1 /*
2 * Copyright (c) 2021, Red Hat, Inc. 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
27 #include "gc/shared/collectorCounters.hpp"
28 #include "gc/shenandoah/shenandoahCollectorPolicy.hpp"
29 #include "gc/shenandoah/shenandoahConcurrentMark.hpp"
30 #include "gc/shenandoah/shenandoahDegeneratedGC.hpp"
31 #include "gc/shenandoah/shenandoahFullGC.hpp"
32 #include "gc/shenandoah/shenandoahGeneration.hpp"
33 #include "gc/shenandoah/shenandoahHeap.inline.hpp"
34 #include "gc/shenandoah/shenandoahMetrics.hpp"
35 #include "gc/shenandoah/shenandoahMonitoringSupport.hpp"
36 #include "gc/shenandoah/shenandoahOopClosures.inline.hpp"
37 #include "gc/shenandoah/shenandoahRootProcessor.inline.hpp"
38 #include "gc/shenandoah/shenandoahSTWMark.hpp"
39 #include "gc/shenandoah/shenandoahUtils.hpp"
40 #include "gc/shenandoah/shenandoahVerifier.hpp"
41 #include "gc/shenandoah/shenandoahYoungGeneration.hpp"
42 #include "gc/shenandoah/shenandoahWorkerPolicy.hpp"
43 #include "gc/shenandoah/shenandoahVMOperations.hpp"
44 #include "runtime/vmThread.hpp"
45 #include "utilities/events.hpp"
46
47 ShenandoahDegenGC::ShenandoahDegenGC(ShenandoahDegenPoint degen_point, ShenandoahGeneration* generation) :
48 ShenandoahGC(),
49 _degen_point(degen_point),
50 _generation(generation),
51 _upgraded_to_full(false) {
52 }
53
54 bool ShenandoahDegenGC::collect(GCCause::Cause cause) {
55 vmop_degenerated();
56 return true;
57 }
58
59 void ShenandoahDegenGC::vmop_degenerated() {
60 TraceCollectorStats tcs(ShenandoahHeap::heap()->monitoring_support()->full_stw_collection_counters());
61 ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::degen_gc_gross);
62 VM_ShenandoahDegeneratedGC degenerated_gc(this);
63 VMThread::execute(°enerated_gc);
64 }
65
66 void ShenandoahDegenGC::entry_degenerated() {
67 char msg[1024];
68 degen_event_message(_degen_point, msg, sizeof(msg));
69 ShenandoahPausePhase gc_phase(msg, ShenandoahPhaseTimings::degen_gc, true /* log_heap_usage */);
70 EventMark em("%s", msg);
71 ShenandoahHeap* const heap = ShenandoahHeap::heap();
72
73 // In case degenerated GC preempted evacuation or update-refs, clear the aging cycle now. No harm in clearing it
74 // redundantly if it is already clear. We don't age during degenerated cycles.
75 heap->set_aging_cycle(false);
76
77 ShenandoahWorkerScope scope(heap->workers(),
78 ShenandoahWorkerPolicy::calc_workers_for_stw_degenerated(),
79 "stw degenerated gc");
80
81 heap->set_degenerated_gc_in_progress(true);
82 op_degenerated();
83 heap->set_degenerated_gc_in_progress(false);
84 }
85
86 void ShenandoahDegenGC::op_degenerated() {
87 ShenandoahHeap* const heap = ShenandoahHeap::heap();
88 // Degenerated GC is STW, but it can also fail. Current mechanics communicates
89 // GC failure via cancelled_concgc() flag. So, if we detect the failure after
90 // some phase, we have to upgrade the Degenerate GC to Full GC.
91 heap->clear_cancelled_gc(true /* clear oom handler */);
92
93 #ifdef ASSERT
94 if (heap->mode()->is_generational()) {
95 if (_generation->generation_mode() == GenerationMode::GLOBAL) {
96 // We can only get to a degenerated global cycle _after_ a concurrent global cycle
97 // has been cancelled. In which case, we expect the concurrent global cycle to have
98 // cancelled the old gc already.
99 assert(!heap->is_old_gc_active(), "Old GC should not be active during global cycle.");
100 }
101
102 if (!heap->is_concurrent_old_mark_in_progress()) {
103 // If we are not marking the old generation, there should be nothing in the old mark queues
104 assert(heap->old_generation()->task_queues()->is_empty(), "Old gen task queues should be empty.");
105 }
106 }
107 #endif
108
109 ShenandoahMetricsSnapshot metrics;
110 metrics.snap_before();
111
112 switch (_degen_point) {
113 // The cases below form the Duff's-like device: it describes the actual GC cycle,
114 // but enters it at different points, depending on which concurrent phase had
115 // degenerated.
116
117 case _degenerated_outside_cycle:
118 // We have degenerated from outside the cycle, which means something is bad with
119 // the heap, most probably heavy humongous fragmentation, or we are very low on free
120 // space. It makes little sense to wait for Full GC to reclaim as much as it can, when
121 // we can do the most aggressive degen cycle, which includes processing references and
122 // class unloading, unless those features are explicitly disabled.
123
124 if (heap->is_concurrent_old_mark_in_progress()) {
125 // We have come straight into a degenerated cycle without running a concurrent cycle
126 // first and the SATB barrier is enabled to support concurrent old marking. The SATB buffer
127 // may hold a mix of old and young pointers. The old pointers need to be transferred
128 // to the old generation mark queues and the young pointers are _not_ part of this
129 // snapshot, so they must be dropped here.
130 heap->transfer_old_pointers_from_satb();
131 }
132
133 // Note that we can only do this for "outside-cycle" degens, otherwise we would risk
134 // changing the cycle parameters mid-cycle during concurrent -> degenerated handover.
135 heap->set_unload_classes((!heap->mode()->is_generational() || _generation->generation_mode() == GLOBAL) && _generation->heuristics()->can_unload_classes());
136
137 if (heap->mode()->is_generational() && (_generation->generation_mode() == YOUNG || (_generation->generation_mode() == GLOBAL && ShenandoahVerify))) {
138 // Swap remembered sets for young, or if the verifier will run during a global collect
139 _generation->swap_remembered_set();
140 }
141
142 case _degenerated_roots:
143 // Degenerated from concurrent root mark, reset the flag for STW mark
144 if (!heap->mode()->is_generational()) {
145 if (heap->is_concurrent_mark_in_progress()) {
146 heap->cancel_concurrent_mark();
147 }
148 } else {
149 if (_generation->is_concurrent_mark_in_progress()) {
150 // We want to allow old generation marking to be punctuated by young collections
151 // (even if they have degenerated). If this is a global cycle, we'd have cancelled
152 // the entire old gc before coming into this switch.
153 _generation->cancel_marking();
154 }
155 }
156
157 if (_degen_point == ShenandoahDegenPoint::_degenerated_roots) {
158 // We only need this if the concurrent cycle has already swapped the card tables.
159 // Marking will use the 'read' table, but interesting pointers may have been
160 // recorded in the 'write' table in the time between the cancelled concurrent cycle
161 // and this degenerated cycle. These pointers need to be included the 'read' table
162 // used to scan the remembered set during the STW mark which follows here.
163 _generation->merge_write_table();
164 }
165
166 op_reset();
167
168 // STW mark
169 op_mark();
170
171 case _degenerated_mark:
172 // No fallthrough. Continue mark, handed over from concurrent mark if
173 // concurrent mark has yet completed
174 if (_degen_point == ShenandoahDegenPoint::_degenerated_mark &&
175 heap->is_concurrent_mark_in_progress()) {
176 op_finish_mark();
177 }
178 assert(!heap->cancelled_gc(), "STW mark can not OOM");
179
180 /* Degen select Collection Set. etc. */
181 op_prepare_evacuation();
182
183 op_cleanup_early();
184
185 case _degenerated_evac:
186
187 if (heap->mode()->is_generational() && _generation->generation_mode() == GLOBAL) {
188 op_global_coalesce_and_fill();
189 }
190
191 // If heuristics thinks we should do the cycle, this flag would be set,
192 // and we can do evacuation. Otherwise, it would be the shortcut cycle.
193 if (heap->is_evacuation_in_progress()) {
194
195 if (_degen_point == _degenerated_evac) {
196 // Degeneration under oom-evac protocol allows the mutator LRB to expose
197 // references to from-space objects. This is okay, in theory, because we
198 // will come to the safepoint here to complete the evacuations and update
199 // the references. However, if the from-space reference is written to a
200 // region that was EC during final mark or was recycled after final mark
201 // it will not have TAMS or UWM updated. Such a region is effectively
202 // skipped during update references which can lead to crashes and corruption
203 // if the from-space reference is accessed.
204 if (UseTLAB) {
205 heap->labs_make_parsable();
206 }
207
208 for (size_t i = 0; i < heap->num_regions(); i++) {
209 ShenandoahHeapRegion* r = heap->get_region(i);
210 if (r->is_active() && r->top() > r->get_update_watermark()) {
211 r->set_update_watermark_at_safepoint(r->top());
212 }
213 }
214 }
215
216 // Degeneration under oom-evac protocol might have left some objects in
217 // collection set un-evacuated. Restart evacuation from the beginning to
218 // capture all objects. For all the objects that are already evacuated,
219 // it would be a simple check, which is supposed to be fast. This is also
220 // safe to do even without degeneration, as CSet iterator is at beginning
221 // in preparation for evacuation anyway.
222 //
223 // Before doing that, we need to make sure we never had any cset-pinned
224 // regions. This may happen if allocation failure happened when evacuating
225 // the about-to-be-pinned object, oom-evac protocol left the object in
226 // the collection set, and then the pin reached the cset region. If we continue
227 // the cycle here, we would trash the cset and alive objects in it. To avoid
228 // it, we fail degeneration right away and slide into Full GC to recover.
229
230 {
231 heap->sync_pinned_region_status();
232 heap->collection_set()->clear_current_index();
233 ShenandoahHeapRegion* r;
234 while ((r = heap->collection_set()->next()) != NULL) {
235 if (r->is_pinned()) {
236 heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
237 op_degenerated_fail();
238 return;
239 }
240 }
241
242 heap->collection_set()->clear_current_index();
243 }
244 op_evacuate();
245 if (heap->cancelled_gc()) {
246 op_degenerated_fail();
247 return;
248 }
249 }
250
251 // If heuristics thinks we should do the cycle, this flag would be set,
252 // and we need to do update-refs. Otherwise, it would be the shortcut cycle.
253 if (heap->has_forwarded_objects()) {
254 op_init_updaterefs();
255 assert(!heap->cancelled_gc(), "STW reference update can not OOM");
256 }
257
258 case _degenerated_updaterefs:
259 if (heap->has_forwarded_objects()) {
260 op_updaterefs();
261 op_update_roots();
262 assert(!heap->cancelled_gc(), "STW reference update can not OOM");
263 }
264
265 if (ClassUnloading) {
266 // Disarm nmethods that armed in concurrent cycle.
267 // In above case, update roots should disarm them
268 ShenandoahCodeRoots::disarm_nmethods();
269 }
270
271 op_cleanup_complete();
272 break;
273 default:
274 ShouldNotReachHere();
275 }
276
277 if (heap->mode()->is_generational()) {
278 // In case degeneration interrupted concurrent evacuation or update references, we need to clean up transient state.
279 // Otherwise, these actions have no effect.
280
281 heap->young_generation()->unadjust_available();
282 heap->old_generation()->unadjust_available();
283 // No need to old_gen->increase_used(). That was done when plabs were allocated, accounting for both old evacs and promotions.
284
285 heap->set_alloc_supplement_reserve(0);
286 heap->set_young_evac_reserve(0);
287 heap->set_old_evac_reserve(0);
288 heap->reset_old_evac_expended();
289 heap->set_promotion_reserve(0);
290
291 }
292
293 if (ShenandoahVerify) {
294 heap->verifier()->verify_after_degenerated();
295 }
296
297 if (VerifyAfterGC) {
298 Universe::verify();
299 }
300
301 metrics.snap_after();
302
303 // Check for futility and fail. There is no reason to do several back-to-back Degenerated cycles,
304 // because that probably means the heap is overloaded and/or fragmented.
305 if (!metrics.is_good_progress()) {
306 heap->notify_gc_no_progress();
307 heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
308 op_degenerated_futile();
309 } else {
310 heap->notify_gc_progress();
311 }
312 }
313
314 void ShenandoahDegenGC::op_reset() {
315 _generation->prepare_gc(false);
316 }
317
318 void ShenandoahDegenGC::op_mark() {
319 assert(!_generation->is_concurrent_mark_in_progress(), "Should be reset");
320 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_mark);
321 ShenandoahSTWMark mark(_generation, false /*full gc*/);
322 mark.mark();
323 }
324
325 void ShenandoahDegenGC::op_finish_mark() {
326 ShenandoahConcurrentMark mark(_generation);
327 mark.finish_mark();
328 }
329
330 void ShenandoahDegenGC::op_prepare_evacuation() {
331 ShenandoahHeap* const heap = ShenandoahHeap::heap();
332 if (ShenandoahVerify) {
333 heap->verifier()->verify_roots_no_forwarded();
334 }
335
336 // STW cleanup weak roots and unload classes
337 heap->parallel_cleaning(false /*full gc*/);
338
339 // Prepare regions and collection set
340 _generation->prepare_regions_and_collection_set(false /*concurrent*/);
341
342 // Retire the TLABs, which will force threads to reacquire their TLABs after the pause.
343 // This is needed for two reasons. Strong one: new allocations would be with new freeset,
344 // which would be outside the collection set, so no cset writes would happen there.
345 // Weaker one: new allocations would happen past update watermark, and so less work would
346 // be needed for reference updates (would update the large filler instead).
347 if (UseTLAB) {
348 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_final_manage_labs);
349 heap->tlabs_retire(false);
350 }
351
352 if (!heap->collection_set()->is_empty()) {
353 heap->set_evacuation_in_progress(true);
354 heap->set_has_forwarded_objects(true);
355
356 if(ShenandoahVerify) {
357 heap->verifier()->verify_during_evacuation();
358 }
359 } else {
360 if (ShenandoahVerify) {
361 heap->verifier()->verify_after_concmark();
362 }
363
364 if (VerifyAfterGC) {
365 Universe::verify();
366 }
367 }
368 }
369
370 void ShenandoahDegenGC::op_cleanup_early() {
371 ShenandoahHeap::heap()->recycle_trash();
372 }
373
374 void ShenandoahDegenGC::op_global_coalesce_and_fill() {
375 ShenandoahHeap::heap()->coalesce_and_fill_old_regions();
376 }
377
378 void ShenandoahDegenGC::op_evacuate() {
379 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_evac);
380 ShenandoahHeap::heap()->evacuate_collection_set(false /* concurrent*/);
381 }
382
383 void ShenandoahDegenGC::op_init_updaterefs() {
384 // Evacuation has completed
385 ShenandoahHeap* const heap = ShenandoahHeap::heap();
386 heap->set_evacuation_in_progress(false);
387 heap->set_concurrent_weak_root_in_progress(false);
388 heap->set_concurrent_strong_root_in_progress(false);
389
390 heap->prepare_update_heap_references(false /*concurrent*/);
391 heap->set_update_refs_in_progress(true);
392 }
393
394 void ShenandoahDegenGC::op_updaterefs() {
395 ShenandoahHeap* const heap = ShenandoahHeap::heap();
396 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_updaterefs);
397 // Handed over from concurrent update references phase
398 heap->update_heap_references(false /*concurrent*/);
399
400 heap->set_update_refs_in_progress(false);
401 heap->set_has_forwarded_objects(false);
402 }
403
404 void ShenandoahDegenGC::op_update_roots() {
405 ShenandoahHeap* const heap = ShenandoahHeap::heap();
406
407 update_roots(false /*full_gc*/);
408
409 heap->update_heap_region_states(false /*concurrent*/);
410
411 if (ShenandoahVerify) {
412 heap->verifier()->verify_after_updaterefs();
413 }
414
415 if (VerifyAfterGC) {
416 Universe::verify();
417 }
418
419 heap->rebuild_free_set(false /*concurrent*/);
420 }
421
422 void ShenandoahDegenGC::op_cleanup_complete() {
423 ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_cleanup_complete);
424 ShenandoahHeap::heap()->recycle_trash();
425 }
426
427 void ShenandoahDegenGC::op_degenerated_fail() {
428 upgrade_to_full();
429 ShenandoahFullGC full_gc;
430 full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
431 }
432
433 void ShenandoahDegenGC::op_degenerated_futile() {
434 upgrade_to_full();
435 ShenandoahFullGC full_gc;
436 full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
437 }
438
439 void ShenandoahDegenGC::degen_event_message(ShenandoahDegenPoint point, char* buf, size_t len) const {
440 jio_snprintf(buf, len, "Pause Degenerated %s GC (%s)", _generation->name(), ShenandoahGC::degen_point_to_string(point));
441 }
442
443 void ShenandoahDegenGC::upgrade_to_full() {
444 log_info(gc)("Degenerate GC upgrading to Full GC");
445 ShenandoahHeap::heap()->shenandoah_policy()->record_degenerated_upgrade_to_full();
446 _upgraded_to_full = true;
447 }
448
449 bool ShenandoahDegenGC::upgraded_to_full() {
450 return _upgraded_to_full;
451 }