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src/hotspot/share/gc/shenandoah/shenandoahDegeneratedGC.cpp

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  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/shenandoahHeap.inline.hpp"
 33 #include "gc/shenandoah/shenandoahMetrics.hpp"
 34 #include "gc/shenandoah/shenandoahMonitoringSupport.hpp"

 35 #include "gc/shenandoah/shenandoahOopClosures.inline.hpp"
 36 #include "gc/shenandoah/shenandoahRootProcessor.inline.hpp"
 37 #include "gc/shenandoah/shenandoahSTWMark.hpp"
 38 #include "gc/shenandoah/shenandoahUtils.hpp"
 39 #include "gc/shenandoah/shenandoahVerifier.hpp"

 40 #include "gc/shenandoah/shenandoahWorkerPolicy.hpp"
 41 #include "gc/shenandoah/shenandoahVMOperations.hpp"
 42 #include "runtime/vmThread.hpp"
 43 #include "utilities/events.hpp"
 44 
 45 ShenandoahDegenGC::ShenandoahDegenGC(ShenandoahDegenPoint degen_point) :
 46   ShenandoahGC(),
 47   _degen_point(degen_point) {


 48 }
 49 
 50 bool ShenandoahDegenGC::collect(GCCause::Cause cause) {
 51   vmop_degenerated();







 52   return true;
 53 }
 54 
 55 void ShenandoahDegenGC::vmop_degenerated() {
 56   TraceCollectorStats tcs(ShenandoahHeap::heap()->monitoring_support()->full_stw_collection_counters());
 57   ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::degen_gc_gross);
 58   VM_ShenandoahDegeneratedGC degenerated_gc(this);
 59   VMThread::execute(&degenerated_gc);
 60 }
 61 
 62 void ShenandoahDegenGC::entry_degenerated() {
 63   const char* msg = degen_event_message(_degen_point);
 64   ShenandoahPausePhase gc_phase(msg, ShenandoahPhaseTimings::degen_gc, true /* log_heap_usage */);
 65   EventMark em("%s", msg);
 66   ShenandoahHeap* const heap = ShenandoahHeap::heap();
 67 
 68   ShenandoahWorkerScope scope(heap->workers(),
 69                               ShenandoahWorkerPolicy::calc_workers_for_stw_degenerated(),
 70                               "stw degenerated gc");
 71 
 72   heap->set_degenerated_gc_in_progress(true);
 73   op_degenerated();
 74   heap->set_degenerated_gc_in_progress(false);
 75 }
 76 
 77 void ShenandoahDegenGC::op_degenerated() {
 78   ShenandoahHeap* const heap = ShenandoahHeap::heap();
 79   // Degenerated GC is STW, but it can also fail. Current mechanics communicates
 80   // GC failure via cancelled_concgc() flag. So, if we detect the failure after
 81   // some phase, we have to upgrade the Degenerate GC to Full GC.
 82   heap->clear_cancelled_gc();




















 83 
 84   ShenandoahMetricsSnapshot metrics;
 85   metrics.snap_before();
 86 
 87   switch (_degen_point) {
 88     // The cases below form the Duff's-like device: it describes the actual GC cycle,
 89     // but enters it at different points, depending on which concurrent phase had
 90     // degenerated.
 91 
 92     case _degenerated_outside_cycle:
 93       // We have degenerated from outside the cycle, which means something is bad with
 94       // the heap, most probably heavy humongous fragmentation, or we are very low on free
 95       // space. It makes little sense to wait for Full GC to reclaim as much as it can, when
 96       // we can do the most aggressive degen cycle, which includes processing references and
 97       // class unloading, unless those features are explicitly disabled.
 98       //
 99 













100       // Degenerated from concurrent root mark, reset the flag for STW mark
101       if (heap->is_concurrent_mark_in_progress()) {
102         ShenandoahConcurrentMark::cancel();
103         heap->set_concurrent_mark_in_progress(false);



















104       }
105 
106       // Note that we can only do this for "outside-cycle" degens, otherwise we would risk
107       // changing the cycle parameters mid-cycle during concurrent -> degenerated handover.
108       heap->set_unload_classes(heap->heuristics()->can_unload_classes());





109 
110       op_reset();
111 
112       // STW mark
113       op_mark();
114 
115     case _degenerated_mark:
116       // No fallthrough. Continue mark, handed over from concurrent mark if
117       // concurrent mark has yet completed
118       if (_degen_point == ShenandoahDegenPoint::_degenerated_mark &&
119           heap->is_concurrent_mark_in_progress()) {
120         op_finish_mark();
121       }
122       assert(!heap->cancelled_gc(), "STW mark can not OOM");
123 
124       /* Degen select Collection Set. etc. */
125       op_prepare_evacuation();
126 
127       op_cleanup_early();
128 

152           }
153         }
154 
155         // Degeneration under oom-evac protocol might have left some objects in
156         // collection set un-evacuated. Restart evacuation from the beginning to
157         // capture all objects. For all the objects that are already evacuated,
158         // it would be a simple check, which is supposed to be fast. This is also
159         // safe to do even without degeneration, as CSet iterator is at beginning
160         // in preparation for evacuation anyway.
161         //
162         // Before doing that, we need to make sure we never had any cset-pinned
163         // regions. This may happen if allocation failure happened when evacuating
164         // the about-to-be-pinned object, oom-evac protocol left the object in
165         // the collection set, and then the pin reached the cset region. If we continue
166         // the cycle here, we would trash the cset and alive objects in it. To avoid
167         // it, we fail degeneration right away and slide into Full GC to recover.
168 
169         {
170           heap->sync_pinned_region_status();
171           heap->collection_set()->clear_current_index();
172 
173           ShenandoahHeapRegion* r;
174           while ((r = heap->collection_set()->next()) != nullptr) {
175             if (r->is_pinned()) {
176               heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
177               op_degenerated_fail();
178               return;
179             }
180           }
181 
182           heap->collection_set()->clear_current_index();
183         }
184         op_evacuate();
185         if (heap->cancelled_gc()) {
186           op_degenerated_fail();
187           return;
188         }
189       }
190 





191       // If heuristics thinks we should do the cycle, this flag would be set,
192       // and we need to do update-refs. Otherwise, it would be the shortcut cycle.
193       if (heap->has_forwarded_objects()) {
194         op_init_updaterefs();
195         assert(!heap->cancelled_gc(), "STW reference update can not OOM");


196       }
197 
198     case _degenerated_updaterefs:
199       if (heap->has_forwarded_objects()) {
200         op_updaterefs();
201         op_update_roots();
202         assert(!heap->cancelled_gc(), "STW reference update can not OOM");
203       }
204 
205       // Disarm nmethods that armed in concurrent cycle.
206       // In above case, update roots should disarm them
207       ShenandoahCodeRoots::disarm_nmethods();
208 







209       op_cleanup_complete();



































210       break;
211     default:
212       ShouldNotReachHere();
213   }
214 








215   if (ShenandoahVerify) {
216     heap->verifier()->verify_after_degenerated();
217   }
218 
219   if (VerifyAfterGC) {
220     Universe::verify();
221   }
222 
223   metrics.snap_after();
224 
225   // Check for futility and fail. There is no reason to do several back-to-back Degenerated cycles,
226   // because that probably means the heap is overloaded and/or fragmented.
227   if (!metrics.is_good_progress()) {
228     heap->notify_gc_no_progress();
229     heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
230     op_degenerated_futile();
231   } else {
232     heap->notify_gc_progress();


233   }
234 }
235 
236 void ShenandoahDegenGC::op_reset() {
237   ShenandoahHeap::heap()->prepare_gc();
238 }
239 
240 void ShenandoahDegenGC::op_mark() {
241   assert(!ShenandoahHeap::heap()->is_concurrent_mark_in_progress(), "Should be reset");
242   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_mark);
243   ShenandoahSTWMark mark(false /*full gc*/);
244   mark.clear();
245   mark.mark();
246 }
247 
248 void ShenandoahDegenGC::op_finish_mark() {
249   ShenandoahConcurrentMark mark;
250   mark.finish_mark();
251 }
252 
253 void ShenandoahDegenGC::op_prepare_evacuation() {
254   ShenandoahHeap* const heap = ShenandoahHeap::heap();
255   if (ShenandoahVerify) {
256     heap->verifier()->verify_roots_no_forwarded();
257   }
258 
259   // STW cleanup weak roots and unload classes
260   heap->parallel_cleaning(false /*full gc*/);

261   // Prepare regions and collection set
262   heap->prepare_regions_and_collection_set(false /*concurrent*/);
263 
264   // Retire the TLABs, which will force threads to reacquire their TLABs after the pause.
265   // This is needed for two reasons. Strong one: new allocations would be with new freeset,
266   // which would be outside the collection set, so no cset writes would happen there.
267   // Weaker one: new allocations would happen past update watermark, and so less work would
268   // be needed for reference updates (would update the large filler instead).
269   if (UseTLAB) {
270     ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_final_manage_labs);
271     heap->tlabs_retire(false);
272   }
273 
274   if (!heap->collection_set()->is_empty()) {









275     heap->set_evacuation_in_progress(true);
276     heap->set_has_forwarded_objects(true);
277 
278     if(ShenandoahVerify) {
279       heap->verifier()->verify_during_evacuation();
280     }


281   } else {
282     if (ShenandoahVerify) {
283       heap->verifier()->verify_after_concmark();
284     }
285 
286     if (VerifyAfterGC) {
287       Universe::verify();
288     }
289   }
290 }
291 
292 void ShenandoahDegenGC::op_cleanup_early() {
293   ShenandoahHeap::heap()->recycle_trash();
294 }
295 
296 void ShenandoahDegenGC::op_evacuate() {
297   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_evac);
298   ShenandoahHeap::heap()->evacuate_collection_set(false /* concurrent*/);
299 }
300 
301 void ShenandoahDegenGC::op_init_updaterefs() {
302   // Evacuation has completed
303   ShenandoahHeap* const heap = ShenandoahHeap::heap();
304   heap->set_evacuation_in_progress(false);
305   heap->set_concurrent_weak_root_in_progress(false);
306   heap->set_concurrent_strong_root_in_progress(false);
307 
308   heap->prepare_update_heap_references(false /*concurrent*/);
309   heap->set_update_refs_in_progress(true);
310 }
311 
312 void ShenandoahDegenGC::op_updaterefs() {
313   ShenandoahHeap* const heap = ShenandoahHeap::heap();
314   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_updaterefs);
315   // Handed over from concurrent update references phase
316   heap->update_heap_references(false /*concurrent*/);
317 
318   heap->set_update_refs_in_progress(false);
319   heap->set_has_forwarded_objects(false);
320 }
321 
322 void ShenandoahDegenGC::op_update_roots() {
323   ShenandoahHeap* const heap = ShenandoahHeap::heap();
324 
325   update_roots(false /*full_gc*/);
326 
327   heap->update_heap_region_states(false /*concurrent*/);
328 
329   if (ShenandoahVerify) {
330     heap->verifier()->verify_after_updaterefs();
331   }
332 
333   if (VerifyAfterGC) {
334     Universe::verify();
335   }
336 
337   heap->rebuild_free_set(false /*concurrent*/);
338 }
339 
340 void ShenandoahDegenGC::op_cleanup_complete() {
341   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_cleanup_complete);
342   ShenandoahHeap::heap()->recycle_trash();
343 }
344 
345 void ShenandoahDegenGC::op_degenerated_fail() {
346   log_info(gc)("Cannot finish degeneration, upgrading to Full GC");
347   ShenandoahHeap::heap()->shenandoah_policy()->record_degenerated_upgrade_to_full();
348 
349   ShenandoahFullGC full_gc;
350   full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
351 }
352 
353 void ShenandoahDegenGC::op_degenerated_futile() {
354   ShenandoahHeap::heap()->shenandoah_policy()->record_degenerated_upgrade_to_full();
355   ShenandoahFullGC full_gc;
356   full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
357 }
358 
359 const char* ShenandoahDegenGC::degen_event_message(ShenandoahDegenPoint point) const {
360   switch (point) {
361     case _degenerated_unset:
362       return "Pause Degenerated GC (<UNSET>)";
363     case _degenerated_outside_cycle:
364       return "Pause Degenerated GC (Outside of Cycle)";


365     case _degenerated_mark:
366       return "Pause Degenerated GC (Mark)";
367     case _degenerated_evac:
368       return "Pause Degenerated GC (Evacuation)";
369     case _degenerated_updaterefs:
370       return "Pause Degenerated GC (Update Refs)";
371     default:
372       ShouldNotReachHere();
373       return "ERROR";
374   }
375 }








  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/shenandoahHeap.inline.hpp"
 35 #include "gc/shenandoah/shenandoahMetrics.hpp"
 36 #include "gc/shenandoah/shenandoahMonitoringSupport.hpp"
 37 #include "gc/shenandoah/shenandoahOldGeneration.hpp"
 38 #include "gc/shenandoah/shenandoahOopClosures.inline.hpp"
 39 #include "gc/shenandoah/shenandoahRootProcessor.inline.hpp"
 40 #include "gc/shenandoah/shenandoahSTWMark.hpp"
 41 #include "gc/shenandoah/shenandoahUtils.hpp"
 42 #include "gc/shenandoah/shenandoahVerifier.hpp"
 43 #include "gc/shenandoah/shenandoahYoungGeneration.hpp"
 44 #include "gc/shenandoah/shenandoahWorkerPolicy.hpp"
 45 #include "gc/shenandoah/shenandoahVMOperations.hpp"
 46 #include "runtime/vmThread.hpp"
 47 #include "utilities/events.hpp"
 48 
 49 ShenandoahDegenGC::ShenandoahDegenGC(ShenandoahDegenPoint degen_point, ShenandoahGeneration* generation) :
 50   ShenandoahGC(),
 51   _degen_point(degen_point),
 52   _generation(generation),
 53   _abbreviated(false) {
 54 }
 55 
 56 bool ShenandoahDegenGC::collect(GCCause::Cause cause) {
 57   vmop_degenerated();
 58   ShenandoahHeap* heap = ShenandoahHeap::heap();
 59   if (heap->mode()->is_generational()) {
 60     bool is_bootstrap_gc = heap->old_generation()->state() == ShenandoahOldGeneration::BOOTSTRAPPING;
 61     heap->mmu_tracker()->record_degenerated(GCId::current(), is_bootstrap_gc);
 62     const char* msg = is_bootstrap_gc? "At end of Degenerated Bootstrap Old GC": "At end of Degenerated Young GC";
 63     heap->log_heap_status(msg);
 64   }
 65   return true;
 66 }
 67 
 68 void ShenandoahDegenGC::vmop_degenerated() {
 69   TraceCollectorStats tcs(ShenandoahHeap::heap()->monitoring_support()->full_stw_collection_counters());
 70   ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::degen_gc_gross);
 71   VM_ShenandoahDegeneratedGC degenerated_gc(this);
 72   VMThread::execute(&degenerated_gc);
 73 }
 74 
 75 void ShenandoahDegenGC::entry_degenerated() {
 76   const char* msg = degen_event_message(_degen_point);
 77   ShenandoahPausePhase gc_phase(msg, ShenandoahPhaseTimings::degen_gc, true /* log_heap_usage */);
 78   EventMark em("%s", msg);
 79   ShenandoahHeap* const heap = ShenandoahHeap::heap();

 80   ShenandoahWorkerScope scope(heap->workers(),
 81                               ShenandoahWorkerPolicy::calc_workers_for_stw_degenerated(),
 82                               "stw degenerated gc");
 83 
 84   heap->set_degenerated_gc_in_progress(true);
 85   op_degenerated();
 86   heap->set_degenerated_gc_in_progress(false);
 87 }
 88 
 89 void ShenandoahDegenGC::op_degenerated() {
 90   ShenandoahHeap* const heap = ShenandoahHeap::heap();
 91   // Degenerated GC is STW, but it can also fail. Current mechanics communicates
 92   // GC failure via cancelled_concgc() flag. So, if we detect the failure after
 93   // some phase, we have to upgrade the Degenerate GC to Full GC.
 94   heap->clear_cancelled_gc(true /* clear oom handler */);
 95 
 96 #ifdef ASSERT
 97   if (heap->mode()->is_generational()) {
 98     ShenandoahOldGeneration* old_generation = heap->old_generation();
 99     if (!heap->is_concurrent_old_mark_in_progress()) {
100       // If we are not marking the old generation, there should be nothing in the old mark queues
101       assert(old_generation->task_queues()->is_empty(), "Old gen task queues should be empty");
102     }
103 
104     if (_generation->is_global()) {
105       // If we are in a global cycle, the old generation should not be marking. It is, however,
106       // allowed to be holding regions for evacuation or coalescing.
107       ShenandoahOldGeneration::State state = old_generation->state();
108       assert(state == ShenandoahOldGeneration::WAITING_FOR_BOOTSTRAP
109              || state == ShenandoahOldGeneration::EVACUATING
110              || state == ShenandoahOldGeneration::FILLING,
111              "Old generation cannot be in state: %s", old_generation->state_name());
112     }
113   }
114 #endif
115 
116   ShenandoahMetricsSnapshot metrics;
117   metrics.snap_before();
118 
119   switch (_degen_point) {
120     // The cases below form the Duff's-like device: it describes the actual GC cycle,
121     // but enters it at different points, depending on which concurrent phase had
122     // degenerated.
123 
124     case _degenerated_outside_cycle:
125       // We have degenerated from outside the cycle, which means something is bad with
126       // the heap, most probably heavy humongous fragmentation, or we are very low on free
127       // space. It makes little sense to wait for Full GC to reclaim as much as it can, when
128       // we can do the most aggressive degen cycle, which includes processing references and
129       // class unloading, unless those features are explicitly disabled.

130 
131       // Note that we can only do this for "outside-cycle" degens, otherwise we would risk
132       // changing the cycle parameters mid-cycle during concurrent -> degenerated handover.
133       heap->set_unload_classes(_generation->heuristics()->can_unload_classes() &&
134                                 (!heap->mode()->is_generational() || _generation->is_global()));
135 
136       if (heap->mode()->is_generational() &&
137             (_generation->is_young() || (_generation->is_global() && ShenandoahVerify))) {
138         // Swap remembered sets for young, or if the verifier will run during a global collect
139         // TODO: This path should not depend on ShenandoahVerify
140         _generation->swap_remembered_set();
141       }
142 
143     case _degenerated_roots:
144       // Degenerated from concurrent root mark, reset the flag for STW mark
145       if (!heap->mode()->is_generational()) {
146         if (heap->is_concurrent_mark_in_progress()) {
147           heap->cancel_concurrent_mark();
148         }
149       } else {
150         if (_generation->is_concurrent_mark_in_progress()) {
151           // We want to allow old generation marking to be punctuated by young collections
152           // (even if they have degenerated). If this is a global cycle, we'd have cancelled
153           // the entire old gc before coming into this switch. Note that cancel_marking on
154           // the generation does NOT abandon incomplete SATB buffers as cancel_concurrent_mark does.
155           // We need to separate out the old pointers which is done below.
156           _generation->cancel_marking();
157         }
158 
159         if (heap->is_concurrent_mark_in_progress()) {
160           // If either old or young marking is in progress, the SATB barrier will be enabled.
161           // The SATB buffer may hold a mix of old and young pointers. The old pointers need to be
162           // transferred to the old generation mark queues and the young pointers are NOT part
163           // of this snapshot, so they must be dropped here. It is safe to drop them here because
164           // we will rescan the roots on this safepoint.
165           heap->transfer_old_pointers_from_satb();
166         }
167       }
168 
169       if (_degen_point == ShenandoahDegenPoint::_degenerated_roots) {
170         // We only need this if the concurrent cycle has already swapped the card tables.
171         // Marking will use the 'read' table, but interesting pointers may have been
172         // recorded in the 'write' table in the time between the cancelled concurrent cycle
173         // and this degenerated cycle. These pointers need to be included the 'read' table
174         // used to scan the remembered set during the STW mark which follows here.
175         _generation->merge_write_table();
176       }
177 
178       op_reset();
179 
180       // STW mark
181       op_mark();
182 
183     case _degenerated_mark:
184       // No fallthrough. Continue mark, handed over from concurrent mark if
185       // concurrent mark has yet completed
186       if (_degen_point == ShenandoahDegenPoint::_degenerated_mark &&
187           heap->is_concurrent_mark_in_progress()) {
188         op_finish_mark();
189       }
190       assert(!heap->cancelled_gc(), "STW mark can not OOM");
191 
192       /* Degen select Collection Set. etc. */
193       op_prepare_evacuation();
194 
195       op_cleanup_early();
196 

220           }
221         }
222 
223         // Degeneration under oom-evac protocol might have left some objects in
224         // collection set un-evacuated. Restart evacuation from the beginning to
225         // capture all objects. For all the objects that are already evacuated,
226         // it would be a simple check, which is supposed to be fast. This is also
227         // safe to do even without degeneration, as CSet iterator is at beginning
228         // in preparation for evacuation anyway.
229         //
230         // Before doing that, we need to make sure we never had any cset-pinned
231         // regions. This may happen if allocation failure happened when evacuating
232         // the about-to-be-pinned object, oom-evac protocol left the object in
233         // the collection set, and then the pin reached the cset region. If we continue
234         // the cycle here, we would trash the cset and alive objects in it. To avoid
235         // it, we fail degeneration right away and slide into Full GC to recover.
236 
237         {
238           heap->sync_pinned_region_status();
239           heap->collection_set()->clear_current_index();

240           ShenandoahHeapRegion* r;
241           while ((r = heap->collection_set()->next()) != nullptr) {
242             if (r->is_pinned()) {
243               heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
244               op_degenerated_fail();
245               return;
246             }
247           }
248 
249           heap->collection_set()->clear_current_index();
250         }
251         op_evacuate();
252         if (heap->cancelled_gc()) {
253           op_degenerated_fail();
254           return;
255         }
256       }
257 
258       // Update collector state regardless of whether or not there are forwarded objects
259       heap->set_evacuation_in_progress(false);
260       heap->set_concurrent_weak_root_in_progress(false);
261       heap->set_concurrent_strong_root_in_progress(false);
262 
263       // If heuristics thinks we should do the cycle, this flag would be set,
264       // and we need to do update-refs. Otherwise, it would be the shortcut cycle.
265       if (heap->has_forwarded_objects()) {
266         op_init_updaterefs();
267         assert(!heap->cancelled_gc(), "STW reference update can not OOM");
268       } else {
269         _abbreviated = true;
270       }
271 
272     case _degenerated_updaterefs:
273       if (heap->has_forwarded_objects()) {
274         op_updaterefs();
275         op_update_roots();
276         assert(!heap->cancelled_gc(), "STW reference update can not OOM");
277       }
278 
279       // Disarm nmethods that armed in concurrent cycle.
280       // In above case, update roots should disarm them
281       ShenandoahCodeRoots::disarm_nmethods();
282 
283       if (heap->mode()->is_generational() && heap->is_concurrent_old_mark_in_progress()) {
284         // This is still necessary for degenerated cycles because the degeneration point may occur
285         // after final mark of the young generation. See ShenandoahConcurrentGC::op_final_updaterefs for
286         // a more detailed explanation.
287         heap->transfer_old_pointers_from_satb();
288       }
289 
290       op_cleanup_complete();
291       // We defer generation resizing actions until after cset regions have been recycled.
292       if (heap->mode()->is_generational()) {
293         size_t old_region_surplus = heap->get_old_region_surplus();
294         size_t old_region_deficit = heap->get_old_region_deficit();
295         bool success;
296         size_t region_xfer;
297         const char* region_destination;
298         if (old_region_surplus) {
299           region_xfer = old_region_surplus;
300           region_destination = "young";
301           success = heap->generation_sizer()->transfer_to_young(old_region_surplus);
302         } else if (old_region_deficit) {
303           region_xfer = old_region_surplus;
304           region_destination = "old";
305           success = heap->generation_sizer()->transfer_to_old(old_region_deficit);
306           if (!success) {
307             heap->old_heuristics()->trigger_cannot_expand();
308           }
309         } else {
310           region_destination = "none";
311           region_xfer = 0;
312           success = true;
313         }
314 
315         size_t young_available = heap->young_generation()->available();
316         size_t old_available = heap->old_generation()->available();
317         log_info(gc, ergo)("After cleanup, %s " SIZE_FORMAT " regions to %s to prepare for next gc, old available: "
318                            SIZE_FORMAT "%s, young_available: " SIZE_FORMAT "%s",
319                            success? "successfully transferred": "failed to transfer", region_xfer, region_destination,
320                            byte_size_in_proper_unit(old_available), proper_unit_for_byte_size(old_available),
321                            byte_size_in_proper_unit(young_available), proper_unit_for_byte_size(young_available));
322 
323         heap->set_old_region_surplus(0);
324         heap->set_old_region_deficit(0);
325       }
326       break;
327     default:
328       ShouldNotReachHere();
329   }
330 
331   if (heap->mode()->is_generational()) {
332     // In case degeneration interrupted concurrent evacuation or update references, we need to clean up transient state.
333     // Otherwise, these actions have no effect.
334     heap->set_young_evac_reserve(0);
335     heap->set_old_evac_reserve(0);
336     heap->set_promoted_reserve(0);
337   }
338 
339   if (ShenandoahVerify) {
340     heap->verifier()->verify_after_degenerated();
341   }
342 
343   if (VerifyAfterGC) {
344     Universe::verify();
345   }
346 
347   metrics.snap_after();
348 
349   // Check for futility and fail. There is no reason to do several back-to-back Degenerated cycles,
350   // because that probably means the heap is overloaded and/or fragmented.
351   if (!metrics.is_good_progress()) {
352     heap->notify_gc_no_progress();
353     heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
354     op_degenerated_futile();
355   } else {
356     heap->notify_gc_progress();
357     heap->shenandoah_policy()->record_success_degenerated(_generation->is_young(), _abbreviated);
358     _generation->heuristics()->record_success_degenerated();
359   }
360 }
361 
362 void ShenandoahDegenGC::op_reset() {
363   _generation->prepare_gc();
364 }
365 
366 void ShenandoahDegenGC::op_mark() {
367   assert(!_generation->is_concurrent_mark_in_progress(), "Should be reset");
368   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_mark);
369   ShenandoahSTWMark mark(_generation, false /*full gc*/);

370   mark.mark();
371 }
372 
373 void ShenandoahDegenGC::op_finish_mark() {
374   ShenandoahConcurrentMark mark(_generation);
375   mark.finish_mark();
376 }
377 
378 void ShenandoahDegenGC::op_prepare_evacuation() {
379   ShenandoahHeap* const heap = ShenandoahHeap::heap();
380   if (ShenandoahVerify) {
381     heap->verifier()->verify_roots_no_forwarded();
382   }
383 
384   // STW cleanup weak roots and unload classes
385   heap->parallel_cleaning(false /*full gc*/);
386 
387   // Prepare regions and collection set
388   _generation->prepare_regions_and_collection_set(false /*concurrent*/);
389 
390   // Retire the TLABs, which will force threads to reacquire their TLABs after the pause.
391   // This is needed for two reasons. Strong one: new allocations would be with new freeset,
392   // which would be outside the collection set, so no cset writes would happen there.
393   // Weaker one: new allocations would happen past update watermark, and so less work would
394   // be needed for reference updates (would update the large filler instead).
395   if (UseTLAB) {
396     ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_final_manage_labs);
397     heap->tlabs_retire(false);
398   }
399 
400   size_t humongous_regions_promoted = heap->get_promotable_humongous_regions();
401   size_t regular_regions_promoted_in_place = heap->get_regular_regions_promoted_in_place();
402   if (!heap->collection_set()->is_empty() || (humongous_regions_promoted + regular_regions_promoted_in_place > 0)) {
403     // Even if the collection set is empty, we need to do evacuation if there are regions to be promoted in place.
404     // Degenerated evacuation takes responsibility for registering objects and setting the remembered set cards to dirty.
405 
406     if (ShenandoahVerify) {
407       heap->verifier()->verify_before_evacuation();
408     }
409 
410     heap->set_evacuation_in_progress(true);

411 
412     if(ShenandoahVerify) {
413       heap->verifier()->verify_during_evacuation();
414     }
415 
416     heap->set_has_forwarded_objects(!heap->collection_set()->is_empty());
417   } else {
418     if (ShenandoahVerify) {
419       heap->verifier()->verify_after_concmark();
420     }
421 
422     if (VerifyAfterGC) {
423       Universe::verify();
424     }
425   }
426 }
427 
428 void ShenandoahDegenGC::op_cleanup_early() {
429   ShenandoahHeap::heap()->recycle_trash();
430 }
431 
432 void ShenandoahDegenGC::op_evacuate() {
433   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_evac);
434   ShenandoahHeap::heap()->evacuate_collection_set(false /* concurrent*/);
435 }
436 
437 void ShenandoahDegenGC::op_init_updaterefs() {
438   // Evacuation has completed
439   ShenandoahHeap* const heap = ShenandoahHeap::heap();




440   heap->prepare_update_heap_references(false /*concurrent*/);
441   heap->set_update_refs_in_progress(true);
442 }
443 
444 void ShenandoahDegenGC::op_updaterefs() {
445   ShenandoahHeap* const heap = ShenandoahHeap::heap();
446   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_updaterefs);
447   // Handed over from concurrent update references phase
448   heap->update_heap_references(false /*concurrent*/);
449 
450   heap->set_update_refs_in_progress(false);
451   heap->set_has_forwarded_objects(false);
452 }
453 
454 void ShenandoahDegenGC::op_update_roots() {
455   ShenandoahHeap* const heap = ShenandoahHeap::heap();
456 
457   update_roots(false /*full_gc*/);
458 
459   heap->update_heap_region_states(false /*concurrent*/);
460 
461   if (ShenandoahVerify) {
462     heap->verifier()->verify_after_updaterefs();
463   }
464 
465   if (VerifyAfterGC) {
466     Universe::verify();
467   }
468 
469   heap->rebuild_free_set(false /*concurrent*/);
470 }
471 
472 void ShenandoahDegenGC::op_cleanup_complete() {
473   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_cleanup_complete);
474   ShenandoahHeap::heap()->recycle_trash();
475 }
476 
477 void ShenandoahDegenGC::op_degenerated_fail() {
478   upgrade_to_full();




479 }
480 
481 void ShenandoahDegenGC::op_degenerated_futile() {
482   upgrade_to_full();


483 }
484 
485 const char* ShenandoahDegenGC::degen_event_message(ShenandoahDegenPoint point) const {
486   switch (point) {
487     case _degenerated_unset:
488       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (<UNSET>)");
489     case _degenerated_outside_cycle:
490       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Outside of Cycle)");
491     case _degenerated_roots:
492       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Roots)");
493     case _degenerated_mark:
494       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Mark)");
495     case _degenerated_evac:
496       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Evacuation)");
497     case _degenerated_updaterefs:
498       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Update Refs)");
499     default:
500       ShouldNotReachHere();
501       SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (?)");
502   }
503 }
504 
505 void ShenandoahDegenGC::upgrade_to_full() {
506   log_info(gc)("Degenerate GC upgrading to Full GC");
507   ShenandoahHeap::heap()->shenandoah_policy()->record_degenerated_upgrade_to_full();
508   ShenandoahFullGC full_gc;
509   full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
510 }
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