 ShenandoahHeap* const _heap;
913 ShenandoahHeapRegionSet** const _worker_slices;
914
915 public:
916 ShenandoahCompactObjectsTask(ShenandoahHeapRegionSet** worker_slices) :
917 WorkerTask("Shenandoah Compact Objects"),
918 _heap(ShenandoahHeap::heap()),
919 _worker_slices(worker_slices) {
920 }
921
922 void work(uint worker_id) {
923 ShenandoahParallelWorkerSession worker_session(worker_id);
924 ShenandoahHeapRegionSetIterator slice(_worker_slices[worker_id]);
925
926 ShenandoahCompactObjectsClosure cl(worker_id);
927 ShenandoahHeapRegion* r = slice.next();
928 while (r != nullptr) {
929 assert(!r->is_humongous(), "must not get humongous regions here");
930 if (r->has_live()) {
931 _heap->marked_object_iterate(r, &cl);
932 }
933 r->set_top(r->new_top());
934 r = slice.next();
935 }
936 }
937 };
938
939 class ShenandoahPostCompactClosure : public ShenandoahHeapRegionClosure {
940 private:
941 ShenandoahHeap* const _heap;
942 bool _is_generational;
943 size_t _young_regions, _young_usage, _young_humongous_waste;
944 size_t _old_regions, _old_usage, _old_humongous_waste;
945
946 public:
947 ShenandoahPostCompactClosure() : _heap(ShenandoahHeap::heap()),
948 _is_generational(_heap->mode()->is_generational()),
949 _young_regions(0),
950 _young_usage(0),
951 _young_humongous_waste(0),
952 _old_regions(0),
953 _old_usage(0),
954 _old_humongous_waste(0)
955 {
956 _heap->free_set()->clear();
957 }
958
959 void heap_region_do(ShenandoahHeapRegion* r) {
960 assert (!r->is_cset(), "cset regions should have been demoted already");
961
962 // Need to reset the complete-top-at-mark-start pointer here because
963 // the complete marking bitmap is no longer valid. This ensures
964 // size-based iteration in marked_object_iterate().
965 // NOTE: See blurb at ShenandoahMCResetCompleteBitmapTask on why we need to skip
966 // pinned regions.
967 if (!r->is_pinned()) {
968 _heap->complete_marking_context()->reset_top_at_mark_start(r);
969 }
970
971 size_t live = r->used();
972
973 // Make empty regions that have been allocated into regular
974 if (r->is_empty() && live > 0) {
975 if (!_is_generational) {
976 r->make_affiliated_maybe();
977 }
978 // else, generational mode compaction has already established affiliation.
979 r->make_regular_bypass();
980 if (ZapUnusedHeapArea) {
981 SpaceMangler::mangle_region(MemRegion(r->top(), r->end()));
982 }
983 }
984
985 // Reclaim regular regions that became empty
986 if (r->is_regular() && live == 0) {
987 r->make_trash();
988 }
989
990 // Recycle all trash regions
991 if (r->is_trash()) {
992 live = 0;
993 r->recycle();
994 } else {
995 if (r->is_old()) {
996 ShenandoahGenerationalFullGC::account_for_region(r, _old_regions, _old_usage, _old_humongous_waste);
997 } else if (r->is_young()) {
998 ShenandoahGenerationalFullGC::account_for_region(r, _young_regions, _young_usage, _young_humongous_waste);
999 }
1000 }
1001 r->set_live_data(live);
1002 r->reset_alloc_metadata();
1003 }
1004
1005 void update_generation_usage() {
1006 if (_is_generational) {
1007 _heap->old_generation()->establish_usage(_old_regions, _old_usage, _old_humongous_waste);
1008 _heap->young_generation()->establish_usage(_young_regions, _young_usage, _young_humongous_waste);
1009 } else {
1010 assert(_old_regions == 0, "Old regions only expected in generational mode");
1011 assert(_old_usage == 0, "Old usage only expected in generational mode");
1012 assert(_old_humongous_waste == 0, "Old humongous waste only expected in generational mode");
1013 }
1014
1015 // In generational mode, global usage should be the sum of young and old. This is also true
1016 // for non-generational modes except that there are no old regions.
1017 _heap->global_generation()->establish_usage(_old_regions + _young_regions,
1018 _old_usage + _young_usage,
1019 _old_humongous_waste + _young_humongous_waste);
1020 }
1021 };
1022
1023 void ShenandoahFullGC::compact_humongous_objects() {
1024 // Compact humongous regions, based on their fwdptr objects.
1025 //
1026 // This code is serial, because doing the in-slice parallel sliding is tricky. In most cases,
1027 // humongous regions are already compacted, and do not require further moves, which alleviates
1028 // sliding costs. We may consider doing this in parallel in the future.
1029
1030 ShenandoahHeap* heap = ShenandoahHeap::heap();
1031
1032 for (size_t c = heap->num_regions(); c > 0; c--) {
1033 ShenandoahHeapRegion* r = heap->get_region(c - 1);
1034 if (r->is_humongous_start()) {
1035 oop old_obj = cast_to_oop(r->bottom());
1036 if (!FullGCForwarding::is_forwarded(old_obj)) {
1037 // No need to move the object, it stays at the same slot
1038 continue;
1039 }
1040 size_t words_size = old_obj->size();
1041 size_t num_regions = ShenandoahHeapRegion::required_regions(words_size * HeapWordSize);
1042
1043 size_t old_start = r->index();
1044 size_t old_end = old_start + num_regions - 1;
1045 size_t new_start = heap->heap_region_index_containing(FullGCForwarding::forwardee(old_obj));
1046 size_t new_end = new_start + num_regions - 1;
1047 assert(old_start != new_start, "must be real move");
1048 assert(r->is_stw_move_allowed(), "Region " SIZE_FORMAT " should be movable", r->index());
1049
1050 log_debug(gc)("Full GC compaction moves humongous object from region " SIZE_FORMAT " to region " SIZE_FORMAT, old_start, new_start);
1051 Copy::aligned_conjoint_words(r->bottom(), heap->get_region(new_start)->bottom(), words_size);
1052 ContinuationGCSupport::relativize_stack_chunk(cast_to_oop<HeapWord*>(r->bottom()));
1053
1054 oop new_obj = cast_to_oop(heap->get_region(new_start)->bottom());
1055 new_obj->init_mark();
1056
1057 {
1058 ShenandoahAffiliation original_affiliation = r->affiliation();
1059 for (size_t c = old_start; c <= old_end; c++) {
1060 ShenandoahHeapRegion* r = heap->get_region(c);
1061 // Leave humongous region affiliation unchanged.
1062 r->make_regular_bypass();
1063 r->set_top(r->bottom());
1064 }
1065
1066 for (size_t c = new_start; c <= new_end; c++) {
1067 ShenandoahHeapRegion* r = heap->get_region(c);
1068 if (c == new_start) {
1069 r->make_humongous_start_bypass(original_affiliation);
1070 } else {
1071 r->make_humongous_cont_bypass(original_affiliation);
1072 }
1073
1074 // Trailing region may be non-full, record the remainder there
1075 size_t remainder = words_size & ShenandoahHeapRegion::region_size_words_mask();
1076 if ((c == new_end) && (remainder != 0)) {
1077 r->set_top(r->bottom() + remainder);
1078 } else {
1079 r->set_top(r->end());
1080 }
1081
1082 r->reset_alloc_metadata();
1083 }
1084 }
1085 }
1086 }
1087 }
1088
1089 // This is slightly different to ShHeap::reset_next_mark_bitmap:
1090 // we need to remain able to walk pinned regions.
1091 // Since pinned region do not move and don't get compacted, we will get holes with
1092 // unreachable objects in them (which may have pointers to unloaded Klasses and thus
1093 // cannot be iterated over using oop->size(). The only way to safely iterate over those is using
1094 // a valid marking bitmap and valid TAMS pointer. This class only resets marking
1095 // bitmaps for un-pinned regions, and later we only reset TAMS for unpinned regions.
1096 class ShenandoahMCResetCompleteBitmapTask : public WorkerTask {
1097 private:
1098 ShenandoahRegionIterator _regions;
1099
1100 public:
1101 ShenandoahMCResetCompleteBitmapTask() :
1102 WorkerTask("Shenandoah Reset Bitmap") {
1103 }
1104
1105 void work(uint worker_id) {
1106 ShenandoahParallelWorkerSession worker_session(worker_id);
1107 ShenandoahHeapRegion* region = _regions.next();
1108 ShenandoahHeap* heap = ShenandoahHeap::heap();
1109 ShenandoahMarkingContext* const ctx = heap->complete_marking_context();
1110 while (region != nullptr) {
1111 if (heap->is_bitmap_slice_committed(region) && !region->is_pinned() && region->has_live()) {
1112 ctx->clear_bitmap(region);
1113 }
1114 region = _regions.next();
1115 }
1116 }
1117 };
1118
1119 void ShenandoahFullGC::phase4_compact_objects(ShenandoahHeapRegionSet** worker_slices) {
1120 GCTraceTime(Info, gc, phases) time("Phase 4: Move objects", _gc_timer);
1121 ShenandoahGCPhase compaction_phase(ShenandoahPhaseTimings::full_gc_copy_objects);
1122
1123 ShenandoahHeap* heap = ShenandoahHeap::heap();
1124
1125 // Compact regular objects first
1126 {
1127 ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_copy_objects_regular);
1128 ShenandoahCompactObjectsTask compact_task(worker_slices);
1129 heap->workers()->run_task(&compact_task);
1130 }
1131
1132 // Compact humongous objects after regular object moves
1133 {
1134 ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_copy_objects_humong);
1135 compact_humongous_objects();
1136 }
1137 }
1138
1139 void ShenandoahFullGC::phase5_epilog() {
1140 GCTraceTime(Info, gc, phases) time("Phase 5: Full GC epilog", _gc_timer);
1141 ShenandoahHeap* heap = ShenandoahHeap::heap();
1142
1143 // Reset complete bitmap. We're about to reset the complete-top-at-mark-start pointer
1144 // and must ensure the bitmap is in sync.
1145 {
1146 ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_copy_objects_reset_complete);
1147 ShenandoahMCResetCompleteBitmapTask task;
1148 heap->workers()->run_task(&task);
1149 }
1150
1151 // Bring regions in proper states after the collection, and set heap properties.
1152 {
1153 ShenandoahGCPhase phase(ShenandoahPhaseTimings::full_gc_copy_objects_rebuild);
1154 ShenandoahPostCompactClosure post_compact;
1155 heap->heap_region_iterate(&post_compact);
1156 post_compact.update_generation_usage();
1157
1158 if (heap->mode()->is_generational()) {
1159 ShenandoahGenerationalFullGC::balance_generations_after_gc(heap);
1160 }
1161
1162 heap->collection_set()->clear();
1163 size_t young_cset_regions, old_cset_regions;
1164 size_t first_old, last_old, num_old;
1165 heap->free_set()->prepare_to_rebuild(young_cset_regions, old_cset_regions, first_old, last_old, num_old);
1166
1167 // We also do not expand old generation size following Full GC because we have scrambled age populations and
1168 // no longer have objects separated by age into distinct regions.
1169 if (heap->mode()->is_generational()) {
1170 ShenandoahGenerationalFullGC::compute_balances();
1171 }
1172
1173 heap->free_set()->finish_rebuild(young_cset_regions, old_cset_regions, num_old);
1174
1175 heap->clear_cancelled_gc(true /* clear oom handler */);
1176 }
1177
1178 _preserved_marks->restore(heap->workers());
1179 _preserved_marks->reclaim();
1180
1181 // We defer generation resizing actions until after cset regions have been recycled. We do this even following an
1182 // abbreviated cycle.
1183 if (heap->mode()->is_generational()) {
1184 ShenandoahGenerationalFullGC::balance_generations_after_rebuilding_free_set();
1185 ShenandoahGenerationalFullGC::rebuild_remembered_set(heap);
1186 }
1187 }