1 /*
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  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/shenandoah/heuristics/shenandoahOldHeuristics.hpp"
 28 #include "gc/shenandoah/shenandoahCollectionSet.hpp"
 29 #include "gc/shenandoah/shenandoahCollectorPolicy.hpp"
 30 #include "gc/shenandoah/shenandoahGenerationalHeap.hpp"
 31 #include "gc/shenandoah/shenandoahHeapRegion.inline.hpp"
 32 #include "gc/shenandoah/shenandoahOldGeneration.hpp"
 33 #include "logging/log.hpp"
 34 #include "utilities/quickSort.hpp"
 35 
 36 uint ShenandoahOldHeuristics::NOT_FOUND = -1U;
 37 
 38 // sort by increasing live (so least live comes first)
 39 int ShenandoahOldHeuristics::compare_by_live(RegionData a, RegionData b) {
 40   if (a._u._live_data < b._u._live_data)
 41     return -1;
 42   else if (a._u._live_data > b._u._live_data)
 43     return 1;
 44   else return 0;
 45 }
 46 
 47 // sort by increasing index
 48 int ShenandoahOldHeuristics::compare_by_index(RegionData a, RegionData b) {
 49   if (a._region->index() < b._region->index()) {
 50     return -1;
 51   } else if (a._region->index() > b._region->index()) {
 52     return 1;
 53   } else {
 54     // quicksort may compare to self during search for pivot
 55     return 0;
 56   }
 57 }
 58 
 59 ShenandoahOldHeuristics::ShenandoahOldHeuristics(ShenandoahOldGeneration* generation, ShenandoahGenerationalHeap* gen_heap) :
 60   ShenandoahHeuristics(generation),
 61   _heap(gen_heap),
 62   _old_gen(generation),
 63   _first_pinned_candidate(NOT_FOUND),
 64   _last_old_collection_candidate(0),
 65   _next_old_collection_candidate(0),
 66   _last_old_region(0),
 67   _live_bytes_in_unprocessed_candidates(0),
 68   _old_generation(generation),
 69   _cannot_expand_trigger(false),
 70   _fragmentation_trigger(false),
 71   _growth_trigger(false),
 72   _fragmentation_density(0.0),
 73   _fragmentation_first_old_region(0),
 74   _fragmentation_last_old_region(0)
 75 {
 76 }
 77 
 78 bool ShenandoahOldHeuristics::prime_collection_set(ShenandoahCollectionSet* collection_set) {
 79   if (unprocessed_old_collection_candidates() == 0) {
 80     return false;
 81   }
 82 
 83   _first_pinned_candidate = NOT_FOUND;
 84 
 85   uint included_old_regions = 0;
 86   size_t evacuated_old_bytes = 0;
 87   size_t collected_old_bytes = 0;
 88 
 89   // If a region is put into the collection set, then this region's free (not yet used) bytes are no longer
 90   // "available" to hold the results of other evacuations.  This may cause a decrease in the remaining amount
 91   // of memory that can still be evacuated.  We address this by reducing the evacuation budget by the amount
 92   // of live memory in that region and by the amount of unallocated memory in that region if the evacuation
 93   // budget is constrained by availability of free memory.
 94   const size_t old_evacuation_reserve = _old_generation->get_evacuation_reserve();
 95   const size_t old_evacuation_budget = (size_t) ((double) old_evacuation_reserve / ShenandoahOldEvacWaste);
 96   size_t unfragmented_available = _old_generation->free_unaffiliated_regions() * ShenandoahHeapRegion::region_size_bytes();
 97   size_t fragmented_available;
 98   size_t excess_fragmented_available;
 99 
100   if (unfragmented_available > old_evacuation_budget) {
101     unfragmented_available = old_evacuation_budget;
102     fragmented_available = 0;
103     excess_fragmented_available = 0;
104   } else {
105     assert(_old_generation->available() >= old_evacuation_budget, "Cannot budget more than is available");
106     fragmented_available = _old_generation->available() - unfragmented_available;
107     assert(fragmented_available + unfragmented_available >= old_evacuation_budget, "Budgets do not add up");
108     if (fragmented_available + unfragmented_available > old_evacuation_budget) {
109       excess_fragmented_available = (fragmented_available + unfragmented_available) - old_evacuation_budget;
110       fragmented_available -= excess_fragmented_available;
111     }
112   }
113 
114   size_t remaining_old_evacuation_budget = old_evacuation_budget;
115   log_info(gc)("Choose old regions for mixed collection: old evacuation budget: " SIZE_FORMAT "%s, candidates: %u",
116                byte_size_in_proper_unit(old_evacuation_budget), proper_unit_for_byte_size(old_evacuation_budget),
117                unprocessed_old_collection_candidates());
118 
119   size_t lost_evacuation_capacity = 0;
120 
121   // The number of old-gen regions that were selected as candidates for collection at the end of the most recent old-gen
122   // concurrent marking phase and have not yet been collected is represented by unprocessed_old_collection_candidates().
123   // Candidate regions are ordered according to increasing amount of live data.  If there is not sufficient room to
124   // evacuate region N, then there is no need to even consider evacuating region N+1.
125   while (unprocessed_old_collection_candidates() > 0) {
126     // Old collection candidates are sorted in order of decreasing garbage contained therein.
127     ShenandoahHeapRegion* r = next_old_collection_candidate();
128     if (r == nullptr) {
129       break;
130     }
131     assert(r->is_regular(), "There should be no humongous regions in the set of mixed-evac candidates");
132 
133     // If region r is evacuated to fragmented memory (to free memory within a partially used region), then we need
134     // to decrease the capacity of the fragmented memory by the scaled loss.
135 
136     size_t live_data_for_evacuation = r->get_live_data_bytes();
137     size_t lost_available = r->free();
138 
139     if ((lost_available > 0) && (excess_fragmented_available > 0)) {
140       if (lost_available < excess_fragmented_available) {
141         excess_fragmented_available -= lost_available;
142         lost_evacuation_capacity -= lost_available;
143         lost_available  = 0;
144       } else {
145         lost_available -= excess_fragmented_available;
146         lost_evacuation_capacity -= excess_fragmented_available;
147         excess_fragmented_available = 0;
148       }
149     }
150     size_t scaled_loss = (size_t) ((double) lost_available / ShenandoahOldEvacWaste);
151     if ((lost_available > 0) && (fragmented_available > 0)) {
152       if (scaled_loss + live_data_for_evacuation < fragmented_available) {
153         fragmented_available -= scaled_loss;
154         scaled_loss = 0;
155       } else {
156         // We will have to allocate this region's evacuation memory from unfragmented memory, so don't bother
157         // to decrement scaled_loss
158       }
159     }
160     if (scaled_loss > 0) {
161       // We were not able to account for the lost free memory within fragmented memory, so we need to take this
162       // allocation out of unfragmented memory.  Unfragmented memory does not need to account for loss of free.
163       if (live_data_for_evacuation > unfragmented_available) {
164         // There is not room to evacuate this region or any that come after it in within the candidates array.
165         break;
166       } else {
167         unfragmented_available -= live_data_for_evacuation;
168       }
169     } else {
170       // Since scaled_loss == 0, we have accounted for the loss of free memory, so we can allocate from either
171       // fragmented or unfragmented available memory.  Use up the fragmented memory budget first.
172       size_t evacuation_need = live_data_for_evacuation;
173 
174       if (evacuation_need > fragmented_available) {
175         evacuation_need -= fragmented_available;
176         fragmented_available = 0;
177       } else {
178         fragmented_available -= evacuation_need;
179         evacuation_need = 0;
180       }
181       if (evacuation_need > unfragmented_available) {
182         // There is not room to evacuate this region or any that come after it in within the candidates array.
183         break;
184       } else {
185         unfragmented_available -= evacuation_need;
186         // dead code: evacuation_need == 0;
187       }
188     }
189     collection_set->add_region(r);
190     included_old_regions++;
191     evacuated_old_bytes += live_data_for_evacuation;
192     collected_old_bytes += r->garbage();
193     consume_old_collection_candidate();
194   }
195 
196   if (_first_pinned_candidate != NOT_FOUND) {
197     // Need to deal with pinned regions
198     slide_pinned_regions_to_front();
199   }
200   decrease_unprocessed_old_collection_candidates_live_memory(evacuated_old_bytes);
201   if (included_old_regions > 0) {
202     log_info(gc)("Old-gen piggyback evac (" UINT32_FORMAT " regions, evacuating " SIZE_FORMAT "%s, reclaiming: " SIZE_FORMAT "%s)",
203                  included_old_regions,
204                  byte_size_in_proper_unit(evacuated_old_bytes), proper_unit_for_byte_size(evacuated_old_bytes),
205                  byte_size_in_proper_unit(collected_old_bytes), proper_unit_for_byte_size(collected_old_bytes));
206   }
207 
208   if (unprocessed_old_collection_candidates() == 0) {
209     // We have added the last of our collection candidates to a mixed collection.
210     // Any triggers that occurred during mixed evacuations may no longer be valid.  They can retrigger if appropriate.
211     clear_triggers();
212 
213     _old_generation->complete_mixed_evacuations();
214   } else if (included_old_regions == 0) {
215     // We have candidates, but none were included for evacuation - are they all pinned?
216     // or did we just not have enough room for any of them in this collection set?
217     // We don't want a region with a stuck pin to prevent subsequent old collections, so
218     // if they are all pinned we transition to a state that will allow us to make these uncollected
219     // (pinned) regions parsable.
220     if (all_candidates_are_pinned()) {
221       log_info(gc)("All candidate regions " UINT32_FORMAT " are pinned", unprocessed_old_collection_candidates());
222       _old_generation->abandon_mixed_evacuations();
223     } else {
224       log_info(gc)("No regions selected for mixed collection. "
225                    "Old evacuation budget: " PROPERFMT ", Remaining evacuation budget: " PROPERFMT
226                    ", Lost capacity: " PROPERFMT
227                    ", Next candidate: " UINT32_FORMAT ", Last candidate: " UINT32_FORMAT,
228                    PROPERFMTARGS(old_evacuation_reserve),
229                    PROPERFMTARGS(remaining_old_evacuation_budget),
230                    PROPERFMTARGS(lost_evacuation_capacity),
231                    _next_old_collection_candidate, _last_old_collection_candidate);
232     }
233   }
234 
235   return (included_old_regions > 0);
236 }
237 
238 bool ShenandoahOldHeuristics::all_candidates_are_pinned() {
239 #ifdef ASSERT
240   if (uint(os::random()) % 100 < ShenandoahCoalesceChance) {
241     return true;
242   }
243 #endif
244 
245   for (uint i = _next_old_collection_candidate; i < _last_old_collection_candidate; ++i) {
246     ShenandoahHeapRegion* region = _region_data[i]._region;
247     if (!region->is_pinned()) {
248       return false;
249     }
250   }
251   return true;
252 }
253 
254 void ShenandoahOldHeuristics::slide_pinned_regions_to_front() {
255   // Find the first unpinned region to the left of the next region that
256   // will be added to the collection set. These regions will have been
257   // added to the cset, so we can use them to hold pointers to regions
258   // that were pinned when the cset was chosen.
259   // [ r p r p p p r r ]
260   //     ^         ^ ^
261   //     |         | | pointer to next region to add to a mixed collection is here.
262   //     |         | first r to the left should be in the collection set now.
263   //     | first pinned region, we don't need to look past this
264   uint write_index = NOT_FOUND;
265   for (uint search = _next_old_collection_candidate - 1; search > _first_pinned_candidate; --search) {
266     ShenandoahHeapRegion* region = _region_data[search]._region;
267     if (!region->is_pinned()) {
268       write_index = search;
269       assert(region->is_cset(), "Expected unpinned region to be added to the collection set.");
270       break;
271     }
272   }
273 
274   // If we could not find an unpinned region, it means there are no slots available
275   // to move up the pinned regions. In this case, we just reset our next index in the
276   // hopes that some of these regions will become unpinned before the next mixed
277   // collection. We may want to bailout of here instead, as it should be quite
278   // rare to have so many pinned regions and may indicate something is wrong.
279   if (write_index == NOT_FOUND) {
280     assert(_first_pinned_candidate != NOT_FOUND, "Should only be here if there are pinned regions.");
281     _next_old_collection_candidate = _first_pinned_candidate;
282     return;
283   }
284 
285   // Find pinned regions to the left and move their pointer into a slot
286   // that was pointing at a region that has been added to the cset (or was pointing
287   // to a pinned region that we've already moved up). We are done when the leftmost
288   // pinned region has been slid up.
289   // [ r p r x p p p r ]
290   //         ^       ^
291   //         |       | next region for mixed collections
292   //         | Write pointer is here. We know this region is already in the cset
293   //         | so we can clobber it with the next pinned region we find.
294   for (int32_t search = (int32_t)write_index - 1; search >= (int32_t)_first_pinned_candidate; --search) {
295     RegionData& skipped = _region_data[search];
296     if (skipped._region->is_pinned()) {
297       RegionData& available_slot = _region_data[write_index];
298       available_slot._region = skipped._region;
299       available_slot._u._live_data = skipped._u._live_data;
300       --write_index;
301     }
302   }
303 
304   // Update to read from the leftmost pinned region. Plus one here because we decremented
305   // the write index to hold the next found pinned region. We are just moving it back now
306   // to point to the first pinned region.
307   _next_old_collection_candidate = write_index + 1;
308 }
309 
310 void ShenandoahOldHeuristics::prepare_for_old_collections() {
311   ShenandoahHeap* heap = ShenandoahHeap::heap();
312 
313   const size_t num_regions = heap->num_regions();
314   size_t cand_idx = 0;
315   size_t immediate_garbage = 0;
316   size_t immediate_regions = 0;
317   size_t live_data = 0;
318 
319   RegionData* candidates = _region_data;
320   for (size_t i = 0; i < num_regions; i++) {
321     ShenandoahHeapRegion* region = heap->get_region(i);
322     if (!region->is_old()) {
323       continue;
324     }
325 
326     size_t garbage = region->garbage();
327     size_t live_bytes = region->get_live_data_bytes();
328     live_data += live_bytes;
329 
330     if (region->is_regular() || region->is_regular_pinned()) {
331         // Only place regular or pinned regions with live data into the candidate set.
332         // Pinned regions cannot be evacuated, but we are not actually choosing candidates
333         // for the collection set here. That happens later during the next young GC cycle,
334         // by which time, the pinned region may no longer be pinned.
335       if (!region->has_live()) {
336         assert(!region->is_pinned(), "Pinned region should have live (pinned) objects.");
337         region->make_trash_immediate();
338         immediate_regions++;
339         immediate_garbage += garbage;
340       } else {
341         region->begin_preemptible_coalesce_and_fill();
342         candidates[cand_idx]._region = region;
343         candidates[cand_idx]._u._live_data = live_bytes;
344         cand_idx++;
345       }
346     } else if (region->is_humongous_start()) {
347       // This will handle humongous start regions whether they are also pinned, or not.
348       // If they are pinned, we expect them to hold live data, so they will not be
349       // turned into immediate garbage.
350       if (!region->has_live()) {
351         assert(!region->is_pinned(), "Pinned region should have live (pinned) objects.");
352         // The humongous object is dead, we can just return this region and the continuations
353         // immediately to the freeset - no evacuations are necessary here. The continuations
354         // will be made into trash by this method, so they'll be skipped by the 'is_regular'
355         // check above, but we still need to count the start region.
356         immediate_regions++;
357         immediate_garbage += garbage;
358         size_t region_count = heap->trash_humongous_region_at(region);
359         log_debug(gc)("Trashed " SIZE_FORMAT " regions for humongous object.", region_count);
360       }
361     } else if (region->is_trash()) {
362       // Count humongous objects made into trash here.
363       immediate_regions++;
364       immediate_garbage += garbage;
365     }
366   }
367 
368   _old_generation->set_live_bytes_after_last_mark(live_data);
369 
370   // TODO: Consider not running mixed collects if we recovered some threshold percentage of memory from immediate garbage.
371   // This would be similar to young and global collections shortcutting evacuation, though we'd probably want a separate
372   // threshold for the old generation.
373 
374   // Unlike young, we are more interested in efficiently packing OLD-gen than in reclaiming garbage first.  We sort by live-data.
375   // Some regular regions may have been promoted in place with no garbage but also with very little live data.  When we "compact"
376   // old-gen, we want to pack these underutilized regions together so we can have more unaffiliated (unfragmented) free regions
377   // in old-gen.
378 
379   QuickSort::sort<RegionData>(candidates, cand_idx, compare_by_live, false);
380 
381   // Any old-gen region that contains (ShenandoahOldGarbageThreshold (default value 25)% garbage or more is to be
382   // added to the list of candidates for subsequent mixed evacuations.
383   //
384   // TODO: allow ShenandoahOldGarbageThreshold to be determined adaptively, by heuristics.
385 
386   const size_t region_size_bytes = ShenandoahHeapRegion::region_size_bytes();
387 
388   // The convention is to collect regions that have more than this amount of garbage.
389   const size_t garbage_threshold = region_size_bytes * ShenandoahOldGarbageThreshold / 100;
390 
391   // Enlightened interpretation: collect regions that have less than this amount of live.
392   const size_t live_threshold = region_size_bytes - garbage_threshold;
393 
394   _last_old_region = (uint)cand_idx;
395   _last_old_collection_candidate = (uint)cand_idx;
396   _next_old_collection_candidate = 0;
397 
398   size_t unfragmented = 0;
399   size_t candidates_garbage = 0;
400 
401   for (size_t i = 0; i < cand_idx; i++) {
402     size_t live = candidates[i]._u._live_data;
403     if (live > live_threshold) {
404       // Candidates are sorted in increasing order of live data, so no regions after this will be below the threshold.
405       _last_old_collection_candidate = (uint)i;
406       break;
407     }
408     size_t region_garbage = candidates[i]._region->garbage();
409     size_t region_free = candidates[i]._region->free();
410     candidates_garbage += region_garbage;
411     unfragmented += region_free;
412   }
413 
414   // defrag_count represents regions that are placed into the old collection set in order to defragment the memory
415   // that we try to "reserve" for humongous allocations.
416   size_t defrag_count = 0;
417   size_t total_uncollected_old_regions = _last_old_region - _last_old_collection_candidate;
418 
419   if (cand_idx > _last_old_collection_candidate) {
420     // Above, we have added into the set of mixed-evacuation candidates all old-gen regions for which the live memory
421     // that they contain is below a particular old-garbage threshold.  Regions that were not selected for the collection
422     // set hold enough live memory that it is not considered efficient (by "garbage-first standards") to compact these
423     // at the current time.
424     //
425     // However, if any of these regions that were rejected from the collection set reside within areas of memory that
426     // might interfere with future humongous allocation requests, we will prioritize them for evacuation at this time.
427     // Humongous allocations target the bottom of the heap.  We want old-gen regions to congregate at the top of the
428     // heap.
429     //
430     // Sort the regions that were initially rejected from the collection set in order of index.  This allows us to
431     // focus our attention on the regions that have low index value (i.e. the old-gen regions at the bottom of the heap).
432     QuickSort::sort<RegionData>(candidates + _last_old_collection_candidate, cand_idx - _last_old_collection_candidate,
433                                 compare_by_index, false);
434 
435     const size_t first_unselected_old_region = candidates[_last_old_collection_candidate]._region->index();
436     const size_t last_unselected_old_region = candidates[cand_idx - 1]._region->index();
437     size_t span_of_uncollected_regions = 1 + last_unselected_old_region - first_unselected_old_region;
438 
439     // Add no more than 1/8 of the existing old-gen regions to the set of mixed evacuation candidates.
440     const int MAX_FRACTION_OF_HUMONGOUS_DEFRAG_REGIONS = 8;
441     const size_t bound_on_additional_regions = cand_idx / MAX_FRACTION_OF_HUMONGOUS_DEFRAG_REGIONS;
442 
443     // The heuristic old_is_fragmented trigger may be seeking to achieve up to 7/8 density.  Allow ourselves to overshoot
444     // that target (at 15/16) so we will not have to do another defragmenting old collection right away.
445     while ((defrag_count < bound_on_additional_regions) &&
446            (total_uncollected_old_regions < 15 * span_of_uncollected_regions / 16)) {
447       ShenandoahHeapRegion* r = candidates[_last_old_collection_candidate]._region;
448       assert(r->is_regular() || r->is_regular_pinned(), "Region " SIZE_FORMAT " has wrong state for collection: %s",
449              r->index(), ShenandoahHeapRegion::region_state_to_string(r->state()));
450       const size_t region_garbage = candidates[_last_old_collection_candidate]._region->garbage();
451       const size_t region_free = r->free();
452       candidates_garbage += region_garbage;
453       unfragmented += region_free;
454       defrag_count++;
455       _last_old_collection_candidate++;
456 
457       // We now have one fewer uncollected regions, and our uncollected span shrinks because we have removed its first region.
458       total_uncollected_old_regions--;
459       span_of_uncollected_regions = 1 + last_unselected_old_region - candidates[_last_old_collection_candidate]._region->index();
460     }
461   }
462 
463   // Note that we do not coalesce and fill occupied humongous regions
464   // HR: humongous regions, RR: regular regions, CF: coalesce and fill regions
465   const size_t collectable_garbage = immediate_garbage + candidates_garbage;
466   const size_t old_candidates = _last_old_collection_candidate;
467   const size_t mixed_evac_live = old_candidates * region_size_bytes - (candidates_garbage + unfragmented);
468   set_unprocessed_old_collection_candidates_live_memory(mixed_evac_live);
469 
470   log_info(gc)("Old-Gen Collectable Garbage: " PROPERFMT " consolidated with free: " PROPERFMT ", over " SIZE_FORMAT " regions",
471                PROPERFMTARGS(collectable_garbage), PROPERFMTARGS(unfragmented), old_candidates);
472   log_info(gc)("Old-Gen Immediate Garbage: " PROPERFMT " over " SIZE_FORMAT " regions",
473               PROPERFMTARGS(immediate_garbage), immediate_regions);
474   log_info(gc)("Old regions selected for defragmentation: " SIZE_FORMAT, defrag_count);
475   log_info(gc)("Old regions not selected: " SIZE_FORMAT, total_uncollected_old_regions);
476 
477   if (unprocessed_old_collection_candidates() > 0) {
478     _old_generation->transition_to(ShenandoahOldGeneration::EVACUATING);
479   } else if (has_coalesce_and_fill_candidates()) {
480     _old_generation->transition_to(ShenandoahOldGeneration::FILLING);
481   } else {
482     _old_generation->transition_to(ShenandoahOldGeneration::WAITING_FOR_BOOTSTRAP);
483   }
484 }
485 
486 size_t ShenandoahOldHeuristics::unprocessed_old_collection_candidates_live_memory() const {
487   return _live_bytes_in_unprocessed_candidates;
488 }
489 
490 void ShenandoahOldHeuristics::set_unprocessed_old_collection_candidates_live_memory(size_t initial_live) {
491   _live_bytes_in_unprocessed_candidates = initial_live;
492 }
493 
494 void ShenandoahOldHeuristics::decrease_unprocessed_old_collection_candidates_live_memory(size_t evacuated_live) {
495   assert(evacuated_live <= _live_bytes_in_unprocessed_candidates, "Cannot evacuate more than was present");
496   _live_bytes_in_unprocessed_candidates -= evacuated_live;
497 }
498 
499 // Used by unit test: test_shenandoahOldHeuristic.cpp
500 uint ShenandoahOldHeuristics::last_old_collection_candidate_index() const {
501   return _last_old_collection_candidate;
502 }
503 
504 uint ShenandoahOldHeuristics::unprocessed_old_collection_candidates() const {
505   return _last_old_collection_candidate - _next_old_collection_candidate;
506 }
507 
508 ShenandoahHeapRegion* ShenandoahOldHeuristics::next_old_collection_candidate() {
509   while (_next_old_collection_candidate < _last_old_collection_candidate) {
510     ShenandoahHeapRegion* next = _region_data[_next_old_collection_candidate]._region;
511     if (!next->is_pinned()) {
512       return next;
513     } else {
514       assert(next->is_pinned(), "sanity");
515       if (_first_pinned_candidate == NOT_FOUND) {
516         _first_pinned_candidate = _next_old_collection_candidate;
517       }
518     }
519 
520     _next_old_collection_candidate++;
521   }
522   return nullptr;
523 }
524 
525 void ShenandoahOldHeuristics::consume_old_collection_candidate() {
526   _next_old_collection_candidate++;
527 }
528 
529 unsigned int ShenandoahOldHeuristics::get_coalesce_and_fill_candidates(ShenandoahHeapRegion** buffer) {
530   uint end = _last_old_region;
531   uint index = _next_old_collection_candidate;
532   while (index < end) {
533     *buffer++ = _region_data[index++]._region;
534   }
535   return (_last_old_region - _next_old_collection_candidate);
536 }
537 
538 void ShenandoahOldHeuristics::abandon_collection_candidates() {
539   _last_old_collection_candidate = 0;
540   _next_old_collection_candidate = 0;
541   _last_old_region = 0;
542 }
543 
544 void ShenandoahOldHeuristics::record_cycle_end() {
545   this->ShenandoahHeuristics::record_cycle_end();
546   clear_triggers();
547 }
548 
549 void ShenandoahOldHeuristics::clear_triggers() {
550   // Clear any triggers that were set during mixed evacuations.  Conditions may be different now that this phase has finished.
551   _cannot_expand_trigger = false;
552   _fragmentation_trigger = false;
553   _growth_trigger = false;
554 }
555 
556 void ShenandoahOldHeuristics::trigger_collection_if_fragmented(size_t first_old_region, size_t last_old_region, size_t old_region_count, size_t num_regions) {
557   if (ShenandoahGenerationalHumongousReserve > 0) {
558     size_t old_region_span = (first_old_region <= last_old_region)? (last_old_region + 1 - first_old_region): 0;
559     size_t allowed_old_gen_span = num_regions - (ShenandoahGenerationalHumongousReserve * num_regions) / 100;
560 
561     // Tolerate lower density if total span is small.  Here's the implementation:
562     //   if old_gen spans more than 100% and density < 75%, trigger old-defrag
563     //   else if old_gen spans more than 87.5% and density < 62.5%, trigger old-defrag
564     //   else if old_gen spans more than 75% and density < 50%, trigger old-defrag
565     //   else if old_gen spans more than 62.5% and density < 37.5%, trigger old-defrag
566     //   else if old_gen spans more than 50% and density < 25%, trigger old-defrag
567     //
568     // A previous implementation was more aggressive in triggering, resulting in degraded throughput when
569     // humongous allocation was not required.
570 
571     size_t old_available = _old_gen->available();
572     size_t region_size_bytes = ShenandoahHeapRegion::region_size_bytes();
573     size_t old_unaffiliated_available = _old_gen->free_unaffiliated_regions() * region_size_bytes;
574     assert(old_available >= old_unaffiliated_available, "sanity");
575     size_t old_fragmented_available = old_available - old_unaffiliated_available;
576 
577     size_t old_bytes_consumed = old_region_count * region_size_bytes - old_fragmented_available;
578     size_t old_bytes_spanned = old_region_span * region_size_bytes;
579     double old_density = ((double) old_bytes_consumed) / old_bytes_spanned;
580 
581     uint eighths = 8;
582     for (uint i = 0; i < 5; i++) {
583       size_t span_threshold = eighths * allowed_old_gen_span / 8;
584       double density_threshold = (eighths - 2) / 8.0;
585       if ((old_region_span >= span_threshold) && (old_density < density_threshold)) {
586         trigger_old_is_fragmented(old_density, first_old_region, last_old_region);
587         return;
588       }
589       eighths--;
590     }
591   }
592 }
593 
594 void ShenandoahOldHeuristics::trigger_collection_if_overgrown() {
595   size_t old_used = _old_gen->used() + _old_gen->get_humongous_waste();
596   size_t trigger_threshold = _old_gen->usage_trigger_threshold();
597   // Detects unsigned arithmetic underflow
598   assert(old_used <= _heap->capacity(),
599          "Old used (" SIZE_FORMAT ", " SIZE_FORMAT") must not be more than heap capacity (" SIZE_FORMAT ")",
600          _old_gen->used(), _old_gen->get_humongous_waste(), _heap->capacity());
601   if (old_used > trigger_threshold) {
602     trigger_old_has_grown();
603   }
604 }
605 
606 void ShenandoahOldHeuristics::trigger_maybe(size_t first_old_region, size_t last_old_region,
607                                             size_t old_region_count, size_t num_regions) {
608   trigger_collection_if_fragmented(first_old_region, last_old_region, old_region_count, num_regions);
609   trigger_collection_if_overgrown();
610 }
611 
612 bool ShenandoahOldHeuristics::should_start_gc() {
613   // Cannot start a new old-gen GC until previous one has finished.
614   //
615   // Future refinement: under certain circumstances, we might be more sophisticated about this choice.
616   // For example, we could choose to abandon the previous old collection before it has completed evacuations.
617   ShenandoahHeap* heap = ShenandoahHeap::heap();
618   if (!_old_generation->can_start_gc() || heap->collection_set()->has_old_regions()) {
619     return false;
620   }
621 
622   if (_cannot_expand_trigger) {
623     const size_t old_gen_capacity = _old_generation->max_capacity();
624     const size_t heap_capacity = heap->capacity();
625     const double percent = percent_of(old_gen_capacity, heap_capacity);
626     log_info(gc)("Trigger (OLD): Expansion failure, current size: " SIZE_FORMAT "%s which is %.1f%% of total heap size",
627                  byte_size_in_proper_unit(old_gen_capacity), proper_unit_for_byte_size(old_gen_capacity), percent);
628     return true;
629   }
630 
631   if (_fragmentation_trigger) {
632     const size_t used = _old_generation->used();
633     const size_t used_regions_size = _old_generation->used_regions_size();
634 
635     // used_regions includes humongous regions
636     const size_t used_regions = _old_generation->used_regions();
637     assert(used_regions_size > used_regions, "Cannot have more used than used regions");
638 
639     size_t first_old_region, last_old_region;
640     double density;
641     get_fragmentation_trigger_reason_for_log_message(density, first_old_region, last_old_region);
642     const size_t span_of_old_regions = (last_old_region >= first_old_region)? last_old_region + 1 - first_old_region: 0;
643     const size_t fragmented_free = used_regions_size - used;
644 
645     log_info(gc)("Trigger (OLD): Old has become fragmented: "
646                  SIZE_FORMAT "%s available bytes spread between range spanned from "
647                  SIZE_FORMAT " to " SIZE_FORMAT " (" SIZE_FORMAT "), density: %.1f%%",
648                  byte_size_in_proper_unit(fragmented_free), proper_unit_for_byte_size(fragmented_free),
649                  first_old_region, last_old_region, span_of_old_regions, density * 100);
650     return true;
651   }
652 
653   if (_growth_trigger) {
654     // Growth may be falsely triggered during mixed evacuations, before the mixed-evacuation candidates have been
655     // evacuated.  Before acting on a false trigger, we check to confirm the trigger condition is still satisfied.
656     const size_t current_usage = _old_generation->used() + _old_generation->get_humongous_waste();
657     const size_t trigger_threshold = _old_generation->usage_trigger_threshold();
658     const size_t heap_size = heap->capacity();
659     const size_t ignore_threshold = (ShenandoahIgnoreOldGrowthBelowPercentage * heap_size) / 100;
660     size_t consecutive_young_cycles;
661     if ((current_usage < ignore_threshold) &&
662         ((consecutive_young_cycles = heap->shenandoah_policy()->consecutive_young_gc_count())
663          < ShenandoahDoNotIgnoreGrowthAfterYoungCycles)) {
664       log_debug(gc)("Ignoring Trigger (OLD): Old has overgrown: usage (" SIZE_FORMAT "%s) is below threshold ("
665                     SIZE_FORMAT "%s) after " SIZE_FORMAT " consecutive completed young GCs",
666                     byte_size_in_proper_unit(current_usage), proper_unit_for_byte_size(current_usage),
667                     byte_size_in_proper_unit(ignore_threshold), proper_unit_for_byte_size(ignore_threshold),
668                     consecutive_young_cycles);
669       _growth_trigger = false;
670     } else if (current_usage > trigger_threshold) {
671       const size_t live_at_previous_old = _old_generation->get_live_bytes_after_last_mark();
672       const double percent_growth = percent_of(current_usage - live_at_previous_old, live_at_previous_old);
673       log_info(gc)("Trigger (OLD): Old has overgrown, live at end of previous OLD marking: "
674                    SIZE_FORMAT "%s, current usage: " SIZE_FORMAT "%s, percent growth: %.1f%%",
675                    byte_size_in_proper_unit(live_at_previous_old), proper_unit_for_byte_size(live_at_previous_old),
676                    byte_size_in_proper_unit(current_usage), proper_unit_for_byte_size(current_usage), percent_growth);
677       return true;
678     } else {
679       // Mixed evacuations have decreased current_usage such that old-growth trigger is no longer relevant.
680       _growth_trigger = false;
681     }
682   }
683 
684   // Otherwise, defer to inherited heuristic for gc trigger.
685   return this->ShenandoahHeuristics::should_start_gc();
686 }
687 
688 void ShenandoahOldHeuristics::record_success_concurrent(bool abbreviated) {
689   // Forget any triggers that occurred while OLD GC was ongoing.  If we really need to start another, it will retrigger.
690   clear_triggers();
691   this->ShenandoahHeuristics::record_success_concurrent(abbreviated);
692 }
693 
694 void ShenandoahOldHeuristics::record_success_degenerated() {
695   // Forget any triggers that occurred while OLD GC was ongoing.  If we really need to start another, it will retrigger.
696   clear_triggers();
697   this->ShenandoahHeuristics::record_success_degenerated();
698 }
699 
700 void ShenandoahOldHeuristics::record_success_full() {
701   // Forget any triggers that occurred while OLD GC was ongoing.  If we really need to start another, it will retrigger.
702   clear_triggers();
703   this->ShenandoahHeuristics::record_success_full();
704 }
705 
706 const char* ShenandoahOldHeuristics::name() {
707   return "Old";
708 }
709 
710 bool ShenandoahOldHeuristics::is_diagnostic() {
711   return false;
712 }
713 
714 bool ShenandoahOldHeuristics::is_experimental() {
715   return true;
716 }
717 
718 void ShenandoahOldHeuristics::choose_collection_set_from_regiondata(ShenandoahCollectionSet* set,
719                                                                     ShenandoahHeuristics::RegionData* data,
720                                                                     size_t data_size, size_t free) {
721   ShouldNotReachHere();
722 }