1 /*
2 * Copyright (c) 2000, 2014, 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/symbolTable.hpp"
27 #include "classfile/systemDictionary.hpp"
28 #include "classfile/vmSymbols.hpp"
29 #include "code/icBuffer.hpp"
30 #include "gc_implementation/shared/collectorCounters.hpp"
31 #include "gc_implementation/shared/gcTrace.hpp"
32 #include "gc_implementation/shared/gcTraceTime.hpp"
33 #include "gc_implementation/shared/vmGCOperations.hpp"
34 #include "gc_interface/collectedHeap.inline.hpp"
35 #include "memory/filemap.hpp"
36 #include "memory/gcLocker.inline.hpp"
37 #include "memory/genCollectedHeap.hpp"
38 #include "memory/genOopClosures.inline.hpp"
39 #include "memory/generation.inline.hpp"
40 #include "memory/generationSpec.hpp"
41 #include "memory/resourceArea.hpp"
42 #include "memory/sharedHeap.hpp"
43 #include "memory/space.hpp"
44 #include "oops/oop.inline.hpp"
45 #include "oops/oop.inline2.hpp"
46 #include "runtime/biasedLocking.hpp"
47 #include "runtime/fprofiler.hpp"
48 #include "runtime/handles.hpp"
49 #include "runtime/handles.inline.hpp"
50 #include "runtime/java.hpp"
51 #include "runtime/vmThread.hpp"
52 #include "services/memoryService.hpp"
53 #include "utilities/vmError.hpp"
54 #include "utilities/workgroup.hpp"
55 #include "utilities/macros.hpp"
56 #if INCLUDE_ALL_GCS
57 #include "gc_implementation/concurrentMarkSweep/concurrentMarkSweepThread.hpp"
58 #include "gc_implementation/concurrentMarkSweep/vmCMSOperations.hpp"
59 #endif // INCLUDE_ALL_GCS
60
61 GenCollectedHeap* GenCollectedHeap::_gch;
62 NOT_PRODUCT(size_t GenCollectedHeap::_skip_header_HeapWords = 0;)
63
64 // The set of potentially parallel tasks in root scanning.
65 enum GCH_strong_roots_tasks {
66 // We probably want to parallelize both of these internally, but for now...
67 GCH_PS_younger_gens,
68 // Leave this one last.
69 GCH_PS_NumElements
70 };
71
72 GenCollectedHeap::GenCollectedHeap(GenCollectorPolicy *policy) :
73 SharedHeap(policy),
74 _gen_policy(policy),
75 _gen_process_roots_tasks(new SubTasksDone(GCH_PS_NumElements)),
76 _full_collections_completed(0)
77 {
78 if (_gen_process_roots_tasks == NULL ||
79 !_gen_process_roots_tasks->valid()) {
80 vm_exit_during_initialization("Failed necessary allocation.");
81 }
82 assert(policy != NULL, "Sanity check");
83 }
84
85 jint GenCollectedHeap::initialize() {
86 CollectedHeap::pre_initialize();
87
88 int i;
89 _n_gens = gen_policy()->number_of_generations();
90
91 // While there are no constraints in the GC code that HeapWordSize
92 // be any particular value, there are multiple other areas in the
93 // system which believe this to be true (e.g. oop->object_size in some
94 // cases incorrectly returns the size in wordSize units rather than
95 // HeapWordSize).
96 guarantee(HeapWordSize == wordSize, "HeapWordSize must equal wordSize");
97
98 // The heap must be at least as aligned as generations.
99 size_t gen_alignment = Generation::GenGrain;
100
101 _gen_specs = gen_policy()->generations();
102
103 // Make sure the sizes are all aligned.
104 for (i = 0; i < _n_gens; i++) {
105 _gen_specs[i]->align(gen_alignment);
106 }
107
108 // Allocate space for the heap.
109
110 char* heap_address;
111 size_t total_reserved = 0;
112 int n_covered_regions = 0;
113 ReservedSpace heap_rs;
114
115 size_t heap_alignment = collector_policy()->heap_alignment();
116
117 heap_address = allocate(heap_alignment, &total_reserved,
118 &n_covered_regions, &heap_rs);
119
120 if (!heap_rs.is_reserved()) {
121 vm_shutdown_during_initialization(
122 "Could not reserve enough space for object heap");
123 return JNI_ENOMEM;
124 }
125
126 initialize_reserved_region((HeapWord*)heap_rs.base(), (HeapWord*)(heap_rs.base() + heap_rs.size()));
127
128 _rem_set = collector_policy()->create_rem_set(reserved_region(), n_covered_regions);
129 set_barrier_set(rem_set()->bs());
130
131 _gch = this;
132
133 ReservedSpace young_rs = heap_rs.first_part(_gen_specs[0]->max_size(), false, false);
134 _young_gen = _gen_specs[0]->init(young_rs, 0, rem_set());
135 heap_rs = heap_rs.last_part(_gen_specs[0]->max_size());
136
137 ReservedSpace old_rs = heap_rs.first_part(_gen_specs[1]->max_size(), false, false);
138 _old_gen = _gen_specs[1]->init(old_rs, 1, rem_set());
139 heap_rs = heap_rs.last_part(_gen_specs[1]->max_size());
140 clear_incremental_collection_failed();
141
142 #if INCLUDE_ALL_GCS
143 // If we are running CMS, create the collector responsible
144 // for collecting the CMS generations.
145 if (collector_policy()->is_concurrent_mark_sweep_policy()) {
146 bool success = create_cms_collector();
147 if (!success) return JNI_ENOMEM;
148 }
149 #endif // INCLUDE_ALL_GCS
150
151 return JNI_OK;
152 }
153
154 char* GenCollectedHeap::allocate(size_t alignment,
155 size_t* _total_reserved,
156 int* _n_covered_regions,
157 ReservedSpace* heap_rs){
158 const char overflow_msg[] = "The size of the object heap + VM data exceeds "
159 "the maximum representable size";
160
161 // Now figure out the total size.
162 size_t total_reserved = 0;
163 int n_covered_regions = 0;
164 const size_t pageSize = UseLargePages ?
165 os::large_page_size() : os::vm_page_size();
166
167 assert(alignment % pageSize == 0, "Must be");
168
169 for (int i = 0; i < _n_gens; i++) {
170 total_reserved += _gen_specs[i]->max_size();
171 if (total_reserved < _gen_specs[i]->max_size()) {
172 vm_exit_during_initialization(overflow_msg);
173 }
174 n_covered_regions += _gen_specs[i]->n_covered_regions();
175 }
176 assert(total_reserved % alignment == 0,
177 err_msg("Gen size; total_reserved=" SIZE_FORMAT ", alignment="
178 SIZE_FORMAT, total_reserved, alignment));
179
180 // Needed until the cardtable is fixed to have the right number
181 // of covered regions.
182 n_covered_regions += 2;
183
184 *_total_reserved = total_reserved;
185 *_n_covered_regions = n_covered_regions;
186
187 *heap_rs = Universe::reserve_heap(total_reserved, alignment);
188 return heap_rs->base();
189 }
190
191 void GenCollectedHeap::post_initialize() {
192 SharedHeap::post_initialize();
193 GenCollectorPolicy *policy = (GenCollectorPolicy *)collector_policy();
194 guarantee(policy->is_generation_policy(), "Illegal policy type");
195 DefNewGeneration* def_new_gen = (DefNewGeneration*) get_gen(0);
196 assert(def_new_gen->kind() == Generation::DefNew ||
197 def_new_gen->kind() == Generation::ParNew,
198 "Wrong generation kind");
199
200 Generation* old_gen = get_gen(1);
201 assert(old_gen->kind() == Generation::ConcurrentMarkSweep ||
202 old_gen->kind() == Generation::MarkSweepCompact,
203 "Wrong generation kind");
204
205 policy->initialize_size_policy(def_new_gen->eden()->capacity(),
206 old_gen->capacity(),
207 def_new_gen->from()->capacity());
208 policy->initialize_gc_policy_counters();
209 }
210
211 void GenCollectedHeap::ref_processing_init() {
212 SharedHeap::ref_processing_init();
213 _young_gen->ref_processor_init();
214 _old_gen->ref_processor_init();
215 }
216
217 size_t GenCollectedHeap::capacity() const {
218 return _young_gen->capacity() + _old_gen->capacity();
219 }
220
221 size_t GenCollectedHeap::used() const {
222 return _young_gen->used() + _old_gen->used();
223 }
224
225 // Save the "used_region" for generations level and lower.
226 void GenCollectedHeap::save_used_regions(int level) {
227 assert(level < _n_gens, "Illegal level parameter");
228 if (level == 1) {
229 _old_gen->save_used_region();
230 }
231 _young_gen->save_used_region();
232 }
233
234 size_t GenCollectedHeap::max_capacity() const {
235 return _young_gen->max_capacity() + _old_gen->max_capacity();
236 }
237
238 // Update the _full_collections_completed counter
239 // at the end of a stop-world full GC.
240 unsigned int GenCollectedHeap::update_full_collections_completed() {
241 MonitorLockerEx ml(FullGCCount_lock, Mutex::_no_safepoint_check_flag);
242 assert(_full_collections_completed <= _total_full_collections,
243 "Can't complete more collections than were started");
244 _full_collections_completed = _total_full_collections;
245 ml.notify_all();
246 return _full_collections_completed;
247 }
248
249 // Update the _full_collections_completed counter, as appropriate,
250 // at the end of a concurrent GC cycle. Note the conditional update
251 // below to allow this method to be called by a concurrent collector
252 // without synchronizing in any manner with the VM thread (which
253 // may already have initiated a STW full collection "concurrently").
254 unsigned int GenCollectedHeap::update_full_collections_completed(unsigned int count) {
255 MonitorLockerEx ml(FullGCCount_lock, Mutex::_no_safepoint_check_flag);
256 assert((_full_collections_completed <= _total_full_collections) &&
257 (count <= _total_full_collections),
258 "Can't complete more collections than were started");
259 if (count > _full_collections_completed) {
260 _full_collections_completed = count;
261 ml.notify_all();
262 }
263 return _full_collections_completed;
264 }
265
266
267 #ifndef PRODUCT
268 // Override of memory state checking method in CollectedHeap:
269 // Some collectors (CMS for example) can't have badHeapWordVal written
270 // in the first two words of an object. (For instance , in the case of
271 // CMS these words hold state used to synchronize between certain
272 // (concurrent) GC steps and direct allocating mutators.)
273 // The skip_header_HeapWords() method below, allows us to skip
274 // over the requisite number of HeapWord's. Note that (for
275 // generational collectors) this means that those many words are
276 // skipped in each object, irrespective of the generation in which
277 // that object lives. The resultant loss of precision seems to be
278 // harmless and the pain of avoiding that imprecision appears somewhat
279 // higher than we are prepared to pay for such rudimentary debugging
280 // support.
281 void GenCollectedHeap::check_for_non_bad_heap_word_value(HeapWord* addr,
282 size_t size) {
283 if (CheckMemoryInitialization && ZapUnusedHeapArea) {
284 // We are asked to check a size in HeapWords,
285 // but the memory is mangled in juint words.
286 juint* start = (juint*) (addr + skip_header_HeapWords());
287 juint* end = (juint*) (addr + size);
288 for (juint* slot = start; slot < end; slot += 1) {
289 assert(*slot == badHeapWordVal,
290 "Found non badHeapWordValue in pre-allocation check");
291 }
292 }
293 }
294 #endif
295
296 HeapWord* GenCollectedHeap::attempt_allocation(size_t size,
297 bool is_tlab,
298 bool first_only) {
299 HeapWord* res = NULL;
300
301 if (_young_gen->should_allocate(size, is_tlab)) {
302 res = _young_gen->allocate(size, is_tlab);
303 if (res != NULL || first_only) {
304 return res;
305 }
306 }
307
308 if (_old_gen->should_allocate(size, is_tlab)) {
309 res = _old_gen->allocate(size, is_tlab);
310 }
311
312 return res;
313 }
314
315 HeapWord* GenCollectedHeap::mem_allocate(size_t size,
316 bool* gc_overhead_limit_was_exceeded) {
317 return collector_policy()->mem_allocate_work(size,
318 false /* is_tlab */,
319 gc_overhead_limit_was_exceeded);
320 }
321
322 bool GenCollectedHeap::must_clear_all_soft_refs() {
323 return _gc_cause == GCCause::_last_ditch_collection;
324 }
325
326 bool GenCollectedHeap::should_do_concurrent_full_gc(GCCause::Cause cause) {
327 return UseConcMarkSweepGC &&
328 ((cause == GCCause::_gc_locker && GCLockerInvokesConcurrent) ||
329 (cause == GCCause::_java_lang_system_gc && ExplicitGCInvokesConcurrent));
330 }
331
332 void GenCollectedHeap::collect_generation(Generation* gen, bool full, size_t size,
333 bool is_tlab, bool run_verification, bool clear_soft_refs) {
334 // Timer for individual generations. Last argument is false: no CR
335 // FIXME: We should try to start the timing earlier to cover more of the GC pause
336 // The PrintGCDetails logging starts before we have incremented the GC id. We will do that later
337 // so we can assume here that the next GC id is what we want.
338 GCTraceTime t1(gen->short_name(), PrintGCDetails, false, NULL, GCId::peek());
339 TraceCollectorStats tcs(gen->counters());
340 TraceMemoryManagerStats tmms(gen->kind(),gc_cause());
341
342 size_t prev_used = gen->used();
343 gen->stat_record()->invocations++;
344 gen->stat_record()->accumulated_time.start();
345
346 // Must be done anew before each collection because
347 // a previous collection will do mangling and will
348 // change top of some spaces.
349 record_gen_tops_before_GC();
350
351 if (PrintGC && Verbose) {
352 gclog_or_tty->print("level=%d invoke=%d size=" SIZE_FORMAT,
353 gen->level(),
354 gen->stat_record()->invocations,
355 size * HeapWordSize);
356 }
357
358 if (run_verification && VerifyBeforeGC) {
359 HandleMark hm; // Discard invalid handles created during verification
360 Universe::verify(" VerifyBeforeGC:");
361 }
362 COMPILER2_PRESENT(DerivedPointerTable::clear());
363
364 // Do collection work
365 {
366 // Note on ref discovery: For what appear to be historical reasons,
367 // GCH enables and disabled (by enqueing) refs discovery.
368 // In the future this should be moved into the generation's
369 // collect method so that ref discovery and enqueueing concerns
370 // are local to a generation. The collect method could return
371 // an appropriate indication in the case that notification on
372 // the ref lock was needed. This will make the treatment of
373 // weak refs more uniform (and indeed remove such concerns
374 // from GCH). XXX
375
376 HandleMark hm; // Discard invalid handles created during gc
377 save_marks(); // save marks for all gens
378 // We want to discover references, but not process them yet.
379 // This mode is disabled in process_discovered_references if the
380 // generation does some collection work, or in
381 // enqueue_discovered_references if the generation returns
382 // without doing any work.
383 ReferenceProcessor* rp = gen->ref_processor();
384 // If the discovery of ("weak") refs in this generation is
385 // atomic wrt other collectors in this configuration, we
386 // are guaranteed to have empty discovered ref lists.
387 if (rp->discovery_is_atomic()) {
388 rp->enable_discovery(true /*verify_disabled*/, true /*verify_no_refs*/);
389 rp->setup_policy(clear_soft_refs);
390 } else {
391 // collect() below will enable discovery as appropriate
392 }
393 gen->collect(full, clear_soft_refs, size, is_tlab);
394 if (!rp->enqueuing_is_done()) {
395 rp->enqueue_discovered_references();
396 } else {
397 rp->set_enqueuing_is_done(false);
398 }
399 rp->verify_no_references_recorded();
400 }
401
402 // Determine if allocation request was met.
403 if (size > 0) {
404 if (!is_tlab || gen->supports_tlab_allocation()) {
405 if (size * HeapWordSize <= gen->unsafe_max_alloc_nogc()) {
406 size = 0;
407 }
408 }
409 }
410
411 COMPILER2_PRESENT(DerivedPointerTable::update_pointers());
412
413 gen->stat_record()->accumulated_time.stop();
414
415 update_gc_stats(gen->level(), full);
416
417 if (run_verification && VerifyAfterGC) {
418 HandleMark hm; // Discard invalid handles created during verification
419 Universe::verify(" VerifyAfterGC:");
420 }
421
422 if (PrintGCDetails) {
423 gclog_or_tty->print(":");
424 gen->print_heap_change(prev_used);
425 }
426 }
427
428 void GenCollectedHeap::do_collection(bool full,
429 bool clear_all_soft_refs,
430 size_t size,
431 bool is_tlab,
432 int max_level) {
433 ResourceMark rm;
434 DEBUG_ONLY(Thread* my_thread = Thread::current();)
435
436 assert(SafepointSynchronize::is_at_safepoint(), "should be at safepoint");
437 assert(my_thread->is_VM_thread() ||
438 my_thread->is_ConcurrentGC_thread(),
439 "incorrect thread type capability");
440 assert(Heap_lock->is_locked(),
441 "the requesting thread should have the Heap_lock");
442 guarantee(!is_gc_active(), "collection is not reentrant");
443 assert(max_level < n_gens(), "sanity check");
444
445 if (GC_locker::check_active_before_gc()) {
446 return; // GC is disabled (e.g. JNI GetXXXCritical operation)
447 }
448
449 const bool do_clear_all_soft_refs = clear_all_soft_refs ||
450 collector_policy()->should_clear_all_soft_refs();
451
452 ClearedAllSoftRefs casr(do_clear_all_soft_refs, collector_policy());
453
454 const size_t metadata_prev_used = MetaspaceAux::used_bytes();
455
456 print_heap_before_gc();
457
458 {
459 FlagSetting fl(_is_gc_active, true);
460
461 bool complete = full && (max_level == (n_gens()-1));
462 const char* gc_cause_prefix = complete ? "Full GC" : "GC";
463 gclog_or_tty->date_stamp(PrintGC && PrintGCDateStamps);
464 TraceCPUTime tcpu(PrintGCDetails, true, gclog_or_tty);
465 // The PrintGCDetails logging starts before we have incremented the GC id. We will do that later
466 // so we can assume here that the next GC id is what we want.
467 GCTraceTime t(GCCauseString(gc_cause_prefix, gc_cause()), PrintGCDetails, false, NULL, GCId::peek());
468
469 gc_prologue(complete);
470 increment_total_collections(complete);
471
472 size_t gch_prev_used = used();
473 bool must_restore_marks_for_biased_locking = false;
474 bool run_verification = total_collections() >= VerifyGCStartAt;
475
476 if (_young_gen->performs_in_place_marking() ||
477 _old_gen->performs_in_place_marking()) {
478 // We want to avoid doing this for
479 // scavenge-only collections where it's unnecessary.
480 must_restore_marks_for_biased_locking = true;
481 BiasedLocking::preserve_marks();
482 }
483
484 bool prepared_for_verification = false;
485 int max_level_collected = 0;
486 if (!(full && _old_gen->full_collects_younger_generations()) &&
487 _young_gen->should_collect(full, size, is_tlab)) {
488 if (run_verification && VerifyGCLevel <= 0 && VerifyBeforeGC) {
489 prepare_for_verify();
490 prepared_for_verification = true;
491 }
492 collect_generation(_young_gen, full, size, is_tlab, run_verification && VerifyGCLevel <= 0, do_clear_all_soft_refs);
493 }
494 if (max_level == 1 && _old_gen->should_collect(full, size, is_tlab)) {
495 if (!complete) {
496 // The full_collections increment was missed above.
497 increment_total_full_collections();
498 }
499 pre_full_gc_dump(NULL); // do any pre full gc dumps
500 if (run_verification && VerifyGCLevel <= 1 && VerifyBeforeGC) {
501 if (!prepared_for_verification) {
502 prepare_for_verify();
503 }
504 }
505 collect_generation(_old_gen, full, size, is_tlab, run_verification && VerifyGCLevel <= 1, do_clear_all_soft_refs);
506 max_level_collected = 1;
507 }
508
509 // Update "complete" boolean wrt what actually transpired --
510 // for instance, a promotion failure could have led to
511 // a whole heap collection.
512 complete = complete || (max_level_collected == n_gens() - 1);
513
514 if (complete) { // We did a "major" collection
515 // FIXME: See comment at pre_full_gc_dump call
516 post_full_gc_dump(NULL); // do any post full gc dumps
517 }
518
519 if (PrintGCDetails) {
520 print_heap_change(gch_prev_used);
521
522 // Print metaspace info for full GC with PrintGCDetails flag.
523 if (complete) {
524 MetaspaceAux::print_metaspace_change(metadata_prev_used);
525 }
526 }
527
528 // Adjust generation sizes.
529 if (max_level_collected == 1) {
530 _old_gen->compute_new_size();
531 }
532 _young_gen->compute_new_size();
533
534 if (complete) {
535 // Delete metaspaces for unloaded class loaders and clean up loader_data graph
536 ClassLoaderDataGraph::purge();
537 MetaspaceAux::verify_metrics();
538 // Resize the metaspace capacity after full collections
539 MetaspaceGC::compute_new_size();
540 update_full_collections_completed();
541 }
542
543 // Track memory usage and detect low memory after GC finishes
544 MemoryService::track_memory_usage();
545
546 gc_epilogue(complete);
547
548 if (must_restore_marks_for_biased_locking) {
549 BiasedLocking::restore_marks();
550 }
551 }
552
553 print_heap_after_gc();
554
555 #ifdef TRACESPINNING
556 ParallelTaskTerminator::print_termination_counts();
557 #endif
558 }
559
560 HeapWord* GenCollectedHeap::satisfy_failed_allocation(size_t size, bool is_tlab) {
561 return collector_policy()->satisfy_failed_allocation(size, is_tlab);
562 }
563
564 void GenCollectedHeap::set_par_threads(uint t) {
565 SharedHeap::set_par_threads(t);
566 _gen_process_roots_tasks->set_n_threads(t);
567 }
568
569 void GenCollectedHeap::
570 gen_process_roots(int level,
571 bool younger_gens_as_roots,
572 bool activate_scope,
573 SharedHeap::ScanningOption so,
574 OopsInGenClosure* not_older_gens,
575 OopsInGenClosure* weak_roots,
576 OopsInGenClosure* older_gens,
577 CLDClosure* cld_closure,
578 CLDClosure* weak_cld_closure,
579 CodeBlobClosure* code_closure) {
580
581 // General roots.
582 SharedHeap::process_roots(activate_scope, so,
583 not_older_gens, weak_roots,
584 cld_closure, weak_cld_closure,
585 code_closure);
586
587 if (younger_gens_as_roots) {
588 if (!_gen_process_roots_tasks->is_task_claimed(GCH_PS_younger_gens)) {
589 if (level == 1) {
590 not_older_gens->set_generation(_young_gen);
591 _young_gen->oop_iterate(not_older_gens);
592 }
593 not_older_gens->reset_generation();
594 }
595 }
596 // When collection is parallel, all threads get to cooperate to do
597 // older-gen scanning.
598 if (level == 0) {
599 older_gens->set_generation(_old_gen);
600 rem_set()->younger_refs_iterate(_old_gen, older_gens);
601 older_gens->reset_generation();
602 }
603
604 _gen_process_roots_tasks->all_tasks_completed();
605 }
606
607 void GenCollectedHeap::
608 gen_process_roots(int level,
609 bool younger_gens_as_roots,
610 bool activate_scope,
611 SharedHeap::ScanningOption so,
612 bool only_strong_roots,
613 OopsInGenClosure* not_older_gens,
614 OopsInGenClosure* older_gens,
615 CLDClosure* cld_closure) {
616
617 const bool is_adjust_phase = !only_strong_roots && !younger_gens_as_roots;
618
619 bool is_moving_collection = false;
620 if (level == 0 || is_adjust_phase) {
621 // young collections are always moving
622 is_moving_collection = true;
623 }
624
625 MarkingCodeBlobClosure mark_code_closure(not_older_gens, is_moving_collection);
626 CodeBlobClosure* code_closure = &mark_code_closure;
627
628 gen_process_roots(level,
629 younger_gens_as_roots,
630 activate_scope, so,
631 not_older_gens, only_strong_roots ? NULL : not_older_gens,
632 older_gens,
633 cld_closure, only_strong_roots ? NULL : cld_closure,
634 code_closure);
635
636 }
637
638 void GenCollectedHeap::gen_process_weak_roots(OopClosure* root_closure) {
639 SharedHeap::process_weak_roots(root_closure);
640 // "Local" "weak" refs
641 _young_gen->ref_processor()->weak_oops_do(root_closure);
642 _old_gen->ref_processor()->weak_oops_do(root_closure);
643 }
644
645 #define GCH_SINCE_SAVE_MARKS_ITERATE_DEFN(OopClosureType, nv_suffix) \
646 void GenCollectedHeap:: \
647 oop_since_save_marks_iterate(int level, \
648 OopClosureType* cur, \
649 OopClosureType* older) { \
650 if (level == 0) { \
651 _young_gen->oop_since_save_marks_iterate##nv_suffix(cur); \
652 _old_gen->oop_since_save_marks_iterate##nv_suffix(older); \
653 } else { \
654 _old_gen->oop_since_save_marks_iterate##nv_suffix(cur); \
655 } \
656 }
657
658 ALL_SINCE_SAVE_MARKS_CLOSURES(GCH_SINCE_SAVE_MARKS_ITERATE_DEFN)
659
660 #undef GCH_SINCE_SAVE_MARKS_ITERATE_DEFN
661
662 bool GenCollectedHeap::no_allocs_since_save_marks(int level) {
663 if (level == 0) {
664 if (!_young_gen->no_allocs_since_save_marks()) return false;
665 }
666 if (!_old_gen->no_allocs_since_save_marks()) return false;
667 return true;
668 }
669
670 bool GenCollectedHeap::supports_inline_contig_alloc() const {
671 return _young_gen->supports_inline_contig_alloc();
672 }
673
674 HeapWord** GenCollectedHeap::top_addr() const {
675 return _young_gen->top_addr();
676 }
677
678 HeapWord** GenCollectedHeap::end_addr() const {
679 return _young_gen->end_addr();
680 }
681
682 // public collection interfaces
683
684 void GenCollectedHeap::collect(GCCause::Cause cause) {
685 if (should_do_concurrent_full_gc(cause)) {
686 #if INCLUDE_ALL_GCS
687 // mostly concurrent full collection
688 collect_mostly_concurrent(cause);
689 #else // INCLUDE_ALL_GCS
690 ShouldNotReachHere();
691 #endif // INCLUDE_ALL_GCS
692 } else if (cause == GCCause::_wb_young_gc) {
693 // minor collection for WhiteBox API
694 collect(cause, 0);
695 } else {
696 #ifdef ASSERT
697 if (cause == GCCause::_scavenge_alot) {
698 // minor collection only
699 collect(cause, 0);
700 } else {
701 // Stop-the-world full collection
702 collect(cause, n_gens() - 1);
703 }
704 #else
705 // Stop-the-world full collection
706 collect(cause, n_gens() - 1);
707 #endif
708 }
709 }
710
711 void GenCollectedHeap::collect(GCCause::Cause cause, int max_level) {
712 // The caller doesn't have the Heap_lock
713 assert(!Heap_lock->owned_by_self(), "this thread should not own the Heap_lock");
714 MutexLocker ml(Heap_lock);
715 collect_locked(cause, max_level);
716 }
717
718 void GenCollectedHeap::collect_locked(GCCause::Cause cause) {
719 // The caller has the Heap_lock
720 assert(Heap_lock->owned_by_self(), "this thread should own the Heap_lock");
721 collect_locked(cause, n_gens() - 1);
722 }
723
724 // this is the private collection interface
725 // The Heap_lock is expected to be held on entry.
726
727 void GenCollectedHeap::collect_locked(GCCause::Cause cause, int max_level) {
728 // Read the GC count while holding the Heap_lock
729 unsigned int gc_count_before = total_collections();
730 unsigned int full_gc_count_before = total_full_collections();
731 {
732 MutexUnlocker mu(Heap_lock); // give up heap lock, execute gets it back
733 VM_GenCollectFull op(gc_count_before, full_gc_count_before,
734 cause, max_level);
735 VMThread::execute(&op);
736 }
737 }
738
739 #if INCLUDE_ALL_GCS
740 bool GenCollectedHeap::create_cms_collector() {
741
742 assert(_old_gen->kind() == Generation::ConcurrentMarkSweep,
743 "Unexpected generation kinds");
744 // Skip two header words in the block content verification
745 NOT_PRODUCT(_skip_header_HeapWords = CMSCollector::skip_header_HeapWords();)
746 CMSCollector* collector = new CMSCollector(
747 (ConcurrentMarkSweepGeneration*)_old_gen,
748 _rem_set->as_CardTableRS(),
749 (ConcurrentMarkSweepPolicy*) collector_policy());
750
751 if (collector == NULL || !collector->completed_initialization()) {
752 if (collector) {
753 delete collector; // Be nice in embedded situation
754 }
755 vm_shutdown_during_initialization("Could not create CMS collector");
756 return false;
757 }
758 return true; // success
759 }
760
761 void GenCollectedHeap::collect_mostly_concurrent(GCCause::Cause cause) {
762 assert(!Heap_lock->owned_by_self(), "Should not own Heap_lock");
763
764 MutexLocker ml(Heap_lock);
765 // Read the GC counts while holding the Heap_lock
766 unsigned int full_gc_count_before = total_full_collections();
767 unsigned int gc_count_before = total_collections();
768 {
769 MutexUnlocker mu(Heap_lock);
770 VM_GenCollectFullConcurrent op(gc_count_before, full_gc_count_before, cause);
771 VMThread::execute(&op);
772 }
773 }
774 #endif // INCLUDE_ALL_GCS
775
776 void GenCollectedHeap::do_full_collection(bool clear_all_soft_refs) {
777 do_full_collection(clear_all_soft_refs, _n_gens - 1);
778 }
779
780 void GenCollectedHeap::do_full_collection(bool clear_all_soft_refs,
781 int max_level) {
782 int local_max_level;
783 if (!incremental_collection_will_fail(false /* don't consult_young */) &&
784 gc_cause() == GCCause::_gc_locker) {
785 local_max_level = 0;
786 } else {
787 local_max_level = max_level;
788 }
789
790 do_collection(true /* full */,
791 clear_all_soft_refs /* clear_all_soft_refs */,
792 0 /* size */,
793 false /* is_tlab */,
794 local_max_level /* max_level */);
795 // Hack XXX FIX ME !!!
796 // A scavenge may not have been attempted, or may have
797 // been attempted and failed, because the old gen was too full
798 if (local_max_level == 0 && gc_cause() == GCCause::_gc_locker &&
799 incremental_collection_will_fail(false /* don't consult_young */)) {
800 if (PrintGCDetails) {
801 gclog_or_tty->print_cr("GC locker: Trying a full collection "
802 "because scavenge failed");
803 }
804 // This time allow the old gen to be collected as well
805 do_collection(true /* full */,
806 clear_all_soft_refs /* clear_all_soft_refs */,
807 0 /* size */,
808 false /* is_tlab */,
809 n_gens() - 1 /* max_level */);
810 }
811 }
812
813 bool GenCollectedHeap::is_in_young(oop p) {
814 bool result = ((HeapWord*)p) < _old_gen->reserved().start();
815 assert(result == _young_gen->is_in_reserved(p),
816 err_msg("incorrect test - result=%d, p=" INTPTR_FORMAT, result, p2i((void*)p)));
817 return result;
818 }
819
820 // Returns "TRUE" iff "p" points into the committed areas of the heap.
821 bool GenCollectedHeap::is_in(const void* p) const {
822 #ifndef ASSERT
823 guarantee(VerifyBeforeGC ||
824 VerifyDuringGC ||
825 VerifyBeforeExit ||
826 VerifyDuringStartup ||
827 PrintAssembly ||
828 tty->count() != 0 || // already printing
829 VerifyAfterGC ||
830 VMError::fatal_error_in_progress(), "too expensive");
831
832 #endif
833 // This might be sped up with a cache of the last generation that
834 // answered yes.
835 if (_young_gen->is_in(p) || _old_gen->is_in(p)) {
836 return true;
837 }
838 // Otherwise...
839 return false;
840 }
841
842 #ifdef ASSERT
843 // Don't implement this by using is_in_young(). This method is used
844 // in some cases to check that is_in_young() is correct.
845 bool GenCollectedHeap::is_in_partial_collection(const void* p) {
846 assert(is_in_reserved(p) || p == NULL,
847 "Does not work if address is non-null and outside of the heap");
848 return p < _young_gen->reserved().end() && p != NULL;
849 }
850 #endif
851
852 void GenCollectedHeap::oop_iterate(ExtendedOopClosure* cl) {
853 _young_gen->oop_iterate(cl);
854 _old_gen->oop_iterate(cl);
855 }
856
857 void GenCollectedHeap::object_iterate(ObjectClosure* cl) {
858 _young_gen->object_iterate(cl);
859 _old_gen->object_iterate(cl);
860 }
861
862 void GenCollectedHeap::safe_object_iterate(ObjectClosure* cl) {
863 _young_gen->safe_object_iterate(cl);
864 _old_gen->safe_object_iterate(cl);
865 }
866
867 Space* GenCollectedHeap::space_containing(const void* addr) const {
868 Space* res = _young_gen->space_containing(addr);
869 if (res != NULL) {
870 return res;
871 }
872 res = _old_gen->space_containing(addr);
873 assert(res != NULL, "Could not find containing space");
874 return res;
875 }
876
877 HeapWord* GenCollectedHeap::block_start(const void* addr) const {
878 assert(is_in_reserved(addr), "block_start of address outside of heap");
879 if (_young_gen->is_in_reserved(addr)) {
880 assert(_young_gen->is_in(addr), "addr should be in allocated part of generation");
881 return _young_gen->block_start(addr);
882 }
883
884 assert(_old_gen->is_in_reserved(addr), "Some generation should contain the address");
885 assert(_old_gen->is_in(addr), "addr should be in allocated part of generation");
886 return _old_gen->block_start(addr);
887 }
888
889 size_t GenCollectedHeap::block_size(const HeapWord* addr) const {
890 assert(is_in_reserved(addr), "block_size of address outside of heap");
891 if (_young_gen->is_in_reserved(addr)) {
892 assert(_young_gen->is_in(addr), "addr should be in allocated part of generation");
893 return _young_gen->block_size(addr);
894 }
895
896 assert(_old_gen->is_in_reserved(addr), "Some generation should contain the address");
897 assert(_old_gen->is_in(addr), "addr should be in allocated part of generation");
898 return _old_gen->block_size(addr);
899 }
900
901 bool GenCollectedHeap::block_is_obj(const HeapWord* addr) const {
902 assert(is_in_reserved(addr), "block_is_obj of address outside of heap");
903 assert(block_start(addr) == addr, "addr must be a block start");
904 if (_young_gen->is_in_reserved(addr)) {
905 return _young_gen->block_is_obj(addr);
906 }
907
908 assert(_old_gen->is_in_reserved(addr), "Some generation should contain the address");
909 return _old_gen->block_is_obj(addr);
910 }
911
912 bool GenCollectedHeap::supports_tlab_allocation() const {
913 assert(!_old_gen->supports_tlab_allocation(), "Old gen supports TLAB allocation?!");
914 return _young_gen->supports_tlab_allocation();
915 }
916
917 size_t GenCollectedHeap::tlab_capacity(Thread* thr) const {
918 assert(!_old_gen->supports_tlab_allocation(), "Old gen supports TLAB allocation?!");
919 if (_young_gen->supports_tlab_allocation()) {
920 return _young_gen->tlab_capacity();
921 }
922 return 0;
923 }
924
925 size_t GenCollectedHeap::tlab_used(Thread* thr) const {
926 assert(!_old_gen->supports_tlab_allocation(), "Old gen supports TLAB allocation?!");
927 if (_young_gen->supports_tlab_allocation()) {
928 return _young_gen->tlab_used();
929 }
930 return 0;
931 }
932
933 size_t GenCollectedHeap::unsafe_max_tlab_alloc(Thread* thr) const {
934 assert(!_old_gen->supports_tlab_allocation(), "Old gen supports TLAB allocation?!");
935 if (_young_gen->supports_tlab_allocation()) {
936 return _young_gen->unsafe_max_tlab_alloc();
937 }
938 return 0;
939 }
940
941 HeapWord* GenCollectedHeap::allocate_new_tlab(size_t size) {
942 bool gc_overhead_limit_was_exceeded;
943 return collector_policy()->mem_allocate_work(size /* size */,
944 true /* is_tlab */,
945 &gc_overhead_limit_was_exceeded);
946 }
947
948 // Requires "*prev_ptr" to be non-NULL. Deletes and a block of minimal size
949 // from the list headed by "*prev_ptr".
950 static ScratchBlock *removeSmallestScratch(ScratchBlock **prev_ptr) {
951 bool first = true;
952 size_t min_size = 0; // "first" makes this conceptually infinite.
953 ScratchBlock **smallest_ptr, *smallest;
954 ScratchBlock *cur = *prev_ptr;
955 while (cur) {
956 assert(*prev_ptr == cur, "just checking");
957 if (first || cur->num_words < min_size) {
958 smallest_ptr = prev_ptr;
959 smallest = cur;
960 min_size = smallest->num_words;
961 first = false;
962 }
963 prev_ptr = &cur->next;
964 cur = cur->next;
965 }
966 smallest = *smallest_ptr;
967 *smallest_ptr = smallest->next;
968 return smallest;
969 }
970
971 // Sort the scratch block list headed by res into decreasing size order,
972 // and set "res" to the result.
973 static void sort_scratch_list(ScratchBlock*& list) {
974 ScratchBlock* sorted = NULL;
975 ScratchBlock* unsorted = list;
976 while (unsorted) {
977 ScratchBlock *smallest = removeSmallestScratch(&unsorted);
978 smallest->next = sorted;
979 sorted = smallest;
980 }
981 list = sorted;
982 }
983
984 ScratchBlock* GenCollectedHeap::gather_scratch(Generation* requestor,
985 size_t max_alloc_words) {
986 ScratchBlock* res = NULL;
987 _young_gen->contribute_scratch(res, requestor, max_alloc_words);
988 _old_gen->contribute_scratch(res, requestor, max_alloc_words);
989 sort_scratch_list(res);
990 return res;
991 }
992
993 void GenCollectedHeap::release_scratch() {
994 _young_gen->reset_scratch();
995 _old_gen->reset_scratch();
996 }
997
998 class GenPrepareForVerifyClosure: public GenCollectedHeap::GenClosure {
999 void do_generation(Generation* gen) {
1000 gen->prepare_for_verify();
1001 }
1002 };
1003
1004 void GenCollectedHeap::prepare_for_verify() {
1005 ensure_parsability(false); // no need to retire TLABs
1006 GenPrepareForVerifyClosure blk;
1007 generation_iterate(&blk, false);
1008 }
1009
1010 void GenCollectedHeap::generation_iterate(GenClosure* cl,
1011 bool old_to_young) {
1012 if (old_to_young) {
1013 cl->do_generation(_old_gen);
1014 cl->do_generation(_young_gen);
1015 } else {
1016 cl->do_generation(_young_gen);
1017 cl->do_generation(_old_gen);
1018 }
1019 }
1020
1021 void GenCollectedHeap::space_iterate(SpaceClosure* cl) {
1022 _young_gen->space_iterate(cl, true);
1023 _old_gen->space_iterate(cl, true);
1024 }
1025
1026 bool GenCollectedHeap::is_maximal_no_gc() const {
1027 return _young_gen->is_maximal_no_gc() && _old_gen->is_maximal_no_gc();
1028 }
1029
1030 void GenCollectedHeap::save_marks() {
1031 _young_gen->save_marks();
1032 _old_gen->save_marks();
1033 }
1034
1035 GenCollectedHeap* GenCollectedHeap::heap() {
1036 assert(_gch != NULL, "Uninitialized access to GenCollectedHeap::heap()");
1037 assert(_gch->kind() == CollectedHeap::GenCollectedHeap, "not a generational heap");
1038 return _gch;
1039 }
1040
1041
1042 void GenCollectedHeap::prepare_for_compaction() {
1043 guarantee(_n_gens = 2, "Wrong number of generations");
1044 Generation* old_gen = _old_gen;
1045 // Start by compacting into same gen.
1046 CompactPoint cp(old_gen);
1047 old_gen->prepare_for_compaction(&cp);
1048 Generation* young_gen = _young_gen;
1049 young_gen->prepare_for_compaction(&cp);
1050 }
1051
1052 GCStats* GenCollectedHeap::gc_stats(int level) const {
1053 if (level == 0) {
1054 return _young_gen->gc_stats();
1055 } else {
1056 return _old_gen->gc_stats();
1057 }
1058 }
1059
1060 void GenCollectedHeap::verify(bool silent, VerifyOption option /* ignored */) {
1061 if (!silent) {
1062 gclog_or_tty->print("%s", _old_gen->name());
1063 gclog_or_tty->print(" ");
1064 }
1065 _old_gen->verify();
1066
1067 if (!silent) {
1068 gclog_or_tty->print("%s", _young_gen->name());
1069 gclog_or_tty->print(" ");
1070 }
1071 _young_gen->verify();
1072
1073 if (!silent) {
1074 gclog_or_tty->print("remset ");
1075 }
1076 rem_set()->verify();
1077 }
1078
1079 void GenCollectedHeap::print_on(outputStream* st) const {
1080 _young_gen->print_on(st);
1081 _old_gen->print_on(st);
1082 MetaspaceAux::print_on(st);
1083 }
1084
1085 void GenCollectedHeap::gc_threads_do(ThreadClosure* tc) const {
1086 if (workers() != NULL) {
1087 workers()->threads_do(tc);
1088 }
1089 #if INCLUDE_ALL_GCS
1090 if (UseConcMarkSweepGC) {
1091 ConcurrentMarkSweepThread::threads_do(tc);
1092 }
1093 #endif // INCLUDE_ALL_GCS
1094 }
1095
1096 void GenCollectedHeap::print_gc_threads_on(outputStream* st) const {
1097 #if INCLUDE_ALL_GCS
1098 if (UseParNewGC) {
1099 workers()->print_worker_threads_on(st);
1100 }
1101 if (UseConcMarkSweepGC) {
1102 ConcurrentMarkSweepThread::print_all_on(st);
1103 }
1104 #endif // INCLUDE_ALL_GCS
1105 }
1106
1107 void GenCollectedHeap::print_on_error(outputStream* st) const {
1108 this->CollectedHeap::print_on_error(st);
1109
1110 #if INCLUDE_ALL_GCS
1111 if (UseConcMarkSweepGC) {
1112 st->cr();
1113 CMSCollector::print_on_error(st);
1114 }
1115 #endif // INCLUDE_ALL_GCS
1116 }
1117
1118 void GenCollectedHeap::print_tracing_info() const {
1119 if (TraceYoungGenTime) {
1120 get_gen(0)->print_summary_info();
1121 }
1122 if (TraceOldGenTime) {
1123 get_gen(1)->print_summary_info();
1124 }
1125 }
1126
1127 void GenCollectedHeap::print_heap_change(size_t prev_used) const {
1128 if (PrintGCDetails && Verbose) {
1129 gclog_or_tty->print(" " SIZE_FORMAT
1130 "->" SIZE_FORMAT
1131 "(" SIZE_FORMAT ")",
1132 prev_used, used(), capacity());
1133 } else {
1134 gclog_or_tty->print(" " SIZE_FORMAT "K"
1135 "->" SIZE_FORMAT "K"
1136 "(" SIZE_FORMAT "K)",
1137 prev_used / K, used() / K, capacity() / K);
1138 }
1139 }
1140
1141 class GenGCPrologueClosure: public GenCollectedHeap::GenClosure {
1142 private:
1143 bool _full;
1144 public:
1145 void do_generation(Generation* gen) {
1146 gen->gc_prologue(_full);
1147 }
1148 GenGCPrologueClosure(bool full) : _full(full) {};
1149 };
1150
1151 void GenCollectedHeap::gc_prologue(bool full) {
1152 assert(InlineCacheBuffer::is_empty(), "should have cleaned up ICBuffer");
1153
1154 always_do_update_barrier = false;
1155 // Fill TLAB's and such
1156 CollectedHeap::accumulate_statistics_all_tlabs();
1157 ensure_parsability(true); // retire TLABs
1158
1159 // Walk generations
1160 GenGCPrologueClosure blk(full);
1161 generation_iterate(&blk, false); // not old-to-young.
1162 };
1163
1164 class GenGCEpilogueClosure: public GenCollectedHeap::GenClosure {
1165 private:
1166 bool _full;
1167 public:
1168 void do_generation(Generation* gen) {
1169 gen->gc_epilogue(_full);
1170 }
1171 GenGCEpilogueClosure(bool full) : _full(full) {};
1172 };
1173
1174 void GenCollectedHeap::gc_epilogue(bool full) {
1175 #ifdef COMPILER2
1176 assert(DerivedPointerTable::is_empty(), "derived pointer present");
1177 size_t actual_gap = pointer_delta((HeapWord*) (max_uintx-3), *(end_addr()));
1178 guarantee(actual_gap > (size_t)FastAllocateSizeLimit, "inline allocation wraps");
1179 #endif /* COMPILER2 */
1180
1181 resize_all_tlabs();
1182
1183 GenGCEpilogueClosure blk(full);
1184 generation_iterate(&blk, false); // not old-to-young.
1185
1186 if (!CleanChunkPoolAsync) {
1187 Chunk::clean_chunk_pool();
1188 }
1189
1190 MetaspaceCounters::update_performance_counters();
1191 CompressedClassSpaceCounters::update_performance_counters();
1192
1193 always_do_update_barrier = UseConcMarkSweepGC;
1194 };
1195
1196 #ifndef PRODUCT
1197 class GenGCSaveTopsBeforeGCClosure: public GenCollectedHeap::GenClosure {
1198 private:
1199 public:
1200 void do_generation(Generation* gen) {
1201 gen->record_spaces_top();
1202 }
1203 };
1204
1205 void GenCollectedHeap::record_gen_tops_before_GC() {
1206 if (ZapUnusedHeapArea) {
1207 GenGCSaveTopsBeforeGCClosure blk;
1208 generation_iterate(&blk, false); // not old-to-young.
1209 }
1210 }
1211 #endif // not PRODUCT
1212
1213 class GenEnsureParsabilityClosure: public GenCollectedHeap::GenClosure {
1214 public:
1215 void do_generation(Generation* gen) {
1216 gen->ensure_parsability();
1217 }
1218 };
1219
1220 void GenCollectedHeap::ensure_parsability(bool retire_tlabs) {
1221 CollectedHeap::ensure_parsability(retire_tlabs);
1222 GenEnsureParsabilityClosure ep_cl;
1223 generation_iterate(&ep_cl, false);
1224 }
1225
1226 oop GenCollectedHeap::handle_failed_promotion(Generation* old_gen,
1227 oop obj,
1228 size_t obj_size) {
1229 guarantee(old_gen->level() == 1, "We only get here with an old generation");
1230 assert(obj_size == (size_t)obj->size(), "bad obj_size passed in");
1231 HeapWord* result = NULL;
1232
1233 result = old_gen->expand_and_allocate(obj_size, false);
1234
1235 if (result != NULL) {
1236 Copy::aligned_disjoint_words((HeapWord*)obj, result, obj_size);
1237 }
1238 return oop(result);
1239 }
1240
1241 class GenTimeOfLastGCClosure: public GenCollectedHeap::GenClosure {
1242 jlong _time; // in ms
1243 jlong _now; // in ms
1244
1245 public:
1246 GenTimeOfLastGCClosure(jlong now) : _time(now), _now(now) { }
1247
1248 jlong time() { return _time; }
1249
1250 void do_generation(Generation* gen) {
1251 _time = MIN2(_time, gen->time_of_last_gc(_now));
1252 }
1253 };
1254
1255 jlong GenCollectedHeap::millis_since_last_gc() {
1256 // We need a monotonically non-decreasing time in ms but
1257 // os::javaTimeMillis() does not guarantee monotonicity.
1258 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
1259 GenTimeOfLastGCClosure tolgc_cl(now);
1260 // iterate over generations getting the oldest
1261 // time that a generation was collected
1262 generation_iterate(&tolgc_cl, false);
1263
1264 // javaTimeNanos() is guaranteed to be monotonically non-decreasing
1265 // provided the underlying platform provides such a time source
1266 // (and it is bug free). So we still have to guard against getting
1267 // back a time later than 'now'.
1268 jlong retVal = now - tolgc_cl.time();
1269 if (retVal < 0) {
1270 NOT_PRODUCT(warning("time warp: "INT64_FORMAT, (int64_t) retVal);)
1271 return 0;
1272 }
1273 return retVal;
1274 }