1 /*
2 * Copyright (c) 2021, Red Hat, Inc. All rights reserved.
3 * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
4 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
5 *
6 * This code is free software; you can redistribute it and/or modify it
7 * under the terms of the GNU General Public License version 2 only, as
8 * published by the Free Software Foundation.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 *
24 */
25
26 #include "precompiled.hpp"
27
28 #include "gc/shared/collectorCounters.hpp"
29 #include "gc/shenandoah/shenandoahCollectorPolicy.hpp"
30 #include "gc/shenandoah/shenandoahConcurrentMark.hpp"
31 #include "gc/shenandoah/shenandoahDegeneratedGC.hpp"
32 #include "gc/shenandoah/shenandoahFullGC.hpp"
33 #include "gc/shenandoah/shenandoahGeneration.hpp"
34 #include "gc/shenandoah/shenandoahGenerationalHeap.hpp"
35 #include "gc/shenandoah/shenandoahHeap.inline.hpp"
36 #include "gc/shenandoah/shenandoahMetrics.hpp"
37 #include "gc/shenandoah/shenandoahMonitoringSupport.hpp"
38 #include "gc/shenandoah/shenandoahOldGeneration.hpp"
39 #include "gc/shenandoah/shenandoahOopClosures.inline.hpp"
40 #include "gc/shenandoah/shenandoahRootProcessor.inline.hpp"
41 #include "gc/shenandoah/shenandoahSTWMark.hpp"
42 #include "gc/shenandoah/shenandoahUtils.hpp"
43 #include "gc/shenandoah/shenandoahVerifier.hpp"
44 #include "gc/shenandoah/shenandoahYoungGeneration.hpp"
45 #include "gc/shenandoah/shenandoahWorkerPolicy.hpp"
46 #include "gc/shenandoah/shenandoahVMOperations.hpp"
47 #include "runtime/vmThread.hpp"
48 #include "utilities/events.hpp"
49
50 ShenandoahDegenGC::ShenandoahDegenGC(ShenandoahDegenPoint degen_point, ShenandoahGeneration* generation) :
51 ShenandoahGC(),
52 _degen_point(degen_point),
53 _generation(generation),
54 _abbreviated(false) {
55 }
56
57 bool ShenandoahDegenGC::collect(GCCause::Cause cause) {
58 vmop_degenerated();
59 ShenandoahHeap* heap = ShenandoahHeap::heap();
60 if (heap->mode()->is_generational()) {
61 bool is_bootstrap_gc = heap->old_generation()->state() == ShenandoahOldGeneration::BOOTSTRAPPING;
62 heap->mmu_tracker()->record_degenerated(GCId::current(), is_bootstrap_gc);
63 const char* msg = is_bootstrap_gc? "At end of Degenerated Bootstrap Old GC": "At end of Degenerated Young GC";
64 heap->log_heap_status(msg);
65 }
66 return true;
67 }
68
69 void ShenandoahDegenGC::vmop_degenerated() {
70 TraceCollectorStats tcs(ShenandoahHeap::heap()->monitoring_support()->full_stw_collection_counters());
71 ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::degen_gc_gross);
72 VM_ShenandoahDegeneratedGC degenerated_gc(this);
73 VMThread::execute(°enerated_gc);
74 }
75
76 void ShenandoahDegenGC::entry_degenerated() {
77 const char* msg = degen_event_message(_degen_point);
78 ShenandoahPausePhase gc_phase(msg, ShenandoahPhaseTimings::degen_gc, true /* log_heap_usage */);
79 EventMark em("%s", msg);
80 ShenandoahHeap* const heap = ShenandoahHeap::heap();
81 ShenandoahWorkerScope scope(heap->workers(),
82 ShenandoahWorkerPolicy::calc_workers_for_stw_degenerated(),
83 "stw degenerated gc");
84
85 heap->set_degenerated_gc_in_progress(true);
86 op_degenerated();
87 heap->set_degenerated_gc_in_progress(false);
88 }
89
90 void ShenandoahDegenGC::op_degenerated() {
91 ShenandoahHeap* const heap = ShenandoahHeap::heap();
92 // Degenerated GC is STW, but it can also fail. Current mechanics communicates
93 // GC failure via cancelled_concgc() flag. So, if we detect the failure after
94 // some phase, we have to upgrade the Degenerate GC to Full GC.
95 heap->clear_cancelled_gc(true /* clear oom handler */);
96
97 #ifdef ASSERT
98 if (heap->mode()->is_generational()) {
99 ShenandoahOldGeneration* old_generation = heap->old_generation();
100 if (!heap->is_concurrent_old_mark_in_progress()) {
101 // If we are not marking the old generation, there should be nothing in the old mark queues
102 assert(old_generation->task_queues()->is_empty(), "Old gen task queues should be empty");
103 }
104
105 if (_generation->is_global()) {
106 // If we are in a global cycle, the old generation should not be marking. It is, however,
107 // allowed to be holding regions for evacuation or coalescing.
108 ShenandoahOldGeneration::State state = old_generation->state();
109 assert(state == ShenandoahOldGeneration::WAITING_FOR_BOOTSTRAP
110 || state == ShenandoahOldGeneration::EVACUATING
111 || state == ShenandoahOldGeneration::FILLING,
112 "Old generation cannot be in state: %s", old_generation->state_name());
113 }
114 }
115 #endif
116
117 ShenandoahMetricsSnapshot metrics;
118 metrics.snap_before();
119
120 switch (_degen_point) {
121 // The cases below form the Duff's-like device: it describes the actual GC cycle,
122 // but enters it at different points, depending on which concurrent phase had
123 // degenerated.
124
125 case _degenerated_outside_cycle:
126 // We have degenerated from outside the cycle, which means something is bad with
127 // the heap, most probably heavy humongous fragmentation, or we are very low on free
128 // space. It makes little sense to wait for Full GC to reclaim as much as it can, when
129 // we can do the most aggressive degen cycle, which includes processing references and
130 // class unloading, unless those features are explicitly disabled.
131
132 // Note that we can only do this for "outside-cycle" degens, otherwise we would risk
133 // changing the cycle parameters mid-cycle during concurrent -> degenerated handover.
134 heap->set_unload_classes(_generation->heuristics()->can_unload_classes() &&
135 (!heap->mode()->is_generational() || _generation->is_global()));
136
137 if (heap->mode()->is_generational() &&
138 (_generation->is_young() || (_generation->is_global() && ShenandoahVerify))) {
139 // Swap remembered sets for young, or if the verifier will run during a global collect
140 // TODO: This path should not depend on ShenandoahVerify
141 _generation->swap_remembered_set();
142 }
143
144 case _degenerated_roots:
145 // Degenerated from concurrent root mark, reset the flag for STW mark
146 if (!heap->mode()->is_generational()) {
147 if (heap->is_concurrent_mark_in_progress()) {
148 heap->cancel_concurrent_mark();
149 }
150 } else {
151 if (_generation->is_concurrent_mark_in_progress()) {
152 // We want to allow old generation marking to be punctuated by young collections
153 // (even if they have degenerated). If this is a global cycle, we'd have cancelled
154 // the entire old gc before coming into this switch. Note that cancel_marking on
155 // the generation does NOT abandon incomplete SATB buffers as cancel_concurrent_mark does.
156 // We need to separate out the old pointers which is done below.
157 _generation->cancel_marking();
158 }
159
160 if (heap->is_concurrent_mark_in_progress()) {
161 // If either old or young marking is in progress, the SATB barrier will be enabled.
162 // The SATB buffer may hold a mix of old and young pointers. The old pointers need to be
163 // transferred to the old generation mark queues and the young pointers are NOT part
164 // of this snapshot, so they must be dropped here. It is safe to drop them here because
165 // we will rescan the roots on this safepoint.
166 heap->transfer_old_pointers_from_satb();
167 }
168 }
169
170 if (_degen_point == ShenandoahDegenPoint::_degenerated_roots) {
171 // We only need this if the concurrent cycle has already swapped the card tables.
172 // Marking will use the 'read' table, but interesting pointers may have been
173 // recorded in the 'write' table in the time between the cancelled concurrent cycle
174 // and this degenerated cycle. These pointers need to be included the 'read' table
175 // used to scan the remembered set during the STW mark which follows here.
176 _generation->merge_write_table();
177 }
178
179 op_reset();
180
181 // STW mark
182 op_mark();
183
184 case _degenerated_mark:
185 // No fallthrough. Continue mark, handed over from concurrent mark if
186 // concurrent mark has yet completed
187 if (_degen_point == ShenandoahDegenPoint::_degenerated_mark &&
188 heap->is_concurrent_mark_in_progress()) {
189 op_finish_mark();
190 }
191 assert(!heap->cancelled_gc(), "STW mark can not OOM");
192
193 /* Degen select Collection Set. etc. */
194 op_prepare_evacuation();
195
196 op_cleanup_early();
197
198 case _degenerated_evac:
199 // If heuristics thinks we should do the cycle, this flag would be set,
200 // and we can do evacuation. Otherwise, it would be the shortcut cycle.
201 if (heap->is_evacuation_in_progress()) {
202
203 if (_degen_point == _degenerated_evac) {
204 // Degeneration under oom-evac protocol allows the mutator LRB to expose
205 // references to from-space objects. This is okay, in theory, because we
206 // will come to the safepoint here to complete the evacuations and update
207 // the references. However, if the from-space reference is written to a
208 // region that was EC during final mark or was recycled after final mark
209 // it will not have TAMS or UWM updated. Such a region is effectively
210 // skipped during update references which can lead to crashes and corruption
211 // if the from-space reference is accessed.
212 if (UseTLAB) {
213 heap->labs_make_parsable();
214 }
215
216 for (size_t i = 0; i < heap->num_regions(); i++) {
217 ShenandoahHeapRegion* r = heap->get_region(i);
218 if (r->is_active() && r->top() > r->get_update_watermark()) {
219 r->set_update_watermark_at_safepoint(r->top());
220 }
221 }
222 }
223
224 // Degeneration under oom-evac protocol might have left some objects in
225 // collection set un-evacuated. Restart evacuation from the beginning to
226 // capture all objects. For all the objects that are already evacuated,
227 // it would be a simple check, which is supposed to be fast. This is also
228 // safe to do even without degeneration, as CSet iterator is at beginning
229 // in preparation for evacuation anyway.
230 //
231 // Before doing that, we need to make sure we never had any cset-pinned
232 // regions. This may happen if allocation failure happened when evacuating
233 // the about-to-be-pinned object, oom-evac protocol left the object in
234 // the collection set, and then the pin reached the cset region. If we continue
235 // the cycle here, we would trash the cset and alive objects in it. To avoid
236 // it, we fail degeneration right away and slide into Full GC to recover.
237
238 {
239 heap->sync_pinned_region_status();
240 heap->collection_set()->clear_current_index();
241 ShenandoahHeapRegion* r;
242 while ((r = heap->collection_set()->next()) != nullptr) {
243 if (r->is_pinned()) {
244 heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
245 op_degenerated_fail();
246 return;
247 }
248 }
249
250 heap->collection_set()->clear_current_index();
251 }
252 op_evacuate();
253 if (heap->cancelled_gc()) {
254 op_degenerated_fail();
255 return;
256 }
257 }
258
259 // Update collector state regardless of whether or not there are forwarded objects
260 heap->set_evacuation_in_progress(false);
261 heap->set_concurrent_weak_root_in_progress(false);
262 heap->set_concurrent_strong_root_in_progress(false);
263
264 // If heuristics thinks we should do the cycle, this flag would be set,
265 // and we need to do update-refs. Otherwise, it would be the shortcut cycle.
266 if (heap->has_forwarded_objects()) {
267 op_init_updaterefs();
268 assert(!heap->cancelled_gc(), "STW reference update can not OOM");
269 } else {
270 _abbreviated = true;
271 }
272
273 case _degenerated_updaterefs:
274 if (heap->has_forwarded_objects()) {
275 op_updaterefs();
276 op_update_roots();
277 assert(!heap->cancelled_gc(), "STW reference update can not OOM");
278 }
279
280 // Disarm nmethods that armed in concurrent cycle.
281 // In above case, update roots should disarm them
282 ShenandoahCodeRoots::disarm_nmethods();
283
284 if (heap->mode()->is_generational() && heap->is_concurrent_old_mark_in_progress()) {
285 // This is still necessary for degenerated cycles because the degeneration point may occur
286 // after final mark of the young generation. See ShenandoahConcurrentGC::op_final_updaterefs for
287 // a more detailed explanation.
288 heap->transfer_old_pointers_from_satb();
289 }
290
291 op_cleanup_complete();
292 // We defer generation resizing actions until after cset regions have been recycled.
293 if (heap->mode()->is_generational()) {
294 auto result = ShenandoahGenerationalHeap::heap()->balance_generations();
295 LogTarget(Info, gc, ergo) lt;
296 if (lt.is_enabled()) {
297 LogStream ls(lt);
298 result.print_on("Degenerated GC", &ls);
299 }
300 }
301 break;
302 default:
303 ShouldNotReachHere();
304 }
305
306 if (heap->mode()->is_generational()) {
307 // In case degeneration interrupted concurrent evacuation or update references, we need to clean up transient state.
308 // Otherwise, these actions have no effect.
309 ShenandoahGenerationalHeap::heap()->reset_generation_reserves();
310 }
311
312 if (ShenandoahVerify) {
313 heap->verifier()->verify_after_degenerated();
314 }
315
316 if (VerifyAfterGC) {
317 Universe::verify();
318 }
319
320 metrics.snap_after();
321
322 // Check for futility and fail. There is no reason to do several back-to-back Degenerated cycles,
323 // because that probably means the heap is overloaded and/or fragmented.
324 if (!metrics.is_good_progress()) {
325 heap->notify_gc_no_progress();
326 heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
327 op_degenerated_futile();
328 } else {
329 heap->notify_gc_progress();
330 heap->shenandoah_policy()->record_success_degenerated(_generation->is_young(), _abbreviated);
331 _generation->heuristics()->record_success_degenerated();
332 }
333 }
334
335 void ShenandoahDegenGC::op_reset() {
336 _generation->prepare_gc();
337 }
338
339 void ShenandoahDegenGC::op_mark() {
340 assert(!_generation->is_concurrent_mark_in_progress(), "Should be reset");
341 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_mark);
342 ShenandoahSTWMark mark(_generation, false /*full gc*/);
343 mark.mark();
344 }
345
346 void ShenandoahDegenGC::op_finish_mark() {
347 ShenandoahConcurrentMark mark(_generation);
348 mark.finish_mark();
349 }
350
351 void ShenandoahDegenGC::op_prepare_evacuation() {
352 ShenandoahHeap* const heap = ShenandoahHeap::heap();
353 if (ShenandoahVerify) {
354 heap->verifier()->verify_roots_no_forwarded();
355 }
356
357 // STW cleanup weak roots and unload classes
358 heap->parallel_cleaning(false /*full gc*/);
359
360 // Prepare regions and collection set
361 _generation->prepare_regions_and_collection_set(false /*concurrent*/);
362
363 // Retire the TLABs, which will force threads to reacquire their TLABs after the pause.
364 // This is needed for two reasons. Strong one: new allocations would be with new freeset,
365 // which would be outside the collection set, so no cset writes would happen there.
366 // Weaker one: new allocations would happen past update watermark, and so less work would
367 // be needed for reference updates (would update the large filler instead).
368 if (UseTLAB) {
369 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_final_manage_labs);
370 heap->tlabs_retire(false);
371 }
372
373 if (!heap->collection_set()->is_empty() || heap->old_generation()->has_in_place_promotions()) {
374 // Even if the collection set is empty, we need to do evacuation if there are regions to be promoted in place.
375 // Degenerated evacuation takes responsibility for registering objects and setting the remembered set cards to dirty.
376
377 if (ShenandoahVerify) {
378 heap->verifier()->verify_before_evacuation();
379 }
380
381 heap->set_evacuation_in_progress(true);
382
383 if(ShenandoahVerify) {
384 heap->verifier()->verify_during_evacuation();
385 }
386
387 heap->set_has_forwarded_objects(!heap->collection_set()->is_empty());
388 } else {
389 if (ShenandoahVerify) {
390 heap->verifier()->verify_after_concmark();
391 }
392
393 if (VerifyAfterGC) {
394 Universe::verify();
395 }
396 }
397 }
398
399 void ShenandoahDegenGC::op_cleanup_early() {
400 ShenandoahHeap::heap()->recycle_trash();
401 }
402
403 void ShenandoahDegenGC::op_evacuate() {
404 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_evac);
405 ShenandoahHeap::heap()->evacuate_collection_set(false /* concurrent*/);
406 }
407
408 void ShenandoahDegenGC::op_init_updaterefs() {
409 // Evacuation has completed
410 ShenandoahHeap* const heap = ShenandoahHeap::heap();
411 heap->prepare_update_heap_references(false /*concurrent*/);
412 heap->set_update_refs_in_progress(true);
413 }
414
415 void ShenandoahDegenGC::op_updaterefs() {
416 ShenandoahHeap* const heap = ShenandoahHeap::heap();
417 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_updaterefs);
418 // Handed over from concurrent update references phase
419 heap->update_heap_references(false /*concurrent*/);
420
421 heap->set_update_refs_in_progress(false);
422 heap->set_has_forwarded_objects(false);
423 }
424
425 void ShenandoahDegenGC::op_update_roots() {
426 ShenandoahHeap* const heap = ShenandoahHeap::heap();
427
428 update_roots(false /*full_gc*/);
429
430 heap->update_heap_region_states(false /*concurrent*/);
431
432 if (ShenandoahVerify) {
433 heap->verifier()->verify_after_updaterefs();
434 }
435
436 if (VerifyAfterGC) {
437 Universe::verify();
438 }
439
440 heap->rebuild_free_set(false /*concurrent*/);
441 }
442
443 void ShenandoahDegenGC::op_cleanup_complete() {
444 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_cleanup_complete);
445 ShenandoahHeap::heap()->recycle_trash();
446 }
447
448 void ShenandoahDegenGC::op_degenerated_fail() {
449 upgrade_to_full();
450 }
451
452 void ShenandoahDegenGC::op_degenerated_futile() {
453 upgrade_to_full();
454 }
455
456 const char* ShenandoahDegenGC::degen_event_message(ShenandoahDegenPoint point) const {
457 switch (point) {
458 case _degenerated_unset:
459 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (<UNSET>)");
460 case _degenerated_outside_cycle:
461 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Outside of Cycle)");
462 case _degenerated_roots:
463 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Roots)");
464 case _degenerated_mark:
465 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Mark)");
466 case _degenerated_evac:
467 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Evacuation)");
468 case _degenerated_updaterefs:
469 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Update Refs)");
470 default:
471 ShouldNotReachHere();
472 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (?)");
473 }
474 }
475
476 void ShenandoahDegenGC::upgrade_to_full() {
477 log_info(gc)("Degenerated GC upgrading to Full GC");
478 ShenandoahHeap::heap()->shenandoah_policy()->record_degenerated_upgrade_to_full();
479 ShenandoahFullGC full_gc;
480 full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
481 }