1 /*
2 * Copyright (c) 2013, 2021, Red Hat, Inc. All rights reserved.
3 * Copyright (C) 2022 THL A29 Limited, a Tencent company. All rights reserved.
4 * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
5 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
6 *
7 * This code is free software; you can redistribute it and/or modify it
8 * under the terms of the GNU General Public License version 2 only, as
9 * published by the Free Software Foundation.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 *
25 */
26
27 #include "precompiled.hpp"
28 #include "gc/shenandoah/mode/shenandoahMode.hpp"
29 #include "gc/shenandoah/shenandoahAsserts.hpp"
30 #include "gc/shenandoah/shenandoahCollectorPolicy.hpp"
31 #include "gc/shenandoah/shenandoahConcurrentGC.hpp"
32 #include "gc/shenandoah/shenandoahGenerationalControlThread.hpp"
33 #include "gc/shenandoah/shenandoahDegeneratedGC.hpp"
34 #include "gc/shenandoah/shenandoahFreeSet.hpp"
35 #include "gc/shenandoah/shenandoahFullGC.hpp"
36 #include "gc/shenandoah/shenandoahGeneration.hpp"
37 #include "gc/shenandoah/shenandoahOldGC.hpp"
38 #include "gc/shenandoah/shenandoahOldGeneration.hpp"
39 #include "gc/shenandoah/shenandoahHeap.inline.hpp"
40 #include "gc/shenandoah/shenandoahMonitoringSupport.hpp"
41 #include "gc/shenandoah/shenandoahPacer.inline.hpp"
42 #include "gc/shenandoah/shenandoahUtils.hpp"
43 #include "gc/shenandoah/shenandoahYoungGeneration.hpp"
44 #include "logging/log.hpp"
45 #include "memory/metaspaceUtils.hpp"
46 #include "memory/metaspaceStats.hpp"
47 #include "runtime/atomic.hpp"
48
49 ShenandoahGenerationalControlThread::ShenandoahGenerationalControlThread() :
50 ShenandoahController(),
51 _control_lock(Mutex::nosafepoint - 2, "ShenandoahControlGC_lock", true),
52 _regulator_lock(Mutex::nosafepoint - 2, "ShenandoahRegulatorGC_lock", true),
53 _requested_gc_cause(GCCause::_no_gc),
54 _requested_generation(GLOBAL),
55 _degen_point(ShenandoahGC::_degenerated_outside_cycle),
56 _degen_generation(nullptr),
57 _mode(none) {
58 shenandoah_assert_generational();
59 set_name("Shenandoah Control Thread");
60 create_and_start();
61 }
62
63 void ShenandoahGenerationalControlThread::run_service() {
64 ShenandoahHeap* const heap = ShenandoahHeap::heap();
65
66 const GCMode default_mode = concurrent_normal;
67 ShenandoahGenerationType generation = GLOBAL;
68
69 double last_shrink_time = os::elapsedTime();
70 uint age_period = 0;
71
72 // Shrink period avoids constantly polling regions for shrinking.
73 // Having a period 10x lower than the delay would mean we hit the
74 // shrinking with lag of less than 1/10-th of true delay.
75 // ShenandoahUncommitDelay is in msecs, but shrink_period is in seconds.
76 const double shrink_period = (double)ShenandoahUncommitDelay / 1000 / 10;
77
78 ShenandoahCollectorPolicy* const policy = heap->shenandoah_policy();
79
80 // Heuristics are notified of allocation failures here and other outcomes
81 // of the cycle. They're also used here to control whether the Nth consecutive
82 // degenerated cycle should be 'promoted' to a full cycle. The decision to
83 // trigger a cycle or not is evaluated on the regulator thread.
84 ShenandoahHeuristics* global_heuristics = heap->global_generation()->heuristics();
85 while (!in_graceful_shutdown() && !should_terminate()) {
86 // Figure out if we have pending requests.
87 const bool alloc_failure_pending = _alloc_failure_gc.is_set();
88 const bool humongous_alloc_failure_pending = _humongous_alloc_failure_gc.is_set();
89
90 GCCause::Cause cause = Atomic::xchg(&_requested_gc_cause, GCCause::_no_gc);
91
92 const bool is_gc_requested = ShenandoahCollectorPolicy::is_requested_gc(cause);
93
94 // This control loop iteration has seen this much allocation.
95 const size_t allocs_seen = reset_allocs_seen();
96
97 // Check if we have seen a new target for soft max heap size.
98 const bool soft_max_changed = heap->check_soft_max_changed();
99
100 // Choose which GC mode to run in. The block below should select a single mode.
101 set_gc_mode(none);
102 ShenandoahGC::ShenandoahDegenPoint degen_point = ShenandoahGC::_degenerated_unset;
103
104 if (alloc_failure_pending) {
105 // Allocation failure takes precedence: we have to deal with it first thing
106 log_info(gc)("Trigger: Handle Allocation Failure");
107
108 cause = GCCause::_allocation_failure;
109
110 // Consume the degen point, and seed it with default value
111 degen_point = _degen_point;
112 _degen_point = ShenandoahGC::_degenerated_outside_cycle;
113
114 if (degen_point == ShenandoahGC::_degenerated_outside_cycle) {
115 _degen_generation = heap->young_generation();
116 } else {
117 assert(_degen_generation != nullptr, "Need to know which generation to resume");
118 }
119
120 ShenandoahHeuristics* heuristics = _degen_generation->heuristics();
121 generation = _degen_generation->type();
122 bool old_gen_evacuation_failed = heap->old_generation()->clear_failed_evacuation();
123
124 // Do not bother with degenerated cycle if old generation evacuation failed or if humongous allocation failed
125 if (ShenandoahDegeneratedGC && heuristics->should_degenerate_cycle() &&
126 !old_gen_evacuation_failed && !humongous_alloc_failure_pending) {
127 heuristics->record_allocation_failure_gc();
128 policy->record_alloc_failure_to_degenerated(degen_point);
129 set_gc_mode(stw_degenerated);
130 } else {
131 // TODO: if humongous_alloc_failure_pending, there might be value in trying a "compacting" degen before
132 // going all the way to full. But it's a lot of work to implement this, and it may not provide value.
133 // A compacting degen can move young regions around without doing full old-gen mark (relying upon the
134 // remembered set scan), so it might be faster than a full gc.
135 //
136 // Longer term, think about how to defragment humongous memory concurrently.
137
138 heuristics->record_allocation_failure_gc();
139 policy->record_alloc_failure_to_full();
140 generation = GLOBAL;
141 set_gc_mode(stw_full);
142 }
143 } else if (is_gc_requested) {
144 generation = GLOBAL;
145 log_info(gc)("Trigger: GC request (%s)", GCCause::to_string(cause));
146 global_heuristics->record_requested_gc();
147
148 if (ShenandoahCollectorPolicy::should_run_full_gc(cause)) {
149 set_gc_mode(stw_full);
150 } else {
151 set_gc_mode(default_mode);
152 // Unload and clean up everything
153 heap->set_unload_classes(global_heuristics->can_unload_classes());
154 }
155 } else {
156 // We should only be here if the regulator requested a cycle or if
157 // there is an old generation mark in progress.
158 if (cause == GCCause::_shenandoah_concurrent_gc) {
159 if (_requested_generation == OLD && heap->old_generation()->is_doing_mixed_evacuations()) {
160 // If a request to start an old cycle arrived while an old cycle was running, but _before_
161 // it chose any regions for evacuation we don't want to start a new old cycle. Rather, we want
162 // the heuristic to run a young collection so that we can evacuate some old regions.
163 assert(!heap->is_concurrent_old_mark_in_progress(), "Should not be running mixed collections and concurrent marking");
164 generation = YOUNG;
165 } else {
166 generation = _requested_generation;
167 }
168
169 // preemption was requested or this is a regular cycle
170 set_gc_mode(default_mode);
171
172 // Don't start a new old marking if there is one already in progress
173 if (generation == OLD && heap->is_concurrent_old_mark_in_progress()) {
174 set_gc_mode(servicing_old);
175 }
176
177 if (generation == GLOBAL) {
178 heap->set_unload_classes(global_heuristics->should_unload_classes());
179 } else {
180 heap->set_unload_classes(false);
181 }
182 } else if (heap->is_concurrent_old_mark_in_progress() || heap->is_prepare_for_old_mark_in_progress()) {
183 // Nobody asked us to do anything, but we have an old-generation mark or old-generation preparation for
184 // mixed evacuation in progress, so resume working on that.
185 log_info(gc)("Resume old GC: marking is%s in progress, preparing is%s in progress",
186 heap->is_concurrent_old_mark_in_progress() ? "" : " NOT",
187 heap->is_prepare_for_old_mark_in_progress() ? "" : " NOT");
188
189 cause = GCCause::_shenandoah_concurrent_gc;
190 generation = OLD;
191 set_gc_mode(servicing_old);
192 heap->set_unload_classes(false);
193 }
194 }
195
196 const bool gc_requested = (gc_mode() != none);
197 assert (!gc_requested || cause != GCCause::_no_gc, "GC cause should be set");
198
199 if (gc_requested) {
200 // Blow away all soft references on this cycle, if handling allocation failure,
201 // either implicit or explicit GC request, or we are requested to do so unconditionally.
202 if (generation == GLOBAL && (alloc_failure_pending || is_gc_requested || ShenandoahAlwaysClearSoftRefs)) {
203 heap->soft_ref_policy()->set_should_clear_all_soft_refs(true);
204 }
205
206 // GC is starting, bump the internal ID
207 update_gc_id();
208
209 heap->reset_bytes_allocated_since_gc_start();
210
211 MetaspaceCombinedStats meta_sizes = MetaspaceUtils::get_combined_statistics();
212
213 // If GC was requested, we are sampling the counters even without actual triggers
214 // from allocation machinery. This captures GC phases more accurately.
215 heap->set_forced_counters_update(true);
216
217 // If GC was requested, we better dump freeset data for performance debugging
218 {
219 ShenandoahHeapLocker locker(heap->lock());
220 heap->free_set()->log_status();
221 }
222 // In case this is a degenerated cycle, remember whether original cycle was aging.
223 const bool was_aging_cycle = heap->is_aging_cycle();
224 heap->set_aging_cycle(false);
225
226 switch (gc_mode()) {
227 case concurrent_normal: {
228 // At this point:
229 // if (generation == YOUNG), this is a normal YOUNG cycle
230 // if (generation == OLD), this is a bootstrap OLD cycle
231 // if (generation == GLOBAL), this is a GLOBAL cycle triggered by System.gc()
232 // In all three cases, we want to age old objects if this is an aging cycle
233 if (age_period-- == 0) {
234 heap->set_aging_cycle(true);
235 age_period = ShenandoahAgingCyclePeriod - 1;
236 }
237 service_concurrent_normal_cycle(heap, generation, cause);
238 break;
239 }
240 case stw_degenerated: {
241 heap->set_aging_cycle(was_aging_cycle);
242 service_stw_degenerated_cycle(cause, degen_point);
243 break;
244 }
245 case stw_full: {
246 if (age_period-- == 0) {
247 heap->set_aging_cycle(true);
248 age_period = ShenandoahAgingCyclePeriod - 1;
249 }
250 service_stw_full_cycle(cause);
251 break;
252 }
253 case servicing_old: {
254 assert(generation == OLD, "Expected old generation here");
255 GCIdMark gc_id_mark;
256 service_concurrent_old_cycle(heap, cause);
257 break;
258 }
259 default:
260 ShouldNotReachHere();
261 }
262
263 // If this was the requested GC cycle, notify waiters about it
264 if (is_gc_requested) {
265 notify_gc_waiters();
266 }
267
268 // If this was the allocation failure GC cycle, notify waiters about it
269 if (alloc_failure_pending) {
270 notify_alloc_failure_waiters();
271 }
272
273 // Report current free set state at the end of cycle, whether
274 // it is a normal completion, or the abort.
275 {
276 ShenandoahHeapLocker locker(heap->lock());
277 heap->free_set()->log_status();
278
279 // Notify Universe about new heap usage. This has implications for
280 // global soft refs policy, and we better report it every time heap
281 // usage goes down.
282 heap->update_capacity_and_used_at_gc();
283
284 // Signal that we have completed a visit to all live objects.
285 heap->record_whole_heap_examined_timestamp();
286 }
287
288 // Disable forced counters update, and update counters one more time
289 // to capture the state at the end of GC session.
290 heap->handle_force_counters_update();
291 heap->set_forced_counters_update(false);
292
293 // Retract forceful part of soft refs policy
294 heap->soft_ref_policy()->set_should_clear_all_soft_refs(false);
295
296 // Clear metaspace oom flag, if current cycle unloaded classes
297 if (heap->unload_classes()) {
298 global_heuristics->clear_metaspace_oom();
299 }
300
301 process_phase_timings(heap);
302
303 // Print Metaspace change following GC (if logging is enabled).
304 MetaspaceUtils::print_metaspace_change(meta_sizes);
305
306 // GC is over, we are at idle now
307 if (ShenandoahPacing) {
308 heap->pacer()->setup_for_idle();
309 }
310 } else {
311 // Report to pacer that we have seen this many words allocated
312 if (ShenandoahPacing && (allocs_seen > 0)) {
313 heap->pacer()->report_alloc(allocs_seen);
314 }
315 }
316
317 const double current = os::elapsedTime();
318
319 if (ShenandoahUncommit && (is_gc_requested || soft_max_changed || (current - last_shrink_time > shrink_period))) {
320 // Explicit GC tries to uncommit everything down to min capacity.
321 // Soft max change tries to uncommit everything down to target capacity.
322 // Periodic uncommit tries to uncommit suitable regions down to min capacity.
323
324 double shrink_before = (is_gc_requested || soft_max_changed) ?
325 current :
326 current - (ShenandoahUncommitDelay / 1000.0);
327
328 size_t shrink_until = soft_max_changed ?
329 heap->soft_max_capacity() :
330 heap->min_capacity();
331
332 heap->maybe_uncommit(shrink_before, shrink_until);
333 heap->phase_timings()->flush_cycle_to_global();
334 last_shrink_time = current;
335 }
336
337 // Wait for ShenandoahControlIntervalMax unless there was an allocation failure or another request was made mid-cycle.
338 if (!is_alloc_failure_gc() && _requested_gc_cause == GCCause::_no_gc) {
339 // The timed wait is necessary because this thread has a responsibility to send
340 // 'alloc_words' to the pacer when it does not perform a GC.
341 MonitorLocker lock(&_control_lock, Mutex::_no_safepoint_check_flag);
342 lock.wait(ShenandoahControlIntervalMax);
343 }
344 }
345
346 // Wait for the actual stop(), can't leave run_service() earlier.
347 while (!should_terminate()) {
348 os::naked_short_sleep(ShenandoahControlIntervalMin);
349 }
350 }
351
352 void ShenandoahGenerationalControlThread::process_phase_timings(const ShenandoahHeap* heap) {
353 // Commit worker statistics to cycle data
354 heap->phase_timings()->flush_par_workers_to_cycle();
355 if (ShenandoahPacing) {
356 heap->pacer()->flush_stats_to_cycle();
357 }
358
359 ShenandoahEvacuationTracker* evac_tracker = heap->evac_tracker();
360 ShenandoahCycleStats evac_stats = evac_tracker->flush_cycle_to_global();
361
362 // Print GC stats for current cycle
363 {
364 LogTarget(Info, gc, stats) lt;
365 if (lt.is_enabled()) {
366 ResourceMark rm;
367 LogStream ls(lt);
368 heap->phase_timings()->print_cycle_on(&ls);
369 evac_tracker->print_evacuations_on(&ls, &evac_stats.workers,
370 &evac_stats.mutators);
371 if (ShenandoahPacing) {
372 heap->pacer()->print_cycle_on(&ls);
373 }
374 }
375 }
376
377 // Commit statistics to globals
378 heap->phase_timings()->flush_cycle_to_global();
379 }
380
381 // Young and old concurrent cycles are initiated by the regulator. Implicit
382 // and explicit GC requests are handled by the controller thread and always
383 // run a global cycle (which is concurrent by default, but may be overridden
384 // by command line options). Old cycles always degenerate to a global cycle.
385 // Young cycles are degenerated to complete the young cycle. Young
386 // and old degen may upgrade to Full GC. Full GC may also be
387 // triggered directly by a System.gc() invocation.
388 //
389 //
390 // +-----+ Idle +-----+-----------+---------------------+
391 // | + | | |
392 // | | | | |
393 // | | v | |
394 // | | Bootstrap Old +-- | ------------+ |
395 // | | + | | |
396 // | | | | | |
397 // | v v v v |
398 // | Resume Old <----------+ Young +--> Young Degen |
399 // | + + ^ + + |
400 // v | | | | | |
401 // Global <-+ | +----------------------------+ | |
402 // + | | |
403 // | v v |
404 // +---> Global Degen +--------------------> Full <----+
405 //
406 void ShenandoahGenerationalControlThread::service_concurrent_normal_cycle(ShenandoahHeap* heap,
407 const ShenandoahGenerationType generation,
408 GCCause::Cause cause) {
409 GCIdMark gc_id_mark;
410 switch (generation) {
411 case YOUNG: {
412 // Run a young cycle. This might or might not, have interrupted an ongoing
413 // concurrent mark in the old generation. We need to think about promotions
414 // in this case. Promoted objects should be above the TAMS in the old regions
415 // they end up in, but we have to be sure we don't promote into any regions
416 // that are in the cset.
417 log_info(gc, ergo)("Start GC cycle (YOUNG)");
418 service_concurrent_cycle(heap->young_generation(), cause, false);
419 break;
420 }
421 case OLD: {
422 log_info(gc, ergo)("Start GC cycle (OLD)");
423 service_concurrent_old_cycle(heap, cause);
424 break;
425 }
426 case GLOBAL: {
427 log_info(gc, ergo)("Start GC cycle (GLOBAL)");
428 service_concurrent_cycle(heap->global_generation(), cause, false);
429 break;
430 }
431 default:
432 ShouldNotReachHere();
433 }
434 }
435
436 void ShenandoahGenerationalControlThread::service_concurrent_old_cycle(ShenandoahHeap* heap, GCCause::Cause &cause) {
437 ShenandoahOldGeneration* old_generation = heap->old_generation();
438 ShenandoahYoungGeneration* young_generation = heap->young_generation();
439 ShenandoahOldGeneration::State original_state = old_generation->state();
440
441 TraceCollectorStats tcs(heap->monitoring_support()->concurrent_collection_counters());
442
443 switch (original_state) {
444 case ShenandoahOldGeneration::FILLING: {
445 _allow_old_preemption.set();
446 old_generation->entry_coalesce_and_fill();
447 _allow_old_preemption.unset();
448
449 // Before bootstrapping begins, we must acknowledge any cancellation request.
450 // If the gc has not been cancelled, this does nothing. If it has been cancelled,
451 // this will clear the cancellation request and exit before starting the bootstrap
452 // phase. This will allow the young GC cycle to proceed normally. If we do not
453 // acknowledge the cancellation request, the subsequent young cycle will observe
454 // the request and essentially cancel itself.
455 if (check_cancellation_or_degen(ShenandoahGC::_degenerated_outside_cycle)) {
456 log_info(gc)("Preparation for old generation cycle was cancelled");
457 return;
458 }
459
460 // Coalescing threads completed and nothing was cancelled. it is safe to transition from this state.
461 old_generation->transition_to(ShenandoahOldGeneration::WAITING_FOR_BOOTSTRAP);
462 return;
463 }
464 case ShenandoahOldGeneration::WAITING_FOR_BOOTSTRAP:
465 old_generation->transition_to(ShenandoahOldGeneration::BOOTSTRAPPING);
466 case ShenandoahOldGeneration::BOOTSTRAPPING: {
467 // Configure the young generation's concurrent mark to put objects in
468 // old regions into the concurrent mark queues associated with the old
469 // generation. The young cycle will run as normal except that rather than
470 // ignore old references it will mark and enqueue them in the old concurrent
471 // task queues but it will not traverse them.
472 set_gc_mode(bootstrapping_old);
473 young_generation->set_old_gen_task_queues(old_generation->task_queues());
474 ShenandoahGCSession session(cause, young_generation);
475 service_concurrent_cycle(heap, young_generation, cause, true);
476 process_phase_timings(heap);
477 if (heap->cancelled_gc()) {
478 // Young generation bootstrap cycle has failed. Concurrent mark for old generation
479 // is going to resume after degenerated bootstrap cycle completes.
480 log_info(gc)("Bootstrap cycle for old generation was cancelled");
481 return;
482 }
483
484 // Reset the degenerated point. Normally this would happen at the top
485 // of the control loop, but here we have just completed a young cycle
486 // which has bootstrapped the old concurrent marking.
487 _degen_point = ShenandoahGC::_degenerated_outside_cycle;
488
489 // From here we will 'resume' the old concurrent mark. This will skip reset
490 // and init mark for the concurrent mark. All of that work will have been
491 // done by the bootstrapping young cycle.
492 set_gc_mode(servicing_old);
493 old_generation->transition_to(ShenandoahOldGeneration::MARKING);
494 }
495 case ShenandoahOldGeneration::MARKING: {
496 ShenandoahGCSession session(cause, old_generation);
497 bool marking_complete = resume_concurrent_old_cycle(old_generation, cause);
498 if (marking_complete) {
499 assert(old_generation->state() != ShenandoahOldGeneration::MARKING, "Should not still be marking");
500 if (original_state == ShenandoahOldGeneration::MARKING) {
501 heap->mmu_tracker()->record_old_marking_increment(true);
502 heap->log_heap_status("At end of Concurrent Old Marking finishing increment");
503 }
504 } else if (original_state == ShenandoahOldGeneration::MARKING) {
505 heap->mmu_tracker()->record_old_marking_increment(false);
506 heap->log_heap_status("At end of Concurrent Old Marking increment");
507 }
508 break;
509 }
510 default:
511 fatal("Unexpected state for old GC: %s", ShenandoahOldGeneration::state_name(old_generation->state()));
512 }
513 }
514
515 bool ShenandoahGenerationalControlThread::resume_concurrent_old_cycle(ShenandoahOldGeneration* generation, GCCause::Cause cause) {
516 assert(ShenandoahHeap::heap()->is_concurrent_old_mark_in_progress(), "Old mark should be in progress");
517 log_debug(gc)("Resuming old generation with " UINT32_FORMAT " marking tasks queued", generation->task_queues()->tasks());
518
519 ShenandoahHeap* heap = ShenandoahHeap::heap();
520
521 // We can only tolerate being cancelled during concurrent marking or during preparation for mixed
522 // evacuation. This flag here (passed by reference) is used to control precisely where the regulator
523 // is allowed to cancel a GC.
524 ShenandoahOldGC gc(generation, _allow_old_preemption);
525 if (gc.collect(cause)) {
526 generation->record_success_concurrent(false);
527 }
528
529 if (heap->cancelled_gc()) {
530 // It's possible the gc cycle was cancelled after the last time
531 // the collection checked for cancellation. In which case, the
532 // old gc cycle is still completed, and we have to deal with this
533 // cancellation. We set the degeneration point to be outside
534 // the cycle because if this is an allocation failure, that is
535 // what must be done (there is no degenerated old cycle). If the
536 // cancellation was due to a heuristic wanting to start a young
537 // cycle, then we are not actually going to a degenerated cycle,
538 // so the degenerated point doesn't matter here.
539 check_cancellation_or_degen(ShenandoahGC::_degenerated_outside_cycle);
540 if (_requested_gc_cause == GCCause::_shenandoah_concurrent_gc) {
541 heap->shenandoah_policy()->record_interrupted_old();
542 }
543 return false;
544 }
545 return true;
546 }
547
548 void ShenandoahGenerationalControlThread::service_concurrent_cycle(ShenandoahGeneration* generation, GCCause::Cause cause, bool do_old_gc_bootstrap) {
549 // Normal cycle goes via all concurrent phases. If allocation failure (af) happens during
550 // any of the concurrent phases, it first degrades to Degenerated GC and completes GC there.
551 // If second allocation failure happens during Degenerated GC cycle (for example, when GC
552 // tries to evac something and no memory is available), cycle degrades to Full GC.
553 //
554 // There are also a shortcut through the normal cycle: immediate garbage shortcut, when
555 // heuristics says there are no regions to compact, and all the collection comes from immediately
556 // reclaimable regions.
557 //
558 // ................................................................................................
559 //
560 // (immediate garbage shortcut) Concurrent GC
561 // /-------------------------------------------\
562 // | |
563 // | |
564 // | |
565 // | v
566 // [START] ----> Conc Mark ----o----> Conc Evac --o--> Conc Update-Refs ---o----> [END]
567 // | | | ^
568 // | (af) | (af) | (af) |
569 // ..................|....................|.................|..............|.......................
570 // | | | |
571 // | | | | Degenerated GC
572 // v v v |
573 // STW Mark ----------> STW Evac ----> STW Update-Refs ----->o
574 // | | | ^
575 // | (af) | (af) | (af) |
576 // ..................|....................|.................|..............|.......................
577 // | | | |
578 // | v | | Full GC
579 // \------------------->o<----------------/ |
580 // | |
581 // v |
582 // Full GC --------------------------/
583 //
584 if (check_cancellation_or_degen(ShenandoahGC::_degenerated_outside_cycle)) return;
585
586 ShenandoahHeap* heap = ShenandoahHeap::heap();
587 ShenandoahGCSession session(cause, generation);
588 TraceCollectorStats tcs(heap->monitoring_support()->concurrent_collection_counters());
589
590 service_concurrent_cycle(heap, generation, cause, do_old_gc_bootstrap);
591 }
592
593 void ShenandoahGenerationalControlThread::service_concurrent_cycle(ShenandoahHeap* heap,
594 ShenandoahGeneration* generation,
595 GCCause::Cause& cause,
596 bool do_old_gc_bootstrap) {
597 assert(!generation->is_old(), "Old GC takes a different control path");
598
599 ShenandoahConcurrentGC gc(generation, do_old_gc_bootstrap);
600 if (gc.collect(cause)) {
601 // Cycle is complete
602 generation->record_success_concurrent(gc.abbreviated());
603 } else {
604 assert(heap->cancelled_gc(), "Must have been cancelled");
605 check_cancellation_or_degen(gc.degen_point());
606
607 // Concurrent young-gen collection degenerates to young
608 // collection. Same for global collections.
609 _degen_generation = generation;
610 }
611 const char* msg;
612 ShenandoahMmuTracker* mmu_tracker = heap->mmu_tracker();
613 if (generation->is_young()) {
614 if (heap->cancelled_gc()) {
615 msg = (do_old_gc_bootstrap) ? "At end of Interrupted Concurrent Bootstrap GC" :
616 "At end of Interrupted Concurrent Young GC";
617 } else {
618 // We only record GC results if GC was successful
619 msg = (do_old_gc_bootstrap) ? "At end of Concurrent Bootstrap GC" :
620 "At end of Concurrent Young GC";
621 if (heap->collection_set()->has_old_regions()) {
622 mmu_tracker->record_mixed(get_gc_id());
623 } else if (do_old_gc_bootstrap) {
624 mmu_tracker->record_bootstrap(get_gc_id());
625 } else {
626 mmu_tracker->record_young(get_gc_id());
627 }
628 }
629 } else {
630 assert(generation->is_global(), "If not young, must be GLOBAL");
631 assert(!do_old_gc_bootstrap, "Do not bootstrap with GLOBAL GC");
632 if (heap->cancelled_gc()) {
633 msg = "At end of Interrupted Concurrent GLOBAL GC";
634 } else {
635 // We only record GC results if GC was successful
636 msg = "At end of Concurrent Global GC";
637 mmu_tracker->record_global(get_gc_id());
638 }
639 }
640 heap->log_heap_status(msg);
641 }
642
643 bool ShenandoahGenerationalControlThread::check_cancellation_or_degen(ShenandoahGC::ShenandoahDegenPoint point) {
644 ShenandoahHeap* heap = ShenandoahHeap::heap();
645 if (!heap->cancelled_gc()) {
646 return false;
647 }
648
649 if (in_graceful_shutdown()) {
650 return true;
651 }
652
653 assert(_degen_point == ShenandoahGC::_degenerated_outside_cycle,
654 "Should not be set yet: %s", ShenandoahGC::degen_point_to_string(_degen_point));
655
656 if (is_alloc_failure_gc()) {
657 _degen_point = point;
658 _preemption_requested.unset();
659 return true;
660 }
661
662 if (_preemption_requested.is_set()) {
663 assert(_requested_generation == YOUNG, "Only young GCs may preempt old.");
664 _preemption_requested.unset();
665
666 // Old generation marking is only cancellable during concurrent marking.
667 // Once final mark is complete, the code does not check again for cancellation.
668 // If old generation was cancelled for an allocation failure, we wouldn't
669 // make it to this case. The calling code is responsible for forcing a
670 // cancellation due to allocation failure into a degenerated cycle.
671 _degen_point = point;
672 heap->clear_cancelled_gc(false /* clear oom handler */);
673 return true;
674 }
675
676 fatal("Cancel GC either for alloc failure GC, or gracefully exiting, or to pause old generation marking");
677 return false;
678 }
679
680 void ShenandoahGenerationalControlThread::stop_service() {
681 // Nothing to do here.
682 }
683
684 void ShenandoahGenerationalControlThread::service_stw_full_cycle(GCCause::Cause cause) {
685 ShenandoahHeap* const heap = ShenandoahHeap::heap();
686
687 GCIdMark gc_id_mark;
688 ShenandoahGCSession session(cause, heap->global_generation());
689
690 ShenandoahFullGC gc;
691 gc.collect(cause);
692 }
693
694 void ShenandoahGenerationalControlThread::service_stw_degenerated_cycle(GCCause::Cause cause,
695 ShenandoahGC::ShenandoahDegenPoint point) {
696 assert(point != ShenandoahGC::_degenerated_unset, "Degenerated point should be set");
697 ShenandoahHeap* const heap = ShenandoahHeap::heap();
698
699 GCIdMark gc_id_mark;
700 ShenandoahGCSession session(cause, _degen_generation);
701
702 ShenandoahDegenGC gc(point, _degen_generation);
703 gc.collect(cause);
704
705 assert(heap->young_generation()->task_queues()->is_empty(), "Unexpected young generation marking tasks");
706 if (_degen_generation->is_global()) {
707 assert(heap->old_generation()->task_queues()->is_empty(), "Unexpected old generation marking tasks");
708 assert(heap->global_generation()->task_queues()->is_empty(), "Unexpected global generation marking tasks");
709 } else {
710 assert(_degen_generation->is_young(), "Expected degenerated young cycle, if not global.");
711 ShenandoahOldGeneration* old = heap->old_generation();
712 if (old->is_bootstrapping()) {
713 old->transition_to(ShenandoahOldGeneration::MARKING);
714 }
715 }
716 }
717
718 void ShenandoahGenerationalControlThread::request_gc(GCCause::Cause cause) {
719 if (ShenandoahCollectorPolicy::should_handle_requested_gc(cause)) {
720 handle_requested_gc(cause);
721 }
722 }
723
724 bool ShenandoahGenerationalControlThread::request_concurrent_gc(ShenandoahGenerationType generation) {
725 if (_preemption_requested.is_set() || _requested_gc_cause != GCCause::_no_gc || ShenandoahHeap::heap()->cancelled_gc()) {
726 // Ignore subsequent requests from the heuristics
727 log_debug(gc, thread)("Reject request for concurrent gc: preemption_requested: %s, gc_requested: %s, gc_cancelled: %s",
728 BOOL_TO_STR(_preemption_requested.is_set()),
729 GCCause::to_string(_requested_gc_cause),
730 BOOL_TO_STR(ShenandoahHeap::heap()->cancelled_gc()));
731 return false;
732 }
733
734 if (gc_mode() == none) {
735 GCCause::Cause existing = Atomic::cmpxchg(&_requested_gc_cause, GCCause::_no_gc, GCCause::_shenandoah_concurrent_gc);
736 if (existing != GCCause::_no_gc) {
737 log_debug(gc, thread)("Reject request for concurrent gc because another gc is pending: %s", GCCause::to_string(existing));
738 return false;
739 }
740
741 _requested_generation = generation;
742 notify_control_thread();
743
744 MonitorLocker ml(&_regulator_lock, Mutex::_no_safepoint_check_flag);
745 while (gc_mode() == none) {
746 ml.wait();
747 }
748 return true;
749 }
750
751 if (preempt_old_marking(generation)) {
752 assert(gc_mode() == servicing_old, "Expected to be servicing old, but was: %s.", gc_mode_name(gc_mode()));
753 GCCause::Cause existing = Atomic::cmpxchg(&_requested_gc_cause, GCCause::_no_gc, GCCause::_shenandoah_concurrent_gc);
754 if (existing != GCCause::_no_gc) {
755 log_debug(gc, thread)("Reject request to interrupt old gc because another gc is pending: %s", GCCause::to_string(existing));
756 return false;
757 }
758
759 log_info(gc)("Preempting old generation mark to allow %s GC", shenandoah_generation_name(generation));
760 _requested_generation = generation;
761 _preemption_requested.set();
762 ShenandoahHeap::heap()->cancel_gc(GCCause::_shenandoah_concurrent_gc);
763 notify_control_thread();
764
765 MonitorLocker ml(&_regulator_lock, Mutex::_no_safepoint_check_flag);
766 while (gc_mode() == servicing_old) {
767 ml.wait();
768 }
769 return true;
770 }
771
772 log_debug(gc, thread)("Reject request for concurrent gc: mode: %s, allow_old_preemption: %s",
773 gc_mode_name(gc_mode()),
774 BOOL_TO_STR(_allow_old_preemption.is_set()));
775 return false;
776 }
777
778 void ShenandoahGenerationalControlThread::notify_control_thread() {
779 MonitorLocker locker(&_control_lock, Mutex::_no_safepoint_check_flag);
780 _control_lock.notify();
781 }
782
783 bool ShenandoahGenerationalControlThread::preempt_old_marking(ShenandoahGenerationType generation) {
784 return (generation == YOUNG) && _allow_old_preemption.try_unset();
785 }
786
787 void ShenandoahGenerationalControlThread::handle_requested_gc(GCCause::Cause cause) {
788 // Make sure we have at least one complete GC cycle before unblocking
789 // from the explicit GC request.
790 //
791 // This is especially important for weak references cleanup and/or native
792 // resources (e.g. DirectByteBuffers) machinery: when explicit GC request
793 // comes very late in the already running cycle, it would miss lots of new
794 // opportunities for cleanup that were made available before the caller
795 // requested the GC.
796
797 MonitorLocker ml(&_gc_waiters_lock);
798 size_t current_gc_id = get_gc_id();
799 size_t required_gc_id = current_gc_id + 1;
800 while (current_gc_id < required_gc_id) {
801 // This races with the regulator thread to start a concurrent gc and the
802 // control thread to clear it at the start of a cycle. Threads here are
803 // allowed to escalate a heuristic's request for concurrent gc.
804 GCCause::Cause existing = Atomic::xchg(&_requested_gc_cause, cause);
805 if (existing != GCCause::_no_gc) {
806 log_debug(gc, thread)("GC request supersedes existing request: %s", GCCause::to_string(existing));
807 }
808
809 notify_control_thread();
810 if (cause != GCCause::_wb_breakpoint) {
811 ml.wait();
812 }
813 current_gc_id = get_gc_id();
814 }
815 }
816
817 void ShenandoahGenerationalControlThread::notify_gc_waiters() {
818 MonitorLocker ml(&_gc_waiters_lock);
819 ml.notify_all();
820 }
821
822 const char* ShenandoahGenerationalControlThread::gc_mode_name(ShenandoahGenerationalControlThread::GCMode mode) {
823 switch (mode) {
824 case none: return "idle";
825 case concurrent_normal: return "normal";
826 case stw_degenerated: return "degenerated";
827 case stw_full: return "full";
828 case servicing_old: return "old";
829 case bootstrapping_old: return "bootstrap";
830 default: return "unknown";
831 }
832 }
833
834 void ShenandoahGenerationalControlThread::set_gc_mode(ShenandoahGenerationalControlThread::GCMode new_mode) {
835 if (_mode != new_mode) {
836 log_info(gc)("Transition from: %s to: %s", gc_mode_name(_mode), gc_mode_name(new_mode));
837 MonitorLocker ml(&_regulator_lock, Mutex::_no_safepoint_check_flag);
838 _mode = new_mode;
839 ml.notify_all();
840 }
841 }