1 /*
2 * Copyright Amazon.com Inc. or its affiliates. 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/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/shenandoahOldGeneration.hpp"
32 #include "gc/shenandoah/shenandoahHeap.inline.hpp"
33 #include "gc/shenandoah/shenandoahHeapRegion.inline.hpp"
34 #include "logging/log.hpp"
35 #include "utilities/quickSort.hpp"
36
37 uint ShenandoahOldHeuristics::NOT_FOUND = -1U;
38
39 // sort by increasing live (so least live comes first)
40 int ShenandoahOldHeuristics::compare_by_live(RegionData a, RegionData b) {
41 if (a.get_livedata() < b.get_livedata()) {
42 return -1;
43 } else if (a.get_livedata() > b.get_livedata()) {
44 return 1;
45 } else {
46 return 0;
47 }
48 }
49
50 // sort by increasing index
51 int ShenandoahOldHeuristics::compare_by_index(RegionData a, RegionData b) {
52 if (a.get_region()->index() < b.get_region()->index()) {
53 return -1;
54 } else if (a.get_region()->index() > b.get_region()->index()) {
55 return 1;
56 } else {
57 // quicksort may compare to self during search for pivot
58 return 0;
59 }
60 }
61
62 ShenandoahOldHeuristics::ShenandoahOldHeuristics(ShenandoahOldGeneration* generation, ShenandoahGenerationalHeap* gen_heap) :
63 ShenandoahHeuristics(generation),
64 _heap(gen_heap),
65 _old_gen(generation),
66 _first_pinned_candidate(NOT_FOUND),
67 _last_old_collection_candidate(0),
68 _next_old_collection_candidate(0),
69 _last_old_region(0),
70 _live_bytes_in_unprocessed_candidates(0),
71 _old_generation(generation),
72 _cannot_expand_trigger(false),
73 _fragmentation_trigger(false),
74 _growth_trigger(false),
75 _fragmentation_density(0.0),
76 _fragmentation_first_old_region(0),
77 _fragmentation_last_old_region(0)
78 {
79 }
80
81 bool ShenandoahOldHeuristics::prime_collection_set(ShenandoahCollectionSet* collection_set) {
82 if (unprocessed_old_collection_candidates() == 0) {
83 return false;
84 }
85
86 if (_old_generation->is_preparing_for_mark()) {
87 // We have unprocessed old collection candidates, but the heuristic has given up on evacuating them.
88 // This is most likely because they were _all_ pinned at the time of the last mixed evacuation (and
89 // this in turn is most likely because there are just one or two candidate regions remaining).
90 log_info(gc, ergo)("Remaining " UINT32_FORMAT " old regions are being coalesced and filled", unprocessed_old_collection_candidates());
91 return false;
92 }
93
94 _first_pinned_candidate = NOT_FOUND;
95
96 uint included_old_regions = 0;
97 size_t evacuated_old_bytes = 0;
98 size_t collected_old_bytes = 0;
99
100 // If a region is put into the collection set, then this region's free (not yet used) bytes are no longer
101 // "available" to hold the results of other evacuations. This may cause a decrease in the remaining amount
102 // of memory that can still be evacuated. We address this by reducing the evacuation budget by the amount
103 // of live memory in that region and by the amount of unallocated memory in that region if the evacuation
104 // budget is constrained by availability of free memory.
105 const size_t old_evacuation_reserve = _old_generation->get_evacuation_reserve();
106 const size_t old_evacuation_budget = (size_t) ((double) old_evacuation_reserve / ShenandoahOldEvacWaste);
107 size_t unfragmented_available = _old_generation->free_unaffiliated_regions() * ShenandoahHeapRegion::region_size_bytes();
108 size_t fragmented_available;
109 size_t excess_fragmented_available;
110
111 if (unfragmented_available > old_evacuation_budget) {
112 unfragmented_available = old_evacuation_budget;
113 fragmented_available = 0;
114 excess_fragmented_available = 0;
115 } else {
116 assert(_old_generation->available() >= old_evacuation_budget, "Cannot budget more than is available");
117 fragmented_available = _old_generation->available() - unfragmented_available;
118 assert(fragmented_available + unfragmented_available >= old_evacuation_budget, "Budgets do not add up");
119 if (fragmented_available + unfragmented_available > old_evacuation_budget) {
120 excess_fragmented_available = (fragmented_available + unfragmented_available) - old_evacuation_budget;
121 fragmented_available -= excess_fragmented_available;
122 }
123 }
124
125 size_t remaining_old_evacuation_budget = old_evacuation_budget;
126 log_debug(gc)("Choose old regions for mixed collection: old evacuation budget: " SIZE_FORMAT "%s, candidates: %u",
127 byte_size_in_proper_unit(old_evacuation_budget), proper_unit_for_byte_size(old_evacuation_budget),
128 unprocessed_old_collection_candidates());
129
130 size_t lost_evacuation_capacity = 0;
131
132 // The number of old-gen regions that were selected as candidates for collection at the end of the most recent old-gen
133 // concurrent marking phase and have not yet been collected is represented by unprocessed_old_collection_candidates().
134 // Candidate regions are ordered according to increasing amount of live data. If there is not sufficient room to
135 // evacuate region N, then there is no need to even consider evacuating region N+1.
136 while (unprocessed_old_collection_candidates() > 0) {
137 // Old collection candidates are sorted in order of decreasing garbage contained therein.
138 ShenandoahHeapRegion* r = next_old_collection_candidate();
139 if (r == nullptr) {
140 break;
141 }
142 assert(r->is_regular(), "There should be no humongous regions in the set of mixed-evac candidates");
143
144 // If region r is evacuated to fragmented memory (to free memory within a partially used region), then we need
145 // to decrease the capacity of the fragmented memory by the scaled loss.
146
147 size_t live_data_for_evacuation = r->get_live_data_bytes();
148 size_t lost_available = r->free();
149
150 if ((lost_available > 0) && (excess_fragmented_available > 0)) {
151 if (lost_available < excess_fragmented_available) {
152 excess_fragmented_available -= lost_available;
153 lost_evacuation_capacity -= lost_available;
154 lost_available = 0;
155 } else {
156 lost_available -= excess_fragmented_available;
157 lost_evacuation_capacity -= excess_fragmented_available;
158 excess_fragmented_available = 0;
159 }
160 }
161 size_t scaled_loss = (size_t) ((double) lost_available / ShenandoahOldEvacWaste);
162 if ((lost_available > 0) && (fragmented_available > 0)) {
163 if (scaled_loss + live_data_for_evacuation < fragmented_available) {
164 fragmented_available -= scaled_loss;
165 scaled_loss = 0;
166 } else {
167 // We will have to allocate this region's evacuation memory from unfragmented memory, so don't bother
168 // to decrement scaled_loss
169 }
170 }
171 if (scaled_loss > 0) {
172 // We were not able to account for the lost free memory within fragmented memory, so we need to take this
173 // allocation out of unfragmented memory. Unfragmented memory does not need to account for loss of free.
174 if (live_data_for_evacuation > unfragmented_available) {
175 // There is not room to evacuate this region or any that come after it in within the candidates array.
176 break;
177 } else {
178 unfragmented_available -= live_data_for_evacuation;
179 }
180 } else {
181 // Since scaled_loss == 0, we have accounted for the loss of free memory, so we can allocate from either
182 // fragmented or unfragmented available memory. Use up the fragmented memory budget first.
183 size_t evacuation_need = live_data_for_evacuation;
184
185 if (evacuation_need > fragmented_available) {
186 evacuation_need -= fragmented_available;
187 fragmented_available = 0;
188 } else {
189 fragmented_available -= evacuation_need;
190 evacuation_need = 0;
191 }
192 if (evacuation_need > unfragmented_available) {
193 // There is not room to evacuate this region or any that come after it in within the candidates array.
194 break;
195 } else {
196 unfragmented_available -= evacuation_need;
197 // dead code: evacuation_need == 0;
198 }
199 }
200 collection_set->add_region(r);
201 included_old_regions++;
202 evacuated_old_bytes += live_data_for_evacuation;
203 collected_old_bytes += r->garbage();
204 consume_old_collection_candidate();
205 }
206
207 if (_first_pinned_candidate != NOT_FOUND) {
208 // Need to deal with pinned regions
209 slide_pinned_regions_to_front();
210 }
211 decrease_unprocessed_old_collection_candidates_live_memory(evacuated_old_bytes);
212 if (included_old_regions > 0) {
213 log_info(gc, ergo)("Old-gen piggyback evac (" UINT32_FORMAT " regions, evacuating " PROPERFMT ", reclaiming: " PROPERFMT ")",
214 included_old_regions, PROPERFMTARGS(evacuated_old_bytes), PROPERFMTARGS(collected_old_bytes));
215 }
216
217 if (unprocessed_old_collection_candidates() == 0) {
218 // We have added the last of our collection candidates to a mixed collection.
219 // Any triggers that occurred during mixed evacuations may no longer be valid. They can retrigger if appropriate.
220 clear_triggers();
221
222 _old_generation->complete_mixed_evacuations();
223 } else if (included_old_regions == 0) {
224 // We have candidates, but none were included for evacuation - are they all pinned?
225 // or did we just not have enough room for any of them in this collection set?
226 // We don't want a region with a stuck pin to prevent subsequent old collections, so
227 // if they are all pinned we transition to a state that will allow us to make these uncollected
228 // (pinned) regions parsable.
229 if (all_candidates_are_pinned()) {
230 log_info(gc, ergo)("All candidate regions " UINT32_FORMAT " are pinned", unprocessed_old_collection_candidates());
231 _old_generation->abandon_mixed_evacuations();
232 } else {
233 log_info(gc, ergo)("No regions selected for mixed collection. "
234 "Old evacuation budget: " PROPERFMT ", Remaining evacuation budget: " PROPERFMT
235 ", Lost capacity: " PROPERFMT
236 ", Next candidate: " UINT32_FORMAT ", Last candidate: " UINT32_FORMAT,
237 PROPERFMTARGS(old_evacuation_reserve),
238 PROPERFMTARGS(remaining_old_evacuation_budget),
239 PROPERFMTARGS(lost_evacuation_capacity),
240 _next_old_collection_candidate, _last_old_collection_candidate);
241 }
242 }
243
244 return (included_old_regions > 0);
245 }
246
247 bool ShenandoahOldHeuristics::all_candidates_are_pinned() {
248 #ifdef ASSERT
249 if (uint(os::random()) % 100 < ShenandoahCoalesceChance) {
250 return true;
251 }
252 #endif
253
254 for (uint i = _next_old_collection_candidate; i < _last_old_collection_candidate; ++i) {
255 ShenandoahHeapRegion* region = _region_data[i].get_region();
256 if (!region->is_pinned()) {
257 return false;
258 }
259 }
260 return true;
261 }
262
263 void ShenandoahOldHeuristics::slide_pinned_regions_to_front() {
264 // Find the first unpinned region to the left of the next region that
265 // will be added to the collection set. These regions will have been
266 // added to the cset, so we can use them to hold pointers to regions
267 // that were pinned when the cset was chosen.
268 // [ r p r p p p r r ]
269 // ^ ^ ^
270 // | | | pointer to next region to add to a mixed collection is here.
271 // | | first r to the left should be in the collection set now.
272 // | first pinned region, we don't need to look past this
273 uint write_index = NOT_FOUND;
274 for (uint search = _next_old_collection_candidate - 1; search > _first_pinned_candidate; --search) {
275 ShenandoahHeapRegion* region = _region_data[search].get_region();
276 if (!region->is_pinned()) {
277 write_index = search;
278 assert(region->is_cset(), "Expected unpinned region to be added to the collection set.");
279 break;
280 }
281 }
282
283 // If we could not find an unpinned region, it means there are no slots available
284 // to move up the pinned regions. In this case, we just reset our next index in the
285 // hopes that some of these regions will become unpinned before the next mixed
286 // collection. We may want to bailout of here instead, as it should be quite
287 // rare to have so many pinned regions and may indicate something is wrong.
288 if (write_index == NOT_FOUND) {
289 assert(_first_pinned_candidate != NOT_FOUND, "Should only be here if there are pinned regions.");
290 _next_old_collection_candidate = _first_pinned_candidate;
291 return;
292 }
293
294 // Find pinned regions to the left and move their pointer into a slot
295 // that was pointing at a region that has been added to the cset (or was pointing
296 // to a pinned region that we've already moved up). We are done when the leftmost
297 // pinned region has been slid up.
298 // [ r p r x p p p r ]
299 // ^ ^
300 // | | next region for mixed collections
301 // | Write pointer is here. We know this region is already in the cset
302 // | so we can clobber it with the next pinned region we find.
303 for (int32_t search = (int32_t)write_index - 1; search >= (int32_t)_first_pinned_candidate; --search) {
304 RegionData& skipped = _region_data[search];
305 if (skipped.get_region()->is_pinned()) {
306 RegionData& available_slot = _region_data[write_index];
307 available_slot.set_region_and_livedata(skipped.get_region(), skipped.get_livedata());
308 --write_index;
309 }
310 }
311
312 // Update to read from the leftmost pinned region. Plus one here because we decremented
313 // the write index to hold the next found pinned region. We are just moving it back now
314 // to point to the first pinned region.
315 _next_old_collection_candidate = write_index + 1;
316 }
317
318 void ShenandoahOldHeuristics::prepare_for_old_collections() {
319 ShenandoahHeap* heap = ShenandoahHeap::heap();
320
321 const size_t num_regions = heap->num_regions();
322 size_t cand_idx = 0;
323 size_t immediate_garbage = 0;
324 size_t immediate_regions = 0;
325 size_t live_data = 0;
326
327 RegionData* candidates = _region_data;
328 for (size_t i = 0; i < num_regions; i++) {
329 ShenandoahHeapRegion* region = heap->get_region(i);
330 if (!region->is_old()) {
331 continue;
332 }
333
334 size_t garbage = region->garbage();
335 size_t live_bytes = region->get_live_data_bytes();
336 live_data += live_bytes;
337
338 if (region->is_regular() || region->is_regular_pinned()) {
339 // Only place regular or pinned regions with live data into the candidate set.
340 // Pinned regions cannot be evacuated, but we are not actually choosing candidates
341 // for the collection set here. That happens later during the next young GC cycle,
342 // by which time, the pinned region may no longer be pinned.
343 if (!region->has_live()) {
344 assert(!region->is_pinned(), "Pinned region should have live (pinned) objects.");
345 region->make_trash_immediate();
346 immediate_regions++;
347 immediate_garbage += garbage;
348 } else {
349 region->begin_preemptible_coalesce_and_fill();
350 candidates[cand_idx].set_region_and_livedata(region, live_bytes);
351 cand_idx++;
352 }
353 } else if (region->is_humongous_start()) {
354 // This will handle humongous start regions whether they are also pinned, or not.
355 // If they are pinned, we expect them to hold live data, so they will not be
356 // turned into immediate garbage.
357 if (!region->has_live()) {
358 assert(!region->is_pinned(), "Pinned region should have live (pinned) objects.");
359 // The humongous object is dead, we can just return this region and the continuations
360 // immediately to the freeset - no evacuations are necessary here. The continuations
361 // will be made into trash by this method, so they'll be skipped by the 'is_regular'
362 // check above, but we still need to count the start region.
363 immediate_regions++;
364 immediate_garbage += garbage;
365 size_t region_count = heap->trash_humongous_region_at(region);
366 log_debug(gc)("Trashed " SIZE_FORMAT " regions for humongous object.", region_count);
367 }
368 } else if (region->is_trash()) {
369 // Count humongous objects made into trash here.
370 immediate_regions++;
371 immediate_garbage += garbage;
372 }
373 }
374
375 _old_generation->set_live_bytes_after_last_mark(live_data);
376
377 // Unlike young, we are more interested in efficiently packing OLD-gen than in reclaiming garbage first. We sort by live-data.
378 // Some regular regions may have been promoted in place with no garbage but also with very little live data. When we "compact"
379 // old-gen, we want to pack these underutilized regions together so we can have more unaffiliated (unfragmented) free regions
380 // in old-gen.
381
382 QuickSort::sort<RegionData>(candidates, cand_idx, compare_by_live, false);
383
384 const size_t region_size_bytes = ShenandoahHeapRegion::region_size_bytes();
385
386 // The convention is to collect regions that have more than this amount of garbage.
387 const size_t garbage_threshold = region_size_bytes * ShenandoahOldGarbageThreshold / 100;
388
389 // Enlightened interpretation: collect regions that have less than this amount of live.
390 const size_t live_threshold = region_size_bytes - garbage_threshold;
391
392 _last_old_region = (uint)cand_idx;
393 _last_old_collection_candidate = (uint)cand_idx;
394 _next_old_collection_candidate = 0;
395
396 size_t unfragmented = 0;
397 size_t candidates_garbage = 0;
398
399 for (size_t i = 0; i < cand_idx; i++) {
400 size_t live = candidates[i].get_livedata();
401 if (live > live_threshold) {
402 // Candidates are sorted in increasing order of live data, so no regions after this will be below the threshold.
403 _last_old_collection_candidate = (uint)i;
404 break;
405 }
406 ShenandoahHeapRegion* r = candidates[i].get_region();
407 size_t region_garbage = r->garbage();
408 size_t region_free = r->free();
409 candidates_garbage += region_garbage;
410 unfragmented += region_free;
411 }
412
413 // defrag_count represents regions that are placed into the old collection set in order to defragment the memory
414 // that we try to "reserve" for humongous allocations.
415 size_t defrag_count = 0;
416 size_t total_uncollected_old_regions = _last_old_region - _last_old_collection_candidate;
417
418 if (cand_idx > _last_old_collection_candidate) {
419 // Above, we have added into the set of mixed-evacuation candidates all old-gen regions for which the live memory
420 // that they contain is below a particular old-garbage threshold. Regions that were not selected for the collection
421 // set hold enough live memory that it is not considered efficient (by "garbage-first standards") to compact these
422 // at the current time.
423 //
424 // However, if any of these regions that were rejected from the collection set reside within areas of memory that
425 // might interfere with future humongous allocation requests, we will prioritize them for evacuation at this time.
426 // Humongous allocations target the bottom of the heap. We want old-gen regions to congregate at the top of the
427 // heap.
428 //
429 // Sort the regions that were initially rejected from the collection set in order of index. This allows us to
430 // focus our attention on the regions that have low index value (i.e. the old-gen regions at the bottom of the heap).
431 QuickSort::sort<RegionData>(candidates + _last_old_collection_candidate, cand_idx - _last_old_collection_candidate,
432 compare_by_index, false);
433
434 const size_t first_unselected_old_region = candidates[_last_old_collection_candidate].get_region()->index();
435 const size_t last_unselected_old_region = candidates[cand_idx - 1].get_region()->index();
436 size_t span_of_uncollected_regions = 1 + last_unselected_old_region - first_unselected_old_region;
437
438 // Add no more than 1/8 of the existing old-gen regions to the set of mixed evacuation candidates.
439 const int MAX_FRACTION_OF_HUMONGOUS_DEFRAG_REGIONS = 8;
440 const size_t bound_on_additional_regions = cand_idx / MAX_FRACTION_OF_HUMONGOUS_DEFRAG_REGIONS;
441
442 // The heuristic old_is_fragmented trigger may be seeking to achieve up to 75% density. Allow ourselves to overshoot
443 // that target (at 7/8) so we will not have to do another defragmenting old collection right away.
444 while ((defrag_count < bound_on_additional_regions) &&
445 (total_uncollected_old_regions < 7 * span_of_uncollected_regions / 8)) {
446 ShenandoahHeapRegion* r = candidates[_last_old_collection_candidate].get_region();
447 assert(r->is_regular() || r->is_regular_pinned(), "Region " SIZE_FORMAT " has wrong state for collection: %s",
448 r->index(), ShenandoahHeapRegion::region_state_to_string(r->state()));
449 const size_t region_garbage = r->garbage();
450 const size_t region_free = r->free();
451 candidates_garbage += region_garbage;
452 unfragmented += region_free;
453 defrag_count++;
454 _last_old_collection_candidate++;
455
456 // We now have one fewer uncollected regions, and our uncollected span shrinks because we have removed its first region.
457 total_uncollected_old_regions--;
458 span_of_uncollected_regions =
459 1 + last_unselected_old_region - candidates[_last_old_collection_candidate].get_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, ergo)("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, ergo)("Old-Gen Immediate Garbage: " PROPERFMT " over " SIZE_FORMAT " regions",
473 PROPERFMTARGS(immediate_garbage), immediate_regions);
474 log_info(gc, ergo)("Old regions selected for defragmentation: " SIZE_FORMAT, defrag_count);
475 log_info(gc, ergo)("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].get_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++].get_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 // This triggers old-gen collection if the number of regions "dedicated" to old generation is much larger than
557 // is required to represent the memory currently used within the old generation. This trigger looks specifically
558 // at density of the old-gen spanned region. A different mechanism triggers old-gen GC if the total number of
559 // old-gen regions (regardless of how close the regions are to one another) grows beyond an anticipated growth target.
560 void ShenandoahOldHeuristics::set_trigger_if_old_is_fragmented(size_t first_old_region, size_t last_old_region,
561 size_t old_region_count, size_t num_regions) {
562 if (ShenandoahGenerationalHumongousReserve > 0) {
563 // Our intent is to pack old-gen memory into the highest-numbered regions of the heap. Count all memory
564 // above first_old_region as the "span" of old generation.
565 size_t old_region_span = (first_old_region <= last_old_region)? (num_regions - first_old_region): 0;
566 // Given that memory at the bottom of the heap is reserved to represent humongous objects, the number of
567 // regions that old_gen is "allowed" to consume is less than the total heap size. The restriction on allowed
568 // span is not strictly enforced. This is a heuristic designed to reduce the likelihood that a humongous
569 // allocation request will require a STW full GC.
570 size_t allowed_old_gen_span = num_regions - (ShenandoahGenerationalHumongousReserve * num_regions) / 100;
571
572 size_t old_available = _old_gen->available() / HeapWordSize;
573 size_t region_size_words = ShenandoahHeapRegion::region_size_words();
574 size_t old_unaffiliated_available = _old_gen->free_unaffiliated_regions() * region_size_words;
575 assert(old_available >= old_unaffiliated_available, "sanity");
576 size_t old_fragmented_available = old_available - old_unaffiliated_available;
577
578 size_t old_words_consumed = old_region_count * region_size_words - old_fragmented_available;
579 size_t old_words_spanned = old_region_span * region_size_words;
580 double old_density = ((double) old_words_consumed) / old_words_spanned;
581
582 double old_span_percent = ((double) old_region_span) / allowed_old_gen_span;
583 if (old_span_percent > 0.50) {
584 // Squaring old_span_percent in the denominator below allows more aggressive triggering when we are
585 // above desired maximum span and less aggressive triggering when we are far below the desired maximum span.
586 double old_span_percent_squared = old_span_percent * old_span_percent;
587 if (old_density / old_span_percent_squared < 0.75) {
588 // We trigger old defragmentation, for example, if:
589 // old_span_percent is 110% and old_density is below 90.8%, or
590 // old_span_percent is 100% and old_density is below 75.0%, or
591 // old_span_percent is 90% and old_density is below 60.8%, or
592 // old_span_percent is 80% and old_density is below 48.0%, or
593 // old_span_percent is 70% and old_density is below 36.8%, or
594 // old_span_percent is 60% and old_density is below 27.0%, or
595 // old_span_percent is 50% and old_density is below 18.8%.
596
597 // Set the fragmentation trigger and related attributes
598 _fragmentation_trigger = true;
599 _fragmentation_density = old_density;
600 _fragmentation_first_old_region = first_old_region;
601 _fragmentation_last_old_region = last_old_region;
602 }
603 }
604 }
605 }
606
607 void ShenandoahOldHeuristics::set_trigger_if_old_is_overgrown() {
608 size_t old_used = _old_gen->used() + _old_gen->get_humongous_waste();
609 size_t trigger_threshold = _old_gen->usage_trigger_threshold();
610 // Detects unsigned arithmetic underflow
611 assert(old_used <= _heap->capacity(),
612 "Old used (" SIZE_FORMAT ", " SIZE_FORMAT") must not be more than heap capacity (" SIZE_FORMAT ")",
613 _old_gen->used(), _old_gen->get_humongous_waste(), _heap->capacity());
614 if (old_used > trigger_threshold) {
615 _growth_trigger = true;
616 }
617 }
618
619 void ShenandoahOldHeuristics::evaluate_triggers(size_t first_old_region, size_t last_old_region,
620 size_t old_region_count, size_t num_regions) {
621 set_trigger_if_old_is_fragmented(first_old_region, last_old_region, old_region_count, num_regions);
622 set_trigger_if_old_is_overgrown();
623 }
624
625 bool ShenandoahOldHeuristics::should_start_gc() {
626 // Cannot start a new old-gen GC until previous one has finished.
627 //
628 // Future refinement: under certain circumstances, we might be more sophisticated about this choice.
629 // For example, we could choose to abandon the previous old collection before it has completed evacuations.
630 ShenandoahHeap* heap = ShenandoahHeap::heap();
631 if (!_old_generation->can_start_gc() || heap->collection_set()->has_old_regions()) {
632 return false;
633 }
634
635 if (_cannot_expand_trigger) {
636 const size_t old_gen_capacity = _old_generation->max_capacity();
637 const size_t heap_capacity = heap->capacity();
638 const double percent = percent_of(old_gen_capacity, heap_capacity);
639 log_trigger("Expansion failure, current size: " SIZE_FORMAT "%s which is %.1f%% of total heap size",
640 byte_size_in_proper_unit(old_gen_capacity), proper_unit_for_byte_size(old_gen_capacity), percent);
641 return true;
642 }
643
644 if (_fragmentation_trigger) {
645 const size_t used = _old_generation->used();
646 const size_t used_regions_size = _old_generation->used_regions_size();
647
648 // used_regions includes humongous regions
649 const size_t used_regions = _old_generation->used_regions();
650 assert(used_regions_size > used_regions, "Cannot have more used than used regions");
651
652 size_t first_old_region, last_old_region;
653 double density;
654 get_fragmentation_trigger_reason_for_log_message(density, first_old_region, last_old_region);
655 const size_t span_of_old_regions = (last_old_region >= first_old_region)? last_old_region + 1 - first_old_region: 0;
656 const size_t fragmented_free = used_regions_size - used;
657
658 log_trigger("Old has become fragmented: "
659 SIZE_FORMAT "%s available bytes spread between range spanned from "
660 SIZE_FORMAT " to " SIZE_FORMAT " (" SIZE_FORMAT "), density: %.1f%%",
661 byte_size_in_proper_unit(fragmented_free), proper_unit_for_byte_size(fragmented_free),
662 first_old_region, last_old_region, span_of_old_regions, density * 100);
663 return true;
664 }
665
666 if (_growth_trigger) {
667 // Growth may be falsely triggered during mixed evacuations, before the mixed-evacuation candidates have been
668 // evacuated. Before acting on a false trigger, we check to confirm the trigger condition is still satisfied.
669 const size_t current_usage = _old_generation->used() + _old_generation->get_humongous_waste();
670 const size_t trigger_threshold = _old_generation->usage_trigger_threshold();
671 const size_t heap_size = heap->capacity();
672 const size_t ignore_threshold = (ShenandoahIgnoreOldGrowthBelowPercentage * heap_size) / 100;
673 size_t consecutive_young_cycles;
674 if ((current_usage < ignore_threshold) &&
675 ((consecutive_young_cycles = heap->shenandoah_policy()->consecutive_young_gc_count())
676 < ShenandoahDoNotIgnoreGrowthAfterYoungCycles)) {
677 log_debug(gc)("Ignoring Trigger: Old has overgrown: usage (" SIZE_FORMAT "%s) is below threshold ("
678 SIZE_FORMAT "%s) after " SIZE_FORMAT " consecutive completed young GCs",
679 byte_size_in_proper_unit(current_usage), proper_unit_for_byte_size(current_usage),
680 byte_size_in_proper_unit(ignore_threshold), proper_unit_for_byte_size(ignore_threshold),
681 consecutive_young_cycles);
682 _growth_trigger = false;
683 } else if (current_usage > trigger_threshold) {
684 const size_t live_at_previous_old = _old_generation->get_live_bytes_after_last_mark();
685 const double percent_growth = percent_of(current_usage - live_at_previous_old, live_at_previous_old);
686 log_trigger("Old has overgrown, live at end of previous OLD marking: "
687 SIZE_FORMAT "%s, current usage: " SIZE_FORMAT "%s, percent growth: %.1f%%",
688 byte_size_in_proper_unit(live_at_previous_old), proper_unit_for_byte_size(live_at_previous_old),
689 byte_size_in_proper_unit(current_usage), proper_unit_for_byte_size(current_usage), percent_growth);
690 return true;
691 } else {
692 // Mixed evacuations have decreased current_usage such that old-growth trigger is no longer relevant.
693 _growth_trigger = false;
694 }
695 }
696
697 // Otherwise, defer to inherited heuristic for gc trigger.
698 return this->ShenandoahHeuristics::should_start_gc();
699 }
700
701 void ShenandoahOldHeuristics::record_success_concurrent() {
702 // Forget any triggers that occurred while OLD GC was ongoing. If we really need to start another, it will retrigger.
703 clear_triggers();
704 this->ShenandoahHeuristics::record_success_concurrent();
705 }
706
707 void ShenandoahOldHeuristics::record_success_degenerated() {
708 // Forget any triggers that occurred while OLD GC was ongoing. If we really need to start another, it will retrigger.
709 clear_triggers();
710 this->ShenandoahHeuristics::record_success_degenerated();
711 }
712
713 void ShenandoahOldHeuristics::record_success_full() {
714 // Forget any triggers that occurred while OLD GC was ongoing. If we really need to start another, it will retrigger.
715 clear_triggers();
716 this->ShenandoahHeuristics::record_success_full();
717 }
718
719 const char* ShenandoahOldHeuristics::name() {
720 return "Old";
721 }
722
723 bool ShenandoahOldHeuristics::is_diagnostic() {
724 return false;
725 }
726
727 bool ShenandoahOldHeuristics::is_experimental() {
728 return true;
729 }
730
731 void ShenandoahOldHeuristics::choose_collection_set_from_regiondata(ShenandoahCollectionSet* set,
732 ShenandoahHeuristics::RegionData* data,
733 size_t data_size, size_t free) {
734 ShouldNotReachHere();
735 }