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 _consecutive_degen_with_bad_progress(0) {
56 }
57
58 bool ShenandoahDegenGC::collect(GCCause::Cause cause) {
59 vmop_degenerated();
60 ShenandoahHeap* heap = ShenandoahHeap::heap();
61 if (heap->mode()->is_generational()) {
62 bool is_bootstrap_gc = heap->old_generation()->is_bootstrapping();
63 heap->mmu_tracker()->record_degenerated(GCId::current(), is_bootstrap_gc);
64 const char* msg = is_bootstrap_gc? "At end of Degenerated Bootstrap Old GC": "At end of Degenerated Young GC";
65 heap->log_heap_status(msg);
66 }
67 return true;
68 }
69
70 void ShenandoahDegenGC::vmop_degenerated() {
71 TraceCollectorStats tcs(ShenandoahHeap::heap()->monitoring_support()->full_stw_collection_counters());
72 ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::degen_gc_gross);
73 VM_ShenandoahDegeneratedGC degenerated_gc(this);
74 VMThread::execute(°enerated_gc);
75 }
76
77 void ShenandoahDegenGC::entry_degenerated() {
78 const char* msg = degen_event_message(_degen_point);
79 ShenandoahPausePhase gc_phase(msg, ShenandoahPhaseTimings::degen_gc, true /* log_heap_usage */);
80 EventMark em("%s", msg);
81 ShenandoahHeap* const heap = ShenandoahHeap::heap();
82 ShenandoahWorkerScope scope(heap->workers(),
83 ShenandoahWorkerPolicy::calc_workers_for_stw_degenerated(),
84 "stw degenerated gc");
85
86 heap->set_degenerated_gc_in_progress(true);
87 op_degenerated();
88 heap->set_degenerated_gc_in_progress(false);
89 {
90 ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::degen_gc_propagate_gc_state);
91 heap->propagate_gc_state_to_all_threads();
92 }
93 }
94
95 void ShenandoahDegenGC::op_degenerated() {
96 ShenandoahHeap* const heap = ShenandoahHeap::heap();
97 // Degenerated GC is STW, but it can also fail. Current mechanics communicates
98 // GC failure via cancelled_concgc() flag. So, if we detect the failure after
99 // some phase, we have to upgrade the Degenerate GC to Full GC.
100 heap->clear_cancelled_gc(true /* clear oom handler */);
101
102 #ifdef ASSERT
103 if (heap->mode()->is_generational()) {
104 ShenandoahOldGeneration* old_generation = heap->old_generation();
105 if (!heap->is_concurrent_old_mark_in_progress()) {
106 // If we are not marking the old generation, there should be nothing in the old mark queues
107 assert(old_generation->task_queues()->is_empty(), "Old gen task queues should be empty");
108 }
109
110 if (_generation->is_global()) {
111 // If we are in a global cycle, the old generation should not be marking. It is, however,
112 // allowed to be holding regions for evacuation or coalescing.
113 assert(old_generation->is_idle()
114 || old_generation->is_doing_mixed_evacuations()
115 || old_generation->is_preparing_for_mark(),
116 "Old generation cannot be in state: %s", old_generation->state_name());
117 }
118 }
119 #endif
120
121 ShenandoahMetricsSnapshot metrics;
122 metrics.snap_before();
123
124 switch (_degen_point) {
125 // The cases below form the Duff's-like device: it describes the actual GC cycle,
126 // but enters it at different points, depending on which concurrent phase had
127 // degenerated.
128
129 case _degenerated_outside_cycle:
130 // We have degenerated from outside the cycle, which means something is bad with
131 // the heap, most probably heavy humongous fragmentation, or we are very low on free
132 // space. It makes little sense to wait for Full GC to reclaim as much as it can, when
133 // we can do the most aggressive degen cycle, which includes processing references and
134 // class unloading, unless those features are explicitly disabled.
135
136 // Note that we can only do this for "outside-cycle" degens, otherwise we would risk
137 // changing the cycle parameters mid-cycle during concurrent -> degenerated handover.
138 heap->set_unload_classes(_generation->heuristics()->can_unload_classes() &&
139 (!heap->mode()->is_generational() || _generation->is_global()));
140
141 if (heap->mode()->is_generational()) {
142 // Clean the read table before swapping it. The end goal here is to have a clean
143 // write table, and to have the read table updated with the previous write table.
144 heap->old_generation()->card_scan()->mark_read_table_as_clean();
145
146 if (_generation->is_young()) {
147 // Swap remembered sets for young
148 _generation->swap_card_tables();
149 }
150 }
151
152 case _degenerated_roots:
153 // Degenerated from concurrent root mark, reset the flag for STW mark
154 if (!heap->mode()->is_generational()) {
155 if (heap->is_concurrent_mark_in_progress()) {
156 heap->cancel_concurrent_mark();
157 }
158 } else {
159 if (_generation->is_concurrent_mark_in_progress()) {
160 // We want to allow old generation marking to be punctuated by young collections
161 // (even if they have degenerated). If this is a global cycle, we'd have cancelled
162 // the entire old gc before coming into this switch. Note that cancel_marking on
163 // the generation does NOT abandon incomplete SATB buffers as cancel_concurrent_mark does.
164 // We need to separate out the old pointers which is done below.
165 _generation->cancel_marking();
166 }
167
168 if (heap->is_concurrent_mark_in_progress()) {
169 // If either old or young marking is in progress, the SATB barrier will be enabled.
170 // The SATB buffer may hold a mix of old and young pointers. The old pointers need to be
171 // transferred to the old generation mark queues and the young pointers are NOT part
172 // of this snapshot, so they must be dropped here. It is safe to drop them here because
173 // we will rescan the roots on this safepoint.
174 heap->old_generation()->transfer_pointers_from_satb();
175 }
176
177 if (_degen_point == ShenandoahDegenPoint::_degenerated_roots) {
178 // We only need this if the concurrent cycle has already swapped the card tables.
179 // Marking will use the 'read' table, but interesting pointers may have been
180 // recorded in the 'write' table in the time between the cancelled concurrent cycle
181 // and this degenerated cycle. These pointers need to be included in the 'read' table
182 // used to scan the remembered set during the STW mark which follows here.
183 _generation->merge_write_table();
184 }
185 }
186
187 op_reset();
188
189 // STW mark
190 op_mark();
191
192 case _degenerated_mark:
193 // No fallthrough. Continue mark, handed over from concurrent mark if
194 // concurrent mark has yet completed
195 if (_degen_point == ShenandoahDegenPoint::_degenerated_mark &&
196 heap->is_concurrent_mark_in_progress()) {
197 op_finish_mark();
198 }
199 assert(!heap->cancelled_gc(), "STW mark can not OOM");
200
201 /* Degen select Collection Set. etc. */
202 op_prepare_evacuation();
203
204 op_cleanup_early();
205
206 case _degenerated_evac:
207 // If heuristics thinks we should do the cycle, this flag would be set,
208 // and we can do evacuation. Otherwise, it would be the shortcut cycle.
209 if (heap->is_evacuation_in_progress()) {
210
211 if (_degen_point == _degenerated_evac) {
212 // Degeneration under oom-evac protocol allows the mutator LRB to expose
213 // references to from-space objects. This is okay, in theory, because we
214 // will come to the safepoint here to complete the evacuations and update
215 // the references. However, if the from-space reference is written to a
216 // region that was EC during final mark or was recycled after final mark
217 // it will not have TAMS or UWM updated. Such a region is effectively
218 // skipped during update references which can lead to crashes and corruption
219 // if the from-space reference is accessed.
220 if (UseTLAB) {
221 heap->labs_make_parsable();
222 }
223
224 for (size_t i = 0; i < heap->num_regions(); i++) {
225 ShenandoahHeapRegion* r = heap->get_region(i);
226 if (r->is_active() && r->top() > r->get_update_watermark()) {
227 r->set_update_watermark_at_safepoint(r->top());
228 }
229 }
230 }
231
232 // Degeneration under oom-evac protocol might have left some objects in
233 // collection set un-evacuated. Restart evacuation from the beginning to
234 // capture all objects. For all the objects that are already evacuated,
235 // it would be a simple check, which is supposed to be fast. This is also
236 // safe to do even without degeneration, as CSet iterator is at beginning
237 // in preparation for evacuation anyway.
238 //
239 // Before doing that, we need to make sure we never had any cset-pinned
240 // regions. This may happen if allocation failure happened when evacuating
241 // the about-to-be-pinned object, oom-evac protocol left the object in
242 // the collection set, and then the pin reached the cset region. If we continue
243 // the cycle here, we would trash the cset and alive objects in it. To avoid
244 // it, we fail degeneration right away and slide into Full GC to recover.
245
246 {
247 heap->sync_pinned_region_status();
248 heap->collection_set()->clear_current_index();
249 ShenandoahHeapRegion* r;
250 while ((r = heap->collection_set()->next()) != nullptr) {
251 if (r->is_pinned()) {
252 heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
253 op_degenerated_fail();
254 return;
255 }
256 }
257
258 heap->collection_set()->clear_current_index();
259 }
260 op_evacuate();
261 if (heap->cancelled_gc()) {
262 op_degenerated_fail();
263 return;
264 }
265 } else if (has_in_place_promotions(heap)) {
266 // We have nothing to evacuate, but there are still regions to promote in place.
267 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_promote_regions);
268 ShenandoahGenerationalHeap::heap()->promote_regions_in_place(false /* concurrent*/);
269 }
270
271 // Update collector state regardless of whether there are forwarded objects
272 heap->set_evacuation_in_progress(false);
273 heap->set_concurrent_weak_root_in_progress(false);
274 heap->set_concurrent_strong_root_in_progress(false);
275
276 // If heuristics thinks we should do the cycle, this flag would be set,
277 // and we need to do update-refs. Otherwise, it would be the shortcut cycle.
278 if (heap->has_forwarded_objects()) {
279 op_init_update_refs();
280 assert(!heap->cancelled_gc(), "STW reference update can not OOM");
281 } else {
282 _abbreviated = true;
283 }
284
285 case _degenerated_update_refs:
286 if (heap->has_forwarded_objects()) {
287 op_update_refs();
288 op_update_roots();
289 assert(!heap->cancelled_gc(), "STW reference update can not OOM");
290 }
291
292 // Disarm nmethods that armed in concurrent cycle.
293 // In above case, update roots should disarm them
294 ShenandoahCodeRoots::disarm_nmethods();
295
296 op_cleanup_complete();
297
298 if (heap->mode()->is_generational()) {
299 ShenandoahGenerationalHeap::heap()->complete_degenerated_cycle();
300 }
301
302 break;
303 default:
304 ShouldNotReachHere();
305 }
306
307 if (ShenandoahVerify) {
308 heap->verifier()->verify_after_degenerated();
309 }
310
311 if (VerifyAfterGC) {
312 Universe::verify();
313 }
314
315 metrics.snap_after();
316
317 // The most common scenario for lack of good progress following a degenerated GC is an accumulation of floating
318 // garbage during the most recently aborted concurrent GC effort. With generational GC, it is far more effective to
319 // reclaim this floating garbage with another degenerated cycle (which focuses on young generation and might require
320 // a pause of 200 ms) rather than a full GC cycle (which may require over 2 seconds with a 10 GB old generation).
321 //
322 // In generational mode, we'll only upgrade to full GC if we've done two degen cycles in a row and both indicated
323 // bad progress. In non-generational mode, we'll preserve the original behavior, which is to upgrade to full
324 // immediately following a degenerated cycle with bad progress. This preserves original behavior of non-generational
325 // Shenandoah so as to avoid introducing "surprising new behavior." It also makes less sense with non-generational
326 // Shenandoah to replace a full GC with a degenerated GC, because both have similar pause times in non-generational
327 // mode.
328 if (!metrics.is_good_progress(_generation)) {
329 _consecutive_degen_with_bad_progress++;
330 } else {
331 _consecutive_degen_with_bad_progress = 0;
332 }
333 if (!heap->mode()->is_generational() ||
334 ((heap->shenandoah_policy()->consecutive_degenerated_gc_count() > 1) && (_consecutive_degen_with_bad_progress >= 2))) {
335 heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
336 op_degenerated_futile();
337 } else {
338 heap->notify_gc_progress();
339 heap->shenandoah_policy()->record_success_degenerated(_generation->is_young(), _abbreviated);
340 _generation->heuristics()->record_success_degenerated();
341 }
342 }
343
344 void ShenandoahDegenGC::op_reset() {
345 _generation->prepare_gc();
346 }
347
348 void ShenandoahDegenGC::op_mark() {
349 assert(!_generation->is_concurrent_mark_in_progress(), "Should be reset");
350 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_mark);
351 ShenandoahSTWMark mark(_generation, false /*full gc*/);
352 mark.mark();
353 }
354
355 void ShenandoahDegenGC::op_finish_mark() {
356 ShenandoahConcurrentMark mark(_generation);
357 mark.finish_mark();
358 }
359
360 void ShenandoahDegenGC::op_prepare_evacuation() {
361 ShenandoahHeap* const heap = ShenandoahHeap::heap();
362 if (ShenandoahVerify) {
363 heap->verifier()->verify_roots_no_forwarded();
364 }
365
366 // STW cleanup weak roots and unload classes
367 heap->parallel_cleaning(false /*full gc*/);
368
369 // Prepare regions and collection set
370 _generation->prepare_regions_and_collection_set(false /*concurrent*/);
371
372 // Retire the TLABs, which will force threads to reacquire their TLABs after the pause.
373 // This is needed for two reasons. Strong one: new allocations would be with new freeset,
374 // which would be outside the collection set, so no cset writes would happen there.
375 // Weaker one: new allocations would happen past update watermark, and so less work would
376 // be needed for reference updates (would update the large filler instead).
377 if (UseTLAB) {
378 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_final_manage_labs);
379 heap->tlabs_retire(false);
380 }
381
382 if (!heap->collection_set()->is_empty()) {
383 if (ShenandoahVerify) {
384 heap->verifier()->verify_before_evacuation();
385 }
386
387 heap->set_evacuation_in_progress(true);
388
389 heap->set_has_forwarded_objects(true);
390 } else {
391 if (ShenandoahVerify) {
392 if (has_in_place_promotions(heap)) {
393 heap->verifier()->verify_after_concmark_with_promotions();
394 } else {
395 heap->verifier()->verify_after_concmark();
396 }
397 }
398
399 if (VerifyAfterGC) {
400 Universe::verify();
401 }
402 }
403 }
404
405 bool ShenandoahDegenGC::has_in_place_promotions(const ShenandoahHeap* heap) const {
406 return heap->mode()->is_generational() && heap->old_generation()->has_in_place_promotions();
407 }
408
409 void ShenandoahDegenGC::op_cleanup_early() {
410 ShenandoahHeap::heap()->recycle_trash();
411 }
412
413 void ShenandoahDegenGC::op_evacuate() {
414 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_evac);
415 ShenandoahHeap::heap()->evacuate_collection_set(false /* concurrent*/);
416 }
417
418 void ShenandoahDegenGC::op_init_update_refs() {
419 // Evacuation has completed
420 ShenandoahHeap* const heap = ShenandoahHeap::heap();
421 heap->prepare_update_heap_references();
422 heap->set_update_refs_in_progress(true);
423 }
424
425 void ShenandoahDegenGC::op_update_refs() {
426 ShenandoahHeap* const heap = ShenandoahHeap::heap();
427 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_update_refs);
428 // Handed over from concurrent update references phase
429 heap->update_heap_references(false /*concurrent*/);
430
431 heap->set_update_refs_in_progress(false);
432 heap->set_has_forwarded_objects(false);
433 }
434
435 void ShenandoahDegenGC::op_update_roots() {
436 ShenandoahHeap* const heap = ShenandoahHeap::heap();
437
438 update_roots(false /*full_gc*/);
439
440 heap->update_heap_region_states(false /*concurrent*/);
441
442 if (ShenandoahVerify) {
443 heap->verifier()->verify_after_update_refs();
444 }
445
446 if (VerifyAfterGC) {
447 Universe::verify();
448 }
449
450 heap->rebuild_free_set(false /*concurrent*/);
451 }
452
453 void ShenandoahDegenGC::op_cleanup_complete() {
454 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_cleanup_complete);
455 ShenandoahHeap::heap()->recycle_trash();
456 }
457
458 void ShenandoahDegenGC::op_degenerated_fail() {
459 upgrade_to_full();
460 }
461
462 void ShenandoahDegenGC::op_degenerated_futile() {
463 upgrade_to_full();
464 }
465
466 const char* ShenandoahDegenGC::degen_event_message(ShenandoahDegenPoint point) const {
467 switch (point) {
468 case _degenerated_unset:
469 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (<UNSET>)");
470 case _degenerated_outside_cycle:
471 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Outside of Cycle)");
472 case _degenerated_roots:
473 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Roots)");
474 case _degenerated_mark:
475 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Mark)");
476 case _degenerated_evac:
477 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Evacuation)");
478 case _degenerated_update_refs:
479 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Update Refs)");
480 default:
481 ShouldNotReachHere();
482 SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (?)");
483 }
484 }
485
486 void ShenandoahDegenGC::upgrade_to_full() {
487 log_info(gc)("Degenerated GC upgrading to Full GC");
488 ShenandoahHeap::heap()->shenandoah_policy()->record_degenerated_upgrade_to_full();
489 ShenandoahFullGC full_gc;
490 full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
491 }