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