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 _filler_array_max_size = align_object_size(filler_array_hdr_size() +
242 max_len / elements_per_word);
243
244 NOT_PRODUCT(_promotion_failure_alot_count = 0;)
245 NOT_PRODUCT(_promotion_failure_alot_gc_number = 0;)
246
247 if (UsePerfData) {
248 EXCEPTION_MARK;
249
250 // create the gc cause jvmstat counters
251 _perf_gc_cause = PerfDataManager::create_string_variable(SUN_GC, "cause",
252 80, GCCause::to_string(_gc_cause), CHECK);
253
254 _perf_gc_lastcause =
255 PerfDataManager::create_string_variable(SUN_GC, "lastCause",
256 80, GCCause::to_string(_gc_lastcause), CHECK);
257 }
258
259 // Create the ring log
260 if (LogEvents) {
261 _gc_heap_log = new GCHeapLog();
262 } else {
263 _gc_heap_log = NULL;
264 }
265 }
266
267 // This interface assumes that it's being called by the
268 // vm thread. It collects the heap assuming that the
269 // heap lock is already held and that we are executing in
270 // the context of the vm thread.
271 void CollectedHeap::collect_as_vm_thread(GCCause::Cause cause) {
272 Thread* thread = Thread::current();
273 assert(thread->is_VM_thread(), "Precondition#1");
274 assert(Heap_lock->is_locked(), "Precondition#2");
275 GCCauseSetter gcs(this, cause);
276 switch (cause) {
277 case GCCause::_heap_inspection:
278 case GCCause::_heap_dump:
279 case GCCause::_metadata_GC_threshold : {
280 HandleMark hm(thread);
281 do_full_collection(false); // don't clear all soft refs
282 break;
283 }
284 case GCCause::_archive_time_gc:
285 case GCCause::_metadata_GC_clear_soft_refs: {
286 HandleMark hm(thread);
287 do_full_collection(true); // do clear all soft refs
288 break;
289 }
290 default:
291 ShouldNotReachHere(); // Unexpected use of this function
292 }
293 }
294
295 MetaWord* CollectedHeap::satisfy_failed_metadata_allocation(ClassLoaderData* loader_data,
296 size_t word_size,
297 Metaspace::MetadataType mdtype) {
298 uint loop_count = 0;
299 uint gc_count = 0;
300 uint full_gc_count = 0;
301
302 assert(!Heap_lock->owned_by_self(), "Should not be holding the Heap_lock");
303
304 do {
305 MetaWord* result = loader_data->metaspace_non_null()->allocate(word_size, mdtype);
306 if (result != NULL) {
307 return result;
308 }
309
310 if (GCLocker::is_active_and_needs_gc()) {
311 // If the GCLocker is active, just expand and allocate.
312 // If that does not succeed, wait if this thread is not
313 // in a critical section itself.
314 result = loader_data->metaspace_non_null()->expand_and_allocate(word_size, mdtype);
315 if (result != NULL) {
316 return result;
317 }
318 JavaThread* jthr = JavaThread::current();
319 if (!jthr->in_critical()) {
320 // Wait for JNI critical section to be exited
321 GCLocker::stall_until_clear();
322 // The GC invoked by the last thread leaving the critical
323 // section will be a young collection and a full collection
324 // is (currently) needed for unloading classes so continue
325 // to the next iteration to get a full GC.
326 continue;
327 } else {
328 if (CheckJNICalls) {
329 fatal("Possible deadlock due to allocating while"
330 " in jni critical section");
331 }
332 return NULL;
333 }
334 }
335
336 { // Need lock to get self consistent gc_count's
337 MutexLocker ml(Heap_lock);
338 gc_count = Universe::heap()->total_collections();
339 full_gc_count = Universe::heap()->total_full_collections();
340 }
341
342 // Generate a VM operation
343 VM_CollectForMetadataAllocation op(loader_data,
344 word_size,
345 mdtype,
346 gc_count,
347 full_gc_count,
348 GCCause::_metadata_GC_threshold);
349 VMThread::execute(&op);
350
351 // If GC was locked out, try again. Check before checking success because the
352 // prologue could have succeeded and the GC still have been locked out.
353 if (op.gc_locked()) {
354 continue;
355 }
356
357 if (op.prologue_succeeded()) {
358 return op.result();
359 }
360 loop_count++;
361 if ((QueuedAllocationWarningCount > 0) &&
362 (loop_count % QueuedAllocationWarningCount == 0)) {
363 log_warning(gc, ergo)("satisfy_failed_metadata_allocation() retries %d times,"
364 " size=" SIZE_FORMAT, loop_count, word_size);
365 }
366 } while (true); // Until a GC is done
367 }
368
369 MemoryUsage CollectedHeap::memory_usage() {
370 return MemoryUsage(InitialHeapSize, used(), capacity(), max_capacity());
371 }
372
373 void CollectedHeap::set_gc_cause(GCCause::Cause v) {
374 if (UsePerfData) {
375 _gc_lastcause = _gc_cause;
376 _perf_gc_lastcause->set_value(GCCause::to_string(_gc_lastcause));
377 _perf_gc_cause->set_value(GCCause::to_string(v));
378 }
379 _gc_cause = v;
380 }
381
382 #ifndef PRODUCT
383 void CollectedHeap::check_for_non_bad_heap_word_value(HeapWord* addr, size_t size) {
384 if (CheckMemoryInitialization && ZapUnusedHeapArea) {
385 // please note mismatch between size (in 32/64 bit words), and ju_addr that always point to a 32 bit word
386 for (juint* ju_addr = reinterpret_cast<juint*>(addr); ju_addr < reinterpret_cast<juint*>(addr + size); ++ju_addr) {
387 assert(*ju_addr == badHeapWordVal, "Found non badHeapWordValue in pre-allocation check");
388 }
389 }
390 }
391 #endif // PRODUCT
392
393 size_t CollectedHeap::max_tlab_size() const {
394 // TLABs can't be bigger than we can fill with a int[Integer.MAX_VALUE].
395 // This restriction could be removed by enabling filling with multiple arrays.
396 // If we compute that the reasonable way as
397 // header_size + ((sizeof(jint) * max_jint) / HeapWordSize)
398 // we'll overflow on the multiply, so we do the divide first.
399 // We actually lose a little by dividing first,
400 // but that just makes the TLAB somewhat smaller than the biggest array,
401 // which is fine, since we'll be able to fill that.
402 size_t max_int_size = typeArrayOopDesc::header_size(T_INT) +
403 sizeof(jint) *
404 ((juint) max_jint / (size_t) HeapWordSize);
405 return align_down(max_int_size, MinObjAlignment);
406 }
407
408 size_t CollectedHeap::filler_array_hdr_size() {
409 return align_object_offset(arrayOopDesc::header_size(T_INT)); // align to Long
410 }
411
412 size_t CollectedHeap::filler_array_min_size() {
413 return align_object_size(filler_array_hdr_size()); // align to MinObjAlignment
414 }
415
416 #ifdef ASSERT
417 void CollectedHeap::fill_args_check(HeapWord* start, size_t words)
418 {
419 assert(words >= min_fill_size(), "too small to fill");
420 assert(is_object_aligned(words), "unaligned size");
421 }
422
423 void CollectedHeap::zap_filler_array(HeapWord* start, size_t words, bool zap)
424 {
425 if (ZapFillerObjects && zap) {
426 Copy::fill_to_words(start + filler_array_hdr_size(),
427 words - filler_array_hdr_size(), 0XDEAFBABE);
428 }
429 }
430 #endif // ASSERT
431
432 void
433 CollectedHeap::fill_with_array(HeapWord* start, size_t words, bool zap)
434 {
435 assert(words >= filler_array_min_size(), "too small for an array");
436 assert(words <= filler_array_max_size(), "too big for a single object");
437
438 const size_t payload_size = words - filler_array_hdr_size();
439 const size_t len = payload_size * HeapWordSize / sizeof(jint);
440 assert((int)len >= 0, "size too large " SIZE_FORMAT " becomes %d", words, (int)len);
441
442 ObjArrayAllocator allocator(Universe::intArrayKlassObj(), words, (int)len, /* do_zero */ false);
443 allocator.initialize(start);
444 DEBUG_ONLY(zap_filler_array(start, words, zap);)
445 }
446
447 void
448 CollectedHeap::fill_with_object_impl(HeapWord* start, size_t words, bool zap)
449 {
450 assert(words <= filler_array_max_size(), "too big for a single object");
451
452 if (words >= filler_array_min_size()) {
453 fill_with_array(start, words, zap);
454 } else if (words > 0) {
455 assert(words == min_fill_size(), "unaligned size");
456 ObjAllocator allocator(vmClasses::Object_klass(), words);
457 allocator.initialize(start);
458 }
459 }
460
461 void CollectedHeap::fill_with_object(HeapWord* start, size_t words, bool zap)
462 {
463 DEBUG_ONLY(fill_args_check(start, words);)
464 HandleMark hm(Thread::current()); // Free handles before leaving.
465 fill_with_object_impl(start, words, zap);
466 }
467
468 void CollectedHeap::fill_with_objects(HeapWord* start, size_t words, bool zap)
469 {
470 DEBUG_ONLY(fill_args_check(start, words);)
471 HandleMark hm(Thread::current()); // Free handles before leaving.
472
473 // Multiple objects may be required depending on the filler array maximum size. Fill
474 // the range up to that with objects that are filler_array_max_size sized. The
475 // remainder is filled with a single object.
476 const size_t min = min_fill_size();
477 const size_t max = filler_array_max_size();
478 while (words > max) {
479 const size_t cur = (words - max) >= min ? max : max - min;
480 fill_with_array(start, cur, zap);
481 start += cur;
482 words -= cur;
483 }
484
485 fill_with_object_impl(start, words, zap);
486 }
487
488 void CollectedHeap::fill_with_dummy_object(HeapWord* start, HeapWord* end, bool zap) {
489 CollectedHeap::fill_with_object(start, end, zap);
490 }
491
492 size_t CollectedHeap::min_dummy_object_size() const {
493 return oopDesc::header_size();
494 }
495
496 size_t CollectedHeap::tlab_alloc_reserve() const {
497 size_t min_size = min_dummy_object_size();
498 return min_size > (size_t)MinObjAlignment ? align_object_size(min_size) : 0;
499 }
500
501 HeapWord* CollectedHeap::allocate_new_tlab(size_t min_size,
502 size_t requested_size,
503 size_t* actual_size) {
504 guarantee(false, "thread-local allocation buffers not supported");
505 return NULL;
506 }
507
508 void CollectedHeap::ensure_parsability(bool retire_tlabs) {
509 assert(SafepointSynchronize::is_at_safepoint() || !is_init_completed(),
510 "Should only be called at a safepoint or at start-up");
511
512 ThreadLocalAllocStats stats;
513
514 for (JavaThreadIteratorWithHandle jtiwh; JavaThread *thread = jtiwh.next();) {
515 BarrierSet::barrier_set()->make_parsable(thread);
516 if (UseTLAB) {
517 if (retire_tlabs) {
518 thread->tlab().retire(&stats);
519 } else {
520 thread->tlab().make_parsable();
521 }
522 }
523 }
524
525 stats.publish();
526 }
527
528 void CollectedHeap::resize_all_tlabs() {
529 assert(SafepointSynchronize::is_at_safepoint() || !is_init_completed(),
530 "Should only resize tlabs at safepoint");
531
532 if (UseTLAB && ResizeTLAB) {
533 for (JavaThreadIteratorWithHandle jtiwh; JavaThread *thread = jtiwh.next(); ) {
534 thread->tlab().resize();
535 }
536 }
537 }
538
539 jlong CollectedHeap::millis_since_last_whole_heap_examined() {
540 return (os::javaTimeNanos() - _last_whole_heap_examined_time_ns) / NANOSECS_PER_MILLISEC;
541 }
542
543 void CollectedHeap::record_whole_heap_examined_timestamp() {
544 _last_whole_heap_examined_time_ns = os::javaTimeNanos();
545 }
546
547 void CollectedHeap::full_gc_dump(GCTimer* timer, bool before) {
548 assert(timer != NULL, "timer is null");
549 if ((HeapDumpBeforeFullGC && before) || (HeapDumpAfterFullGC && !before)) {
550 GCTraceTime(Info, gc) tm(before ? "Heap Dump (before full gc)" : "Heap Dump (after full gc)", timer);
551 HeapDumper::dump_heap();
552 }
553
554 LogTarget(Trace, gc, classhisto) lt;
555 if (lt.is_enabled()) {
556 GCTraceTime(Trace, gc, classhisto) tm(before ? "Class Histogram (before full gc)" : "Class Histogram (after full gc)", timer);
557 ResourceMark rm;
558 LogStream ls(lt);
559 VM_GC_HeapInspection inspector(&ls, false /* ! full gc */);
560 inspector.doit();
561 }
562 }
563
564 void CollectedHeap::pre_full_gc_dump(GCTimer* timer) {
565 full_gc_dump(timer, true);
566 }
567
568 void CollectedHeap::post_full_gc_dump(GCTimer* timer) {
569 full_gc_dump(timer, false);
570 }
571
572 void CollectedHeap::initialize_reserved_region(const ReservedHeapSpace& rs) {
573 // It is important to do this in a way such that concurrent readers can't
574 // temporarily think something is in the heap. (Seen this happen in asserts.)
575 _reserved.set_word_size(0);
576 _reserved.set_start((HeapWord*)rs.base());
577 _reserved.set_end((HeapWord*)rs.end());
578 }
579
580 void CollectedHeap::post_initialize() {
581 StringDedup::initialize();
582 initialize_serviceability();
583 }
584
585 #ifndef PRODUCT
586
587 bool CollectedHeap::promotion_should_fail(volatile size_t* count) {
588 // Access to count is not atomic; the value does not have to be exact.
589 if (PromotionFailureALot) {
590 const size_t gc_num = total_collections();
591 const size_t elapsed_gcs = gc_num - _promotion_failure_alot_gc_number;
592 if (elapsed_gcs >= PromotionFailureALotInterval) {
593 // Test for unsigned arithmetic wrap-around.
594 if (++*count >= PromotionFailureALotCount) {
595 *count = 0;
596 return true;
597 }
598 }
599 }
600 return false;
601 }
602
603 bool CollectedHeap::promotion_should_fail() {
604 return promotion_should_fail(&_promotion_failure_alot_count);
605 }
606
607 void CollectedHeap::reset_promotion_should_fail(volatile size_t* count) {
608 if (PromotionFailureALot) {
609 _promotion_failure_alot_gc_number = total_collections();
610 *count = 0;
611 }
612 }
613
614 void CollectedHeap::reset_promotion_should_fail() {
615 reset_promotion_should_fail(&_promotion_failure_alot_count);
616 }
617
618 #endif // #ifndef PRODUCT
619
620 bool CollectedHeap::supports_object_pinning() const {
621 return false;
622 }
623
624 oop CollectedHeap::pin_object(JavaThread* thread, oop obj) {
625 ShouldNotReachHere();
626 return NULL;
627 }
628
629 void CollectedHeap::unpin_object(JavaThread* thread, oop obj) {
630 ShouldNotReachHere();
631 }
632
633 bool CollectedHeap::is_archived_object(oop object) const {
634 return false;
635 }
636
637 uint32_t CollectedHeap::hash_oop(oop obj) const {
638 const uintptr_t addr = cast_from_oop<uintptr_t>(obj);
639 return static_cast<uint32_t>(addr >> LogMinObjAlignment);
640 }
641
642 // It's the caller's responsibility to ensure glitch-freedom
643 // (if required).
644 void CollectedHeap::update_capacity_and_used_at_gc() {
645 _capacity_at_last_gc = capacity();
646 _used_at_last_gc = used();
647 }