1 /*
2 * Copyright (c) 2001, 2021, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/classLoaderData.hpp"
27 #include "classfile/vmClasses.hpp"
28 #include "gc/shared/allocTracer.hpp"
29 #include "gc/shared/barrierSet.hpp"
30 #include "gc/shared/collectedHeap.hpp"
31 #include "gc/shared/collectedHeap.inline.hpp"
32 #include "gc/shared/gcLocker.inline.hpp"
33 #include "gc/shared/gcHeapSummary.hpp"
34 #include "gc/shared/stringdedup/stringDedup.hpp"
35 #include "gc/shared/gcTrace.hpp"
36 #include "gc/shared/gcTraceTime.inline.hpp"
37 #include "gc/shared/gcVMOperations.hpp"
38 #include "gc/shared/gcWhen.hpp"
39 #include "gc/shared/gc_globals.hpp"
40 #include "gc/shared/memAllocator.hpp"
41 #include "gc/shared/tlab_globals.hpp"
42 #include "logging/log.hpp"
43 #include "logging/logStream.hpp"
44 #include "memory/classLoaderMetaspace.hpp"
45 #include "memory/metaspaceUtils.hpp"
46 #include "memory/resourceArea.hpp"
47 #include "memory/universe.hpp"
48 #include "oops/instanceMirrorKlass.hpp"
49 #include "oops/oop.inline.hpp"
50 #include "runtime/handles.inline.hpp"
51 #include "runtime/init.hpp"
52 #include "runtime/perfData.hpp"
53 #include "runtime/thread.inline.hpp"
54 #include "runtime/threadSMR.hpp"
55 #include "runtime/vmThread.hpp"
56 #include "services/heapDumper.hpp"
57 #include "utilities/align.hpp"
58 #include "utilities/copy.hpp"
59 #include "utilities/events.hpp"
60
61 class ClassLoaderData;
62
63 size_t CollectedHeap::_filler_array_max_size = 0;
64
65 class GCMessage : public FormatBuffer<1024> {
66 public:
67 bool is_before;
68 };
69
70 template <>
71 void EventLogBase<GCMessage>::print(outputStream* st, GCMessage& m) {
72 st->print_cr("GC heap %s", m.is_before ? "before" : "after");
73 st->print_raw(m);
74 }
75
76 class GCHeapLog : public EventLogBase<GCMessage> {
77 private:
78 void log_heap(CollectedHeap* heap, bool before);
79
80 public:
81 GCHeapLog() : EventLogBase<GCMessage>("GC Heap History", "gc") {}
82
83 void log_heap_before(CollectedHeap* heap) {
84 log_heap(heap, true);
85 }
86 void log_heap_after(CollectedHeap* heap) {
87 log_heap(heap, false);
88 }
89 };
90
91 void GCHeapLog::log_heap(CollectedHeap* heap, bool before) {
92 if (!should_log()) {
93 return;
94 }
95
96 double timestamp = fetch_timestamp();
97 MutexLocker ml(&_mutex, Mutex::_no_safepoint_check_flag);
98 int index = compute_log_index();
99 _records[index].thread = NULL; // Its the GC thread so it's not that interesting.
100 _records[index].timestamp = timestamp;
101 _records[index].data.is_before = before;
102 stringStream st(_records[index].data.buffer(), _records[index].data.size());
103
104 st.print_cr("{Heap %s GC invocations=%u (full %u):",
105 before ? "before" : "after",
106 heap->total_collections(),
107 heap->total_full_collections());
108
109 heap->print_on(&st);
110 st.print_cr("}");
111 }
112
113 ParallelObjectIterator::ParallelObjectIterator(uint thread_num) :
114 _impl(Universe::heap()->parallel_object_iterator(thread_num))
115 {}
116
117 ParallelObjectIterator::~ParallelObjectIterator() {
118 delete _impl;
119 }
120
121 void ParallelObjectIterator::object_iterate(ObjectClosure* cl, uint worker_id) {
122 _impl->object_iterate(cl, worker_id);
123 }
124
125 size_t CollectedHeap::unused() const {
126 MutexLocker ml(Heap_lock);
127 return capacity() - used();
128 }
129
130 VirtualSpaceSummary CollectedHeap::create_heap_space_summary() {
131 size_t capacity_in_words = capacity() / HeapWordSize;
132
133 return VirtualSpaceSummary(
134 _reserved.start(), _reserved.start() + capacity_in_words, _reserved.end());
135 }
136
137 GCHeapSummary CollectedHeap::create_heap_summary() {
138 VirtualSpaceSummary heap_space = create_heap_space_summary();
139 return GCHeapSummary(heap_space, used());
140 }
141
142 MetaspaceSummary CollectedHeap::create_metaspace_summary() {
143 const MetaspaceChunkFreeListSummary& ms_chunk_free_list_summary =
144 MetaspaceUtils::chunk_free_list_summary(Metaspace::NonClassType);
145 const MetaspaceChunkFreeListSummary& class_chunk_free_list_summary =
146 MetaspaceUtils::chunk_free_list_summary(Metaspace::ClassType);
147 return MetaspaceSummary(MetaspaceGC::capacity_until_GC(),
148 MetaspaceUtils::get_combined_statistics(),
149 ms_chunk_free_list_summary, class_chunk_free_list_summary);
150 }
151
152 void CollectedHeap::print_heap_before_gc() {
153 LogTarget(Debug, gc, heap) lt;
154 if (lt.is_enabled()) {
155 LogStream ls(lt);
156 ls.print_cr("Heap before GC invocations=%u (full %u):", total_collections(), total_full_collections());
157 ResourceMark rm;
158 print_on(&ls);
159 }
160
161 if (_gc_heap_log != NULL) {
162 _gc_heap_log->log_heap_before(this);
163 }
164 }
165
166 void CollectedHeap::print_heap_after_gc() {
167 LogTarget(Debug, gc, heap) lt;
168 if (lt.is_enabled()) {
169 LogStream ls(lt);
170 ls.print_cr("Heap after GC invocations=%u (full %u):", total_collections(), total_full_collections());
171 ResourceMark rm;
172 print_on(&ls);
173 }
174
175 if (_gc_heap_log != NULL) {
176 _gc_heap_log->log_heap_after(this);
177 }
178 }
179
180 void CollectedHeap::print() const { print_on(tty); }
181
182 void CollectedHeap::print_on_error(outputStream* st) const {
183 st->print_cr("Heap:");
184 print_extended_on(st);
185 st->cr();
186
187 BarrierSet* bs = BarrierSet::barrier_set();
188 if (bs != NULL) {
189 bs->print_on(st);
190 }
191 }
192
193 void CollectedHeap::trace_heap(GCWhen::Type when, const GCTracer* gc_tracer) {
194 const GCHeapSummary& heap_summary = create_heap_summary();
195 gc_tracer->report_gc_heap_summary(when, heap_summary);
196
197 const MetaspaceSummary& metaspace_summary = create_metaspace_summary();
198 gc_tracer->report_metaspace_summary(when, metaspace_summary);
199 }
200
201 void CollectedHeap::trace_heap_before_gc(const GCTracer* gc_tracer) {
202 trace_heap(GCWhen::BeforeGC, gc_tracer);
203 }
204
205 void CollectedHeap::trace_heap_after_gc(const GCTracer* gc_tracer) {
206 trace_heap(GCWhen::AfterGC, gc_tracer);
207 }
208
209 // Default implementation, for collectors that don't support the feature.
210 bool CollectedHeap::supports_concurrent_gc_breakpoints() const {
211 return false;
212 }
213
214 bool CollectedHeap::is_oop(oop object) const {
215 if (!is_object_aligned(object)) {
216 return false;
217 }
218
219 if (!is_in(object)) {
220 return false;
221 }
222
223 return true;
224 }
225
226 // Memory state functions.
227
228
229 CollectedHeap::CollectedHeap() :
230 _capacity_at_last_gc(0),
231 _used_at_last_gc(0),
232 _is_gc_active(false),
233 _last_whole_heap_examined_time_ns(os::javaTimeNanos()),
234 _total_collections(0),
235 _total_full_collections(0),
236 _gc_cause(GCCause::_no_gc),
237 _gc_lastcause(GCCause::_no_gc)
238 {
239 const size_t max_len = size_t(arrayOopDesc::max_array_length(T_INT));
240 const size_t elements_per_word = HeapWordSize / sizeof(jint);
241 int header_size_in_bytes = arrayOopDesc::base_offset_in_bytes(T_INT);
242 assert(header_size_in_bytes % sizeof(jint) == 0, "must be aligned to int");
243 int header_size_in_ints = header_size_in_bytes / sizeof(jint);
244 _filler_array_max_size = align_object_size((header_size_in_ints + max_len) / elements_per_word);
245
246 NOT_PRODUCT(_promotion_failure_alot_count = 0;)
247 NOT_PRODUCT(_promotion_failure_alot_gc_number = 0;)
248
249 if (UsePerfData) {
250 EXCEPTION_MARK;
251
252 // create the gc cause jvmstat counters
253 _perf_gc_cause = PerfDataManager::create_string_variable(SUN_GC, "cause",
254 80, GCCause::to_string(_gc_cause), CHECK);
255
256 _perf_gc_lastcause =
257 PerfDataManager::create_string_variable(SUN_GC, "lastCause",
258 80, GCCause::to_string(_gc_lastcause), CHECK);
259 }
260
261 // Create the ring log
262 if (LogEvents) {
263 _gc_heap_log = new GCHeapLog();
264 } else {
265 _gc_heap_log = NULL;
266 }
267 }
268
269 // This interface assumes that it's being called by the
270 // vm thread. It collects the heap assuming that the
271 // heap lock is already held and that we are executing in
272 // the context of the vm thread.
273 void CollectedHeap::collect_as_vm_thread(GCCause::Cause cause) {
274 Thread* thread = Thread::current();
275 assert(thread->is_VM_thread(), "Precondition#1");
276 assert(Heap_lock->is_locked(), "Precondition#2");
277 GCCauseSetter gcs(this, cause);
278 switch (cause) {
279 case GCCause::_heap_inspection:
280 case GCCause::_heap_dump:
281 case GCCause::_metadata_GC_threshold : {
282 HandleMark hm(thread);
283 do_full_collection(false); // don't clear all soft refs
284 break;
285 }
286 case GCCause::_archive_time_gc:
287 case GCCause::_metadata_GC_clear_soft_refs: {
288 HandleMark hm(thread);
289 do_full_collection(true); // do clear all soft refs
290 break;
291 }
292 default:
293 ShouldNotReachHere(); // Unexpected use of this function
294 }
295 }
296
297 MetaWord* CollectedHeap::satisfy_failed_metadata_allocation(ClassLoaderData* loader_data,
298 size_t word_size,
299 Metaspace::MetadataType mdtype) {
300 uint loop_count = 0;
301 uint gc_count = 0;
302 uint full_gc_count = 0;
303
304 assert(!Heap_lock->owned_by_self(), "Should not be holding the Heap_lock");
305
306 do {
307 MetaWord* result = loader_data->metaspace_non_null()->allocate(word_size, mdtype);
308 if (result != NULL) {
309 return result;
310 }
311
312 if (GCLocker::is_active_and_needs_gc()) {
313 // If the GCLocker is active, just expand and allocate.
314 // If that does not succeed, wait if this thread is not
315 // in a critical section itself.
316 result = loader_data->metaspace_non_null()->expand_and_allocate(word_size, mdtype);
317 if (result != NULL) {
318 return result;
319 }
320 JavaThread* jthr = JavaThread::current();
321 if (!jthr->in_critical()) {
322 // Wait for JNI critical section to be exited
323 GCLocker::stall_until_clear();
324 // The GC invoked by the last thread leaving the critical
325 // section will be a young collection and a full collection
326 // is (currently) needed for unloading classes so continue
327 // to the next iteration to get a full GC.
328 continue;
329 } else {
330 if (CheckJNICalls) {
331 fatal("Possible deadlock due to allocating while"
332 " in jni critical section");
333 }
334 return NULL;
335 }
336 }
337
338 { // Need lock to get self consistent gc_count's
339 MutexLocker ml(Heap_lock);
340 gc_count = Universe::heap()->total_collections();
341 full_gc_count = Universe::heap()->total_full_collections();
342 }
343
344 // Generate a VM operation
345 VM_CollectForMetadataAllocation op(loader_data,
346 word_size,
347 mdtype,
348 gc_count,
349 full_gc_count,
350 GCCause::_metadata_GC_threshold);
351 VMThread::execute(&op);
352
353 // If GC was locked out, try again. Check before checking success because the
354 // prologue could have succeeded and the GC still have been locked out.
355 if (op.gc_locked()) {
356 continue;
357 }
358
359 if (op.prologue_succeeded()) {
360 return op.result();
361 }
362 loop_count++;
363 if ((QueuedAllocationWarningCount > 0) &&
364 (loop_count % QueuedAllocationWarningCount == 0)) {
365 log_warning(gc, ergo)("satisfy_failed_metadata_allocation() retries %d times,"
366 " size=" SIZE_FORMAT, loop_count, word_size);
367 }
368 } while (true); // Until a GC is done
369 }
370
371 MemoryUsage CollectedHeap::memory_usage() {
372 return MemoryUsage(InitialHeapSize, used(), capacity(), max_capacity());
373 }
374
375 void CollectedHeap::set_gc_cause(GCCause::Cause v) {
376 if (UsePerfData) {
377 _gc_lastcause = _gc_cause;
378 _perf_gc_lastcause->set_value(GCCause::to_string(_gc_lastcause));
379 _perf_gc_cause->set_value(GCCause::to_string(v));
380 }
381 _gc_cause = v;
382 }
383
384 size_t CollectedHeap::max_tlab_size() const {
385 // TLABs can't be bigger than we can fill with a int[Integer.MAX_VALUE].
386 // This restriction could be removed by enabling filling with multiple arrays.
387 // If we compute that the reasonable way as
388 // header_size + ((sizeof(jint) * max_jint) / HeapWordSize)
389 // we'll overflow on the multiply, so we do the divide first.
390 // We actually lose a little by dividing first,
391 // but that just makes the TLAB somewhat smaller than the biggest array,
392 // which is fine, since we'll be able to fill that.
393 int header_size_in_bytes = typeArrayOopDesc::base_offset_in_bytes(T_INT);
394 assert(header_size_in_bytes % sizeof(jint) == 0, "header size must align to int");
395 size_t max_int_size = header_size_in_bytes / HeapWordSize +
396 sizeof(jint) *
397 ((juint) max_jint / (size_t) HeapWordSize);
398 return align_down(max_int_size, MinObjAlignment);
399 }
400
401 size_t CollectedHeap::filler_array_min_size() {
402 int aligned_header_size_words = align_up(arrayOopDesc::base_offset_in_bytes(T_INT), HeapWordSize) / HeapWordSize;
403 return align_object_size(aligned_header_size_words); // align to MinObjAlignment
404 }
405
406 #ifdef ASSERT
407 void CollectedHeap::fill_args_check(HeapWord* start, size_t words)
408 {
409 assert(words >= min_fill_size(), "too small to fill");
410 assert(is_object_aligned(words), "unaligned size");
411 }
412
413 void CollectedHeap::zap_filler_array(HeapWord* start, size_t words, bool zap)
414 {
415 if (ZapFillerObjects && zap) {
416 int payload_start = align_up(arrayOopDesc::base_offset_in_bytes(T_INT), HeapWordSize) / HeapWordSize;
417 Copy::fill_to_words(start + payload_start,
418 words - payload_start, 0XDEAFBABE);
419 }
420 }
421 #endif // ASSERT
422
423 void
424 CollectedHeap::fill_with_array(HeapWord* start, size_t words, bool zap)
425 {
426 assert(words >= filler_array_min_size(), "too small for an array");
427 assert(words <= filler_array_max_size(), "too big for a single object");
428
429 const size_t payload_size_bytes = words * HeapWordSize - arrayOopDesc::base_offset_in_bytes(T_INT);
430 assert(payload_size_bytes % sizeof(jint) == 0, "must be int aligned");
431 const size_t len = payload_size_bytes / sizeof(jint);
432 assert((int)len >= 0, "size too large " SIZE_FORMAT " becomes %d", words, (int)len);
433
434 ObjArrayAllocator allocator(Universe::intArrayKlassObj(), words, (int)len, /* do_zero */ false);
435 allocator.initialize(start);
436 DEBUG_ONLY(zap_filler_array(start, words, zap);)
437 }
438
439 void
440 CollectedHeap::fill_with_object_impl(HeapWord* start, size_t words, bool zap)
441 {
442 assert(words <= filler_array_max_size(), "too big for a single object");
443
444 if (words >= filler_array_min_size()) {
445 fill_with_array(start, words, zap);
446 } else if (words > 0) {
447 assert(words == min_fill_size(), "unaligned size");
448 ObjAllocator allocator(vmClasses::Object_klass(), words);
449 allocator.initialize(start);
450 }
451 }
452
453 void CollectedHeap::fill_with_object(HeapWord* start, size_t words, bool zap)
454 {
455 DEBUG_ONLY(fill_args_check(start, words);)
456 HandleMark hm(Thread::current()); // Free handles before leaving.
457 fill_with_object_impl(start, words, zap);
458 }
459
460 void CollectedHeap::fill_with_objects(HeapWord* start, size_t words, bool zap)
461 {
462 DEBUG_ONLY(fill_args_check(start, words);)
463 HandleMark hm(Thread::current()); // Free handles before leaving.
464
465 // Multiple objects may be required depending on the filler array maximum size. Fill
466 // the range up to that with objects that are filler_array_max_size sized. The
467 // remainder is filled with a single object.
468 const size_t min = min_fill_size();
469 const size_t max = filler_array_max_size();
470 while (words > max) {
471 const size_t cur = (words - max) >= min ? max : max - min;
472 fill_with_array(start, cur, zap);
473 start += cur;
474 words -= cur;
475 }
476
477 fill_with_object_impl(start, words, zap);
478 }
479
480 void CollectedHeap::fill_with_dummy_object(HeapWord* start, HeapWord* end, bool zap) {
481 CollectedHeap::fill_with_object(start, end, zap);
482 }
483
484 size_t CollectedHeap::min_dummy_object_size() const {
485 return oopDesc::header_size();
486 }
487
488 size_t CollectedHeap::tlab_alloc_reserve() const {
489 size_t min_size = min_dummy_object_size();
490 return min_size > (size_t)MinObjAlignment ? align_object_size(min_size) : 0;
491 }
492
493 HeapWord* CollectedHeap::allocate_new_tlab(size_t min_size,
494 size_t requested_size,
495 size_t* actual_size) {
496 guarantee(false, "thread-local allocation buffers not supported");
497 return NULL;
498 }
499
500 void CollectedHeap::ensure_parsability(bool retire_tlabs) {
501 assert(SafepointSynchronize::is_at_safepoint() || !is_init_completed(),
502 "Should only be called at a safepoint or at start-up");
503
504 ThreadLocalAllocStats stats;
505
506 for (JavaThreadIteratorWithHandle jtiwh; JavaThread *thread = jtiwh.next();) {
507 BarrierSet::barrier_set()->make_parsable(thread);
508 if (UseTLAB) {
509 if (retire_tlabs) {
510 thread->tlab().retire(&stats);
511 } else {
512 thread->tlab().make_parsable();
513 }
514 }
515 }
516
517 stats.publish();
518 }
519
520 void CollectedHeap::resize_all_tlabs() {
521 assert(SafepointSynchronize::is_at_safepoint() || !is_init_completed(),
522 "Should only resize tlabs at safepoint");
523
524 if (UseTLAB && ResizeTLAB) {
525 for (JavaThreadIteratorWithHandle jtiwh; JavaThread *thread = jtiwh.next(); ) {
526 thread->tlab().resize();
527 }
528 }
529 }
530
531 jlong CollectedHeap::millis_since_last_whole_heap_examined() {
532 return (os::javaTimeNanos() - _last_whole_heap_examined_time_ns) / NANOSECS_PER_MILLISEC;
533 }
534
535 void CollectedHeap::record_whole_heap_examined_timestamp() {
536 _last_whole_heap_examined_time_ns = os::javaTimeNanos();
537 }
538
539 void CollectedHeap::full_gc_dump(GCTimer* timer, bool before) {
540 assert(timer != NULL, "timer is null");
541 if ((HeapDumpBeforeFullGC && before) || (HeapDumpAfterFullGC && !before)) {
542 GCTraceTime(Info, gc) tm(before ? "Heap Dump (before full gc)" : "Heap Dump (after full gc)", timer);
543 HeapDumper::dump_heap();
544 }
545
546 LogTarget(Trace, gc, classhisto) lt;
547 if (lt.is_enabled()) {
548 GCTraceTime(Trace, gc, classhisto) tm(before ? "Class Histogram (before full gc)" : "Class Histogram (after full gc)", timer);
549 ResourceMark rm;
550 LogStream ls(lt);
551 VM_GC_HeapInspection inspector(&ls, false /* ! full gc */);
552 inspector.doit();
553 }
554 }
555
556 void CollectedHeap::pre_full_gc_dump(GCTimer* timer) {
557 full_gc_dump(timer, true);
558 }
559
560 void CollectedHeap::post_full_gc_dump(GCTimer* timer) {
561 full_gc_dump(timer, false);
562 }
563
564 void CollectedHeap::initialize_reserved_region(const ReservedHeapSpace& rs) {
565 // It is important to do this in a way such that concurrent readers can't
566 // temporarily think something is in the heap. (Seen this happen in asserts.)
567 _reserved.set_word_size(0);
568 _reserved.set_start((HeapWord*)rs.base());
569 _reserved.set_end((HeapWord*)rs.end());
570 }
571
572 void CollectedHeap::post_initialize() {
573 StringDedup::initialize();
574 initialize_serviceability();
575 }
576
577 #ifndef PRODUCT
578
579 bool CollectedHeap::promotion_should_fail(volatile size_t* count) {
580 // Access to count is not atomic; the value does not have to be exact.
581 if (PromotionFailureALot) {
582 const size_t gc_num = total_collections();
583 const size_t elapsed_gcs = gc_num - _promotion_failure_alot_gc_number;
584 if (elapsed_gcs >= PromotionFailureALotInterval) {
585 // Test for unsigned arithmetic wrap-around.
586 if (++*count >= PromotionFailureALotCount) {
587 *count = 0;
588 return true;
589 }
590 }
591 }
592 return false;
593 }
594
595 bool CollectedHeap::promotion_should_fail() {
596 return promotion_should_fail(&_promotion_failure_alot_count);
597 }
598
599 void CollectedHeap::reset_promotion_should_fail(volatile size_t* count) {
600 if (PromotionFailureALot) {
601 _promotion_failure_alot_gc_number = total_collections();
602 *count = 0;
603 }
604 }
605
606 void CollectedHeap::reset_promotion_should_fail() {
607 reset_promotion_should_fail(&_promotion_failure_alot_count);
608 }
609
610 #endif // #ifndef PRODUCT
611
612 bool CollectedHeap::supports_object_pinning() const {
613 return false;
614 }
615
616 oop CollectedHeap::pin_object(JavaThread* thread, oop obj) {
617 ShouldNotReachHere();
618 return NULL;
619 }
620
621 void CollectedHeap::unpin_object(JavaThread* thread, oop obj) {
622 ShouldNotReachHere();
623 }
624
625 bool CollectedHeap::is_archived_object(oop object) const {
626 return false;
627 }
628
629 uint32_t CollectedHeap::hash_oop(oop obj) const {
630 const uintptr_t addr = cast_from_oop<uintptr_t>(obj);
631 return static_cast<uint32_t>(addr >> LogMinObjAlignment);
632 }
633
634 // It's the caller's responsibility to ensure glitch-freedom
635 // (if required).
636 void CollectedHeap::update_capacity_and_used_at_gc() {
637 _capacity_at_last_gc = capacity();
638 _used_at_last_gc = used();
639 }