1 /*
2 * Copyright (c) 2001, 2024, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "cds/cdsConfig.hpp"
27 #include "classfile/classLoaderData.hpp"
28 #include "classfile/vmClasses.hpp"
29 #include "gc/shared/allocTracer.hpp"
30 #include "gc/shared/barrierSet.hpp"
31 #include "gc/shared/collectedHeap.hpp"
32 #include "gc/shared/collectedHeap.inline.hpp"
33 #include "gc/shared/gcLocker.inline.hpp"
34 #include "gc/shared/gcHeapSummary.hpp"
35 #include "gc/shared/stringdedup/stringDedup.hpp"
36 #include "gc/shared/gcTrace.hpp"
37 #include "gc/shared/gcTraceTime.inline.hpp"
38 #include "gc/shared/gcVMOperations.hpp"
39 #include "gc/shared/gcWhen.hpp"
40 #include "gc/shared/gc_globals.hpp"
41 #include "gc/shared/memAllocator.hpp"
42 #include "gc/shared/tlab_globals.hpp"
43 #include "logging/log.hpp"
44 #include "logging/logStream.hpp"
45 #include "memory/classLoaderMetaspace.hpp"
46 #include "memory/metaspace.hpp"
47 #include "memory/metaspaceUtils.hpp"
48 #include "memory/resourceArea.hpp"
49 #include "memory/universe.hpp"
50 #include "oops/instanceMirrorKlass.hpp"
51 #include "oops/oop.inline.hpp"
52 #include "runtime/handles.inline.hpp"
53 #include "runtime/init.hpp"
54 #include "runtime/javaThread.hpp"
55 #include "runtime/perfData.hpp"
56 #include "runtime/threadSMR.hpp"
57 #include "runtime/vmThread.hpp"
58 #include "services/heapDumper.hpp"
59 #include "utilities/align.hpp"
60 #include "utilities/copy.hpp"
61 #include "utilities/events.hpp"
62
63 class ClassLoaderData;
64
65 size_t CollectedHeap::_lab_alignment_reserve = SIZE_MAX;
66 Klass* CollectedHeap::_filler_object_klass = nullptr;
67 size_t CollectedHeap::_filler_array_max_size = 0;
68 size_t CollectedHeap::_stack_chunk_max_size = 0;
69
70 class GCMessage : public FormatBuffer<1024> {
71 public:
72 bool is_before;
73 };
74
75 template <>
76 void EventLogBase<GCMessage>::print(outputStream* st, GCMessage& m) {
77 st->print_cr("GC heap %s", m.is_before ? "before" : "after");
78 st->print_raw(m);
79 }
80
81 class GCHeapLog : public EventLogBase<GCMessage> {
82 private:
83 void log_heap(CollectedHeap* heap, bool before);
84
85 public:
86 GCHeapLog() : EventLogBase<GCMessage>("GC Heap History", "gc") {}
87
88 void log_heap_before(CollectedHeap* heap) {
89 log_heap(heap, true);
90 }
91 void log_heap_after(CollectedHeap* heap) {
92 log_heap(heap, false);
93 }
94 };
95
96 void GCHeapLog::log_heap(CollectedHeap* heap, bool before) {
97 if (!should_log()) {
98 return;
99 }
100
101 double timestamp = fetch_timestamp();
102 MutexLocker ml(&_mutex, Mutex::_no_safepoint_check_flag);
103 int index = compute_log_index();
104 _records[index].thread = nullptr; // Its the GC thread so it's not that interesting.
105 _records[index].timestamp = timestamp;
106 _records[index].data.is_before = before;
107 stringStream st(_records[index].data.buffer(), _records[index].data.size());
108
109 st.print_cr("{Heap %s GC invocations=%u (full %u):",
110 before ? "before" : "after",
111 heap->total_collections(),
112 heap->total_full_collections());
113
114 heap->print_on(&st);
115 st.print_cr("}");
116 }
117
118 ParallelObjectIterator::ParallelObjectIterator(uint thread_num) :
119 _impl(Universe::heap()->parallel_object_iterator(thread_num))
120 {}
121
122 ParallelObjectIterator::~ParallelObjectIterator() {
123 delete _impl;
124 }
125
126 void ParallelObjectIterator::object_iterate(ObjectClosure* cl, uint worker_id) {
127 _impl->object_iterate(cl, worker_id);
128 }
129
130 size_t CollectedHeap::unused() const {
131 MutexLocker ml(Heap_lock);
132 return capacity() - used();
133 }
134
135 VirtualSpaceSummary CollectedHeap::create_heap_space_summary() {
136 size_t capacity_in_words = capacity() / HeapWordSize;
137
138 return VirtualSpaceSummary(
139 _reserved.start(), _reserved.start() + capacity_in_words, _reserved.end());
140 }
141
142 GCHeapSummary CollectedHeap::create_heap_summary() {
143 VirtualSpaceSummary heap_space = create_heap_space_summary();
144 return GCHeapSummary(heap_space, used());
145 }
146
147 MetaspaceSummary CollectedHeap::create_metaspace_summary() {
148 const MetaspaceChunkFreeListSummary& ms_chunk_free_list_summary =
149 MetaspaceUtils::chunk_free_list_summary(Metaspace::NonClassType);
150 const MetaspaceChunkFreeListSummary& class_chunk_free_list_summary =
151 MetaspaceUtils::chunk_free_list_summary(Metaspace::ClassType);
152 return MetaspaceSummary(MetaspaceGC::capacity_until_GC(),
153 MetaspaceUtils::get_combined_statistics(),
154 ms_chunk_free_list_summary, class_chunk_free_list_summary);
155 }
156
157 bool CollectedHeap::contains_null(const oop* p) const {
158 return *p == nullptr;
159 }
160
161 void CollectedHeap::print_heap_before_gc() {
162 LogTarget(Debug, gc, heap) lt;
163 if (lt.is_enabled()) {
164 LogStream ls(lt);
165 ls.print_cr("Heap before GC invocations=%u (full %u):", total_collections(), total_full_collections());
166 ResourceMark rm;
167 print_on(&ls);
168 }
169
170 if (_gc_heap_log != nullptr) {
171 _gc_heap_log->log_heap_before(this);
172 }
173 }
174
175 void CollectedHeap::print_heap_after_gc() {
176 LogTarget(Debug, gc, heap) lt;
177 if (lt.is_enabled()) {
178 LogStream ls(lt);
179 ls.print_cr("Heap after GC invocations=%u (full %u):", total_collections(), total_full_collections());
180 ResourceMark rm;
181 print_on(&ls);
182 }
183
184 if (_gc_heap_log != nullptr) {
185 _gc_heap_log->log_heap_after(this);
186 }
187 }
188
189 void CollectedHeap::print() const { print_on(tty); }
190
191 void CollectedHeap::print_on_error(outputStream* st) const {
192 st->print_cr("Heap:");
193 print_extended_on(st);
194 st->cr();
195
196 BarrierSet* bs = BarrierSet::barrier_set();
197 if (bs != nullptr) {
198 bs->print_on(st);
199 }
200 }
201
202 void CollectedHeap::trace_heap(GCWhen::Type when, const GCTracer* gc_tracer) {
203 const GCHeapSummary& heap_summary = create_heap_summary();
204 gc_tracer->report_gc_heap_summary(when, heap_summary);
205
206 const MetaspaceSummary& metaspace_summary = create_metaspace_summary();
207 gc_tracer->report_metaspace_summary(when, metaspace_summary);
208 }
209
210 void CollectedHeap::trace_heap_before_gc(const GCTracer* gc_tracer) {
211 trace_heap(GCWhen::BeforeGC, gc_tracer);
212 }
213
214 void CollectedHeap::trace_heap_after_gc(const GCTracer* gc_tracer) {
215 trace_heap(GCWhen::AfterGC, gc_tracer);
216 }
217
218 // Default implementation, for collectors that don't support the feature.
219 bool CollectedHeap::supports_concurrent_gc_breakpoints() const {
220 return false;
221 }
222
223 bool CollectedHeap::is_oop(oop object) const {
224 if (!is_object_aligned(object)) {
225 return false;
226 }
227
228 if (!is_in(object)) {
229 return false;
230 }
231
232 // With compact headers, we can't safely access the class, due
233 // to possibly forwarded objects.
234 if (!UseCompactObjectHeaders && !Metaspace::contains(object->klass_without_asserts())) {
235 return false;
236 }
237
238 return true;
239 }
240
241 // Memory state functions.
242
243
244 CollectedHeap::CollectedHeap() :
245 _capacity_at_last_gc(0),
246 _used_at_last_gc(0),
247 _soft_ref_policy(),
248 _is_gc_active(false),
249 _last_whole_heap_examined_time_ns(os::javaTimeNanos()),
250 _total_collections(0),
251 _total_full_collections(0),
252 _gc_cause(GCCause::_no_gc),
253 _gc_lastcause(GCCause::_no_gc)
254 {
255 // If the minimum object size is greater than MinObjAlignment, we can
256 // end up with a shard at the end of the buffer that's smaller than
257 // the smallest object. We can't allow that because the buffer must
258 // look like it's full of objects when we retire it, so we make
259 // sure we have enough space for a filler int array object.
260 size_t min_size = min_dummy_object_size();
261 _lab_alignment_reserve = min_size > (size_t)MinObjAlignment ? align_object_size(min_size) : 0;
262
263 const size_t max_len = size_t(arrayOopDesc::max_array_length(T_INT));
264 const size_t elements_per_word = HeapWordSize / sizeof(jint);
265 _filler_array_max_size = align_object_size(filler_array_hdr_size() +
266 max_len / elements_per_word);
267
268 NOT_PRODUCT(_promotion_failure_alot_count = 0;)
269 NOT_PRODUCT(_promotion_failure_alot_gc_number = 0;)
270
271 if (UsePerfData) {
272 EXCEPTION_MARK;
273
274 // create the gc cause jvmstat counters
275 _perf_gc_cause = PerfDataManager::create_string_variable(SUN_GC, "cause",
276 80, GCCause::to_string(_gc_cause), CHECK);
277
278 _perf_gc_lastcause =
279 PerfDataManager::create_string_variable(SUN_GC, "lastCause",
280 80, GCCause::to_string(_gc_lastcause), CHECK);
281 }
282
283 // Create the ring log
284 if (LogEvents) {
285 _gc_heap_log = new GCHeapLog();
286 } else {
287 _gc_heap_log = nullptr;
288 }
289 }
290
291 // This interface assumes that it's being called by the
292 // vm thread. It collects the heap assuming that the
293 // heap lock is already held and that we are executing in
294 // the context of the vm thread.
295 void CollectedHeap::collect_as_vm_thread(GCCause::Cause cause) {
296 Thread* thread = Thread::current();
297 assert(thread->is_VM_thread(), "Precondition#1");
298 assert(Heap_lock->is_locked(), "Precondition#2");
299 GCCauseSetter gcs(this, cause);
300 switch (cause) {
301 case GCCause::_codecache_GC_threshold:
302 case GCCause::_codecache_GC_aggressive:
303 case GCCause::_heap_inspection:
304 case GCCause::_heap_dump:
305 case GCCause::_metadata_GC_threshold: {
306 HandleMark hm(thread);
307 do_full_collection(false); // don't clear all soft refs
308 break;
309 }
310 case GCCause::_metadata_GC_clear_soft_refs: {
311 HandleMark hm(thread);
312 do_full_collection(true); // do clear all soft refs
313 break;
314 }
315 default:
316 ShouldNotReachHere(); // Unexpected use of this function
317 }
318 }
319
320 MetaWord* CollectedHeap::satisfy_failed_metadata_allocation(ClassLoaderData* loader_data,
321 size_t word_size,
322 Metaspace::MetadataType mdtype) {
323 uint loop_count = 0;
324 uint gc_count = 0;
325 uint full_gc_count = 0;
326
327 assert(!Heap_lock->owned_by_self(), "Should not be holding the Heap_lock");
328
329 do {
330 MetaWord* result = loader_data->metaspace_non_null()->allocate(word_size, mdtype);
331 if (result != nullptr) {
332 return result;
333 }
334
335 if (GCLocker::is_active_and_needs_gc()) {
336 // If the GCLocker is active, just expand and allocate.
337 // If that does not succeed, wait if this thread is not
338 // in a critical section itself.
339 result = loader_data->metaspace_non_null()->expand_and_allocate(word_size, mdtype);
340 if (result != nullptr) {
341 return result;
342 }
343 JavaThread* jthr = JavaThread::current();
344 if (!jthr->in_critical()) {
345 // Wait for JNI critical section to be exited
346 GCLocker::stall_until_clear();
347 // The GC invoked by the last thread leaving the critical
348 // section will be a young collection and a full collection
349 // is (currently) needed for unloading classes so continue
350 // to the next iteration to get a full GC.
351 continue;
352 } else {
353 if (CheckJNICalls) {
354 fatal("Possible deadlock due to allocating while"
355 " in jni critical section");
356 }
357 return nullptr;
358 }
359 }
360
361 { // Need lock to get self consistent gc_count's
362 MutexLocker ml(Heap_lock);
363 gc_count = Universe::heap()->total_collections();
364 full_gc_count = Universe::heap()->total_full_collections();
365 }
366
367 // Generate a VM operation
368 VM_CollectForMetadataAllocation op(loader_data,
369 word_size,
370 mdtype,
371 gc_count,
372 full_gc_count,
373 GCCause::_metadata_GC_threshold);
374 VMThread::execute(&op);
375
376 // If GC was locked out, try again. Check before checking success because the
377 // prologue could have succeeded and the GC still have been locked out.
378 if (op.gc_locked()) {
379 continue;
380 }
381
382 if (op.prologue_succeeded()) {
383 return op.result();
384 }
385 loop_count++;
386 if ((QueuedAllocationWarningCount > 0) &&
387 (loop_count % QueuedAllocationWarningCount == 0)) {
388 log_warning(gc, ergo)("satisfy_failed_metadata_allocation() retries %d times,"
389 " size=" SIZE_FORMAT, loop_count, word_size);
390 }
391 } while (true); // Until a GC is done
392 }
393
394 MemoryUsage CollectedHeap::memory_usage() {
395 return MemoryUsage(InitialHeapSize, used(), capacity(), max_capacity());
396 }
397
398 void CollectedHeap::set_gc_cause(GCCause::Cause v) {
399 if (UsePerfData) {
400 _gc_lastcause = _gc_cause;
401 _perf_gc_lastcause->set_value(GCCause::to_string(_gc_lastcause));
402 _perf_gc_cause->set_value(GCCause::to_string(v));
403 }
404 _gc_cause = v;
405 }
406
407 // Returns the header size in words aligned to the requirements of the
408 // array object type.
409 static int int_array_header_size() {
410 size_t typesize_in_bytes = arrayOopDesc::header_size_in_bytes();
411 return (int)align_up(typesize_in_bytes, HeapWordSize)/HeapWordSize;
412 }
413
414 size_t CollectedHeap::max_tlab_size() const {
415 // TLABs can't be bigger than we can fill with a int[Integer.MAX_VALUE].
416 // This restriction could be removed by enabling filling with multiple arrays.
417 // If we compute that the reasonable way as
418 // header_size + ((sizeof(jint) * max_jint) / HeapWordSize)
419 // we'll overflow on the multiply, so we do the divide first.
420 // We actually lose a little by dividing first,
421 // but that just makes the TLAB somewhat smaller than the biggest array,
422 // which is fine, since we'll be able to fill that.
423 size_t max_int_size = int_array_header_size() +
424 sizeof(jint) *
425 ((juint) max_jint / (size_t) HeapWordSize);
426 return align_down(max_int_size, MinObjAlignment);
427 }
428
429 size_t CollectedHeap::filler_array_hdr_size() {
430 return align_object_offset(int_array_header_size()); // align to Long
431 }
432
433 size_t CollectedHeap::filler_array_min_size() {
434 return align_object_size(filler_array_hdr_size()); // align to MinObjAlignment
435 }
436
437 void CollectedHeap::zap_filler_array_with(HeapWord* start, size_t words, juint value) {
438 Copy::fill_to_words(start + filler_array_hdr_size(),
439 words - filler_array_hdr_size(), value);
440 }
441
442 #ifdef ASSERT
443 void CollectedHeap::fill_args_check(HeapWord* start, size_t words)
444 {
445 assert(words >= min_fill_size(), "too small to fill");
446 assert(is_object_aligned(words), "unaligned size");
447 }
448
449 void CollectedHeap::zap_filler_array(HeapWord* start, size_t words, bool zap)
450 {
451 if (ZapFillerObjects && zap) {
452 zap_filler_array_with(start, words, 0XDEAFBABE);
453 }
454 }
455 #endif // ASSERT
456
457 void
458 CollectedHeap::fill_with_array(HeapWord* start, size_t words, bool zap)
459 {
460 assert(words >= filler_array_min_size(), "too small for an array");
461 assert(words <= filler_array_max_size(), "too big for a single object");
462
463 const size_t payload_size = words - filler_array_hdr_size();
464 const size_t len = payload_size * HeapWordSize / sizeof(jint);
465 assert((int)len >= 0, "size too large " SIZE_FORMAT " becomes %d", words, (int)len);
466
467 ObjArrayAllocator allocator(Universe::fillerArrayKlass(), words, (int)len, /* do_zero */ false);
468 allocator.initialize(start);
469 if (CDSConfig::is_dumping_heap()) {
470 // This array is written into the CDS archive. Make sure it
471 // has deterministic contents.
472 zap_filler_array_with(start, words, 0);
473 } else {
474 DEBUG_ONLY(zap_filler_array(start, words, zap);)
475 }
476 }
477
478 void
479 CollectedHeap::fill_with_object_impl(HeapWord* start, size_t words, bool zap)
480 {
481 assert(words <= filler_array_max_size(), "too big for a single object");
482
483 if (words >= filler_array_min_size()) {
484 fill_with_array(start, words, zap);
485 } else if (words > 0) {
486 assert(words == min_fill_size(), "unaligned size");
487 ObjAllocator allocator(CollectedHeap::filler_object_klass(), words);
488 allocator.initialize(start);
489 }
490 }
491
492 void CollectedHeap::fill_with_object(HeapWord* start, size_t words, bool zap)
493 {
494 DEBUG_ONLY(fill_args_check(start, words);)
495 HandleMark hm(Thread::current()); // Free handles before leaving.
496 fill_with_object_impl(start, words, zap);
497 }
498
499 void CollectedHeap::fill_with_objects(HeapWord* start, size_t words, bool zap)
500 {
501 DEBUG_ONLY(fill_args_check(start, words);)
502 HandleMark hm(Thread::current()); // Free handles before leaving.
503
504 // Multiple objects may be required depending on the filler array maximum size. Fill
505 // the range up to that with objects that are filler_array_max_size sized. The
506 // remainder is filled with a single object.
507 const size_t min = min_fill_size();
508 const size_t max = filler_array_max_size();
509 while (words > max) {
510 const size_t cur = (words - max) >= min ? max : max - min;
511 fill_with_array(start, cur, zap);
512 start += cur;
513 words -= cur;
514 }
515
516 fill_with_object_impl(start, words, zap);
517 }
518
519 void CollectedHeap::fill_with_dummy_object(HeapWord* start, HeapWord* end, bool zap) {
520 CollectedHeap::fill_with_object(start, end, zap);
521 }
522
523 void CollectedHeap::ensure_parsability(bool retire_tlabs) {
524 assert(SafepointSynchronize::is_at_safepoint() || !is_init_completed(),
525 "Should only be called at a safepoint or at start-up");
526
527 ThreadLocalAllocStats stats;
528
529 for (JavaThreadIteratorWithHandle jtiwh; JavaThread *thread = jtiwh.next();) {
530 BarrierSet::barrier_set()->make_parsable(thread);
531 if (UseTLAB) {
532 if (retire_tlabs) {
533 thread->tlab().retire(&stats);
534 } else {
535 thread->tlab().make_parsable();
536 }
537 }
538 }
539
540 stats.publish();
541 }
542
543 void CollectedHeap::resize_all_tlabs() {
544 assert(SafepointSynchronize::is_at_safepoint() || !is_init_completed(),
545 "Should only resize tlabs at safepoint");
546
547 if (UseTLAB && ResizeTLAB) {
548 for (JavaThreadIteratorWithHandle jtiwh; JavaThread *thread = jtiwh.next(); ) {
549 thread->tlab().resize();
550 }
551 }
552 }
553
554 jlong CollectedHeap::millis_since_last_whole_heap_examined() {
555 return (os::javaTimeNanos() - _last_whole_heap_examined_time_ns) / NANOSECS_PER_MILLISEC;
556 }
557
558 void CollectedHeap::record_whole_heap_examined_timestamp() {
559 _last_whole_heap_examined_time_ns = os::javaTimeNanos();
560 }
561
562 void CollectedHeap::full_gc_dump(GCTimer* timer, bool before) {
563 assert(timer != nullptr, "timer is null");
564 static uint count = 0;
565 if ((HeapDumpBeforeFullGC && before) || (HeapDumpAfterFullGC && !before)) {
566 if (FullGCHeapDumpLimit == 0 || count < FullGCHeapDumpLimit) {
567 GCTraceTime(Info, gc) tm(before ? "Heap Dump (before full gc)" : "Heap Dump (after full gc)", timer);
568 HeapDumper::dump_heap();
569 count++;
570 }
571 }
572
573 LogTarget(Trace, gc, classhisto) lt;
574 if (lt.is_enabled()) {
575 GCTraceTime(Trace, gc, classhisto) tm(before ? "Class Histogram (before full gc)" : "Class Histogram (after full gc)", timer);
576 ResourceMark rm;
577 LogStream ls(lt);
578 VM_GC_HeapInspection inspector(&ls, false /* ! full gc */);
579 inspector.doit();
580 }
581 }
582
583 void CollectedHeap::pre_full_gc_dump(GCTimer* timer) {
584 full_gc_dump(timer, true);
585 }
586
587 void CollectedHeap::post_full_gc_dump(GCTimer* timer) {
588 full_gc_dump(timer, false);
589 }
590
591 void CollectedHeap::initialize_reserved_region(const ReservedHeapSpace& rs) {
592 // It is important to do this in a way such that concurrent readers can't
593 // temporarily think something is in the heap. (Seen this happen in asserts.)
594 _reserved.set_word_size(0);
595 _reserved.set_start((HeapWord*)rs.base());
596 _reserved.set_end((HeapWord*)rs.end());
597 }
598
599 void CollectedHeap::post_initialize() {
600 StringDedup::initialize();
601 initialize_serviceability();
602 }
603
604 #ifndef PRODUCT
605
606 bool CollectedHeap::promotion_should_fail(volatile size_t* count) {
607 // Access to count is not atomic; the value does not have to be exact.
608 if (PromotionFailureALot) {
609 const size_t gc_num = total_collections();
610 const size_t elapsed_gcs = gc_num - _promotion_failure_alot_gc_number;
611 if (elapsed_gcs >= PromotionFailureALotInterval) {
612 // Test for unsigned arithmetic wrap-around.
613 if (++*count >= PromotionFailureALotCount) {
614 *count = 0;
615 return true;
616 }
617 }
618 }
619 return false;
620 }
621
622 bool CollectedHeap::promotion_should_fail() {
623 return promotion_should_fail(&_promotion_failure_alot_count);
624 }
625
626 void CollectedHeap::reset_promotion_should_fail(volatile size_t* count) {
627 if (PromotionFailureALot) {
628 _promotion_failure_alot_gc_number = total_collections();
629 *count = 0;
630 }
631 }
632
633 void CollectedHeap::reset_promotion_should_fail() {
634 reset_promotion_should_fail(&_promotion_failure_alot_count);
635 }
636
637 #endif // #ifndef PRODUCT
638
639 // It's the caller's responsibility to ensure glitch-freedom
640 // (if required).
641 void CollectedHeap::update_capacity_and_used_at_gc() {
642 _capacity_at_last_gc = capacity();
643 _used_at_last_gc = used();
644 }