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/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);
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);
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() {
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();
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 }