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.get_livedata() < b.get_livedata()) {
41 return -1;
42 } else if (a.get_livedata() > b.get_livedata()) {
43 return 1;
44 } else {
45 return 0;
46 }
47 }
48
49 // sort by increasing index
50 int ShenandoahOldHeuristics::compare_by_index(RegionData a, RegionData b) {
51 if (a.get_region()->index() < b.get_region()->index()) {
52 return -1;
53 } else if (a.get_region()->index() > b.get_region()->index()) {
54 return 1;
55 } else {
56 // quicksort may compare to self during search for pivot
57 return 0;
58 }
59 }
60
61 ShenandoahOldHeuristics::ShenandoahOldHeuristics(ShenandoahOldGeneration* generation, ShenandoahGenerationalHeap* gen_heap) :
62 ShenandoahHeuristics(generation),
63 _heap(gen_heap),
64 _old_gen(generation),
65 _first_pinned_candidate(NOT_FOUND),
66 _last_old_collection_candidate(0),
67 _next_old_collection_candidate(0),
68 _last_old_region(0),
69 _live_bytes_in_unprocessed_candidates(0),
70 _old_generation(generation),
71 _cannot_expand_trigger(false),
72 _fragmentation_trigger(false),
73 _growth_trigger(false),
74 _fragmentation_density(0.0),
75 _fragmentation_first_old_region(0),
76 _fragmentation_last_old_region(0)
77 {
78 }
79
80 bool ShenandoahOldHeuristics::prime_collection_set(ShenandoahCollectionSet* collection_set) {
81 if (unprocessed_old_collection_candidates() == 0) {
82 return false;
83 }
84
85 _first_pinned_candidate = NOT_FOUND;
86
87 uint included_old_regions = 0;
88 size_t evacuated_old_bytes = 0;
89 size_t collected_old_bytes = 0;
90
91 // If a region is put into the collection set, then this region's free (not yet used) bytes are no longer
92 // "available" to hold the results of other evacuations. This may cause a decrease in the remaining amount
93 // of memory that can still be evacuated. We address this by reducing the evacuation budget by the amount
94 // of live memory in that region and by the amount of unallocated memory in that region if the evacuation
95 // budget is constrained by availability of free memory.
96 const size_t old_evacuation_reserve = _old_generation->get_evacuation_reserve();
97 const size_t old_evacuation_budget = (size_t) ((double) old_evacuation_reserve / ShenandoahOldEvacWaste);
98 size_t unfragmented_available = _old_generation->free_unaffiliated_regions() * ShenandoahHeapRegion::region_size_bytes();
99 size_t fragmented_available;
100 size_t excess_fragmented_available;
101
102 if (unfragmented_available > old_evacuation_budget) {
103 unfragmented_available = old_evacuation_budget;
104 fragmented_available = 0;
105 excess_fragmented_available = 0;
106 } else {
107 assert(_old_generation->available() >= old_evacuation_budget, "Cannot budget more than is available");
108 fragmented_available = _old_generation->available() - unfragmented_available;
109 assert(fragmented_available + unfragmented_available >= old_evacuation_budget, "Budgets do not add up");
110 if (fragmented_available + unfragmented_available > old_evacuation_budget) {
111 excess_fragmented_available = (fragmented_available + unfragmented_available) - old_evacuation_budget;
112 fragmented_available -= excess_fragmented_available;
113 }
114 }
115
116 size_t remaining_old_evacuation_budget = old_evacuation_budget;
117 log_info(gc)("Choose old regions for mixed collection: old evacuation budget: " SIZE_FORMAT "%s, candidates: %u",
118 byte_size_in_proper_unit(old_evacuation_budget), proper_unit_for_byte_size(old_evacuation_budget),
119 unprocessed_old_collection_candidates());
120
121 size_t lost_evacuation_capacity = 0;
122
123 // The number of old-gen regions that were selected as candidates for collection at the end of the most recent old-gen
124 // concurrent marking phase and have not yet been collected is represented by unprocessed_old_collection_candidates().
125 // Candidate regions are ordered according to increasing amount of live data. If there is not sufficient room to
126 // evacuate region N, then there is no need to even consider evacuating region N+1.
127 while (unprocessed_old_collection_candidates() > 0) {
128 // Old collection candidates are sorted in order of decreasing garbage contained therein.
129 ShenandoahHeapRegion* r = next_old_collection_candidate();
130 if (r == nullptr) {
131 break;
132 }
133 assert(r->is_regular(), "There should be no humongous regions in the set of mixed-evac candidates");
134
135 // If region r is evacuated to fragmented memory (to free memory within a partially used region), then we need
136 // to decrease the capacity of the fragmented memory by the scaled loss.
137
138 size_t live_data_for_evacuation = r->get_live_data_bytes();
139 size_t lost_available = r->free();
140
141 if ((lost_available > 0) && (excess_fragmented_available > 0)) {
142 if (lost_available < excess_fragmented_available) {
143 excess_fragmented_available -= lost_available;
144 lost_evacuation_capacity -= lost_available;
145 lost_available = 0;
146 } else {
147 lost_available -= excess_fragmented_available;
148 lost_evacuation_capacity -= excess_fragmented_available;
149 excess_fragmented_available = 0;
150 }
151 }
152 size_t scaled_loss = (size_t) ((double) lost_available / ShenandoahOldEvacWaste);
153 if ((lost_available > 0) && (fragmented_available > 0)) {
154 if (scaled_loss + live_data_for_evacuation < fragmented_available) {
155 fragmented_available -= scaled_loss;
156 scaled_loss = 0;
157 } else {
158 // We will have to allocate this region's evacuation memory from unfragmented memory, so don't bother
159 // to decrement scaled_loss
160 }
161 }
162 if (scaled_loss > 0) {
163 // We were not able to account for the lost free memory within fragmented memory, so we need to take this
164 // allocation out of unfragmented memory. Unfragmented memory does not need to account for loss of free.
165 if (live_data_for_evacuation > unfragmented_available) {
166 // There is not room to evacuate this region or any that come after it in within the candidates array.
167 break;
168 } else {
169 unfragmented_available -= live_data_for_evacuation;
170 }
171 } else {
172 // Since scaled_loss == 0, we have accounted for the loss of free memory, so we can allocate from either
173 // fragmented or unfragmented available memory. Use up the fragmented memory budget first.
174 size_t evacuation_need = live_data_for_evacuation;
175
176 if (evacuation_need > fragmented_available) {
177 evacuation_need -= fragmented_available;
178 fragmented_available = 0;
179 } else {
180 fragmented_available -= evacuation_need;
181 evacuation_need = 0;
182 }
183 if (evacuation_need > unfragmented_available) {
184 // There is not room to evacuate this region or any that come after it in within the candidates array.
185 break;
186 } else {
187 unfragmented_available -= evacuation_need;
188 // dead code: evacuation_need == 0;
189 }
190 }
191 collection_set->add_region(r);
192 included_old_regions++;
193 evacuated_old_bytes += live_data_for_evacuation;
194 collected_old_bytes += r->garbage();
195 consume_old_collection_candidate();
196 }
197
198 if (_first_pinned_candidate != NOT_FOUND) {
199 // Need to deal with pinned regions
200 slide_pinned_regions_to_front();
201 }
202 decrease_unprocessed_old_collection_candidates_live_memory(evacuated_old_bytes);
203 if (included_old_regions > 0) {
204 log_info(gc)("Old-gen piggyback evac (" UINT32_FORMAT " regions, evacuating " SIZE_FORMAT "%s, reclaiming: " SIZE_FORMAT "%s)",
205 included_old_regions,
206 byte_size_in_proper_unit(evacuated_old_bytes), proper_unit_for_byte_size(evacuated_old_bytes),
207 byte_size_in_proper_unit(collected_old_bytes), proper_unit_for_byte_size(collected_old_bytes));
208 }
209
210 if (unprocessed_old_collection_candidates() == 0) {
211 // We have added the last of our collection candidates to a mixed collection.
212 // Any triggers that occurred during mixed evacuations may no longer be valid. They can retrigger if appropriate.
213 clear_triggers();
214
215 _old_generation->complete_mixed_evacuations();
216 } else if (included_old_regions == 0) {
217 // We have candidates, but none were included for evacuation - are they all pinned?
218 // or did we just not have enough room for any of them in this collection set?
219 // We don't want a region with a stuck pin to prevent subsequent old collections, so
220 // if they are all pinned we transition to a state that will allow us to make these uncollected
221 // (pinned) regions parsable.
222 if (all_candidates_are_pinned()) {
223 log_info(gc)("All candidate regions " UINT32_FORMAT " are pinned", unprocessed_old_collection_candidates());
224 _old_generation->abandon_mixed_evacuations();
225 } else {
226 log_info(gc)("No regions selected for mixed collection. "
227 "Old evacuation budget: " PROPERFMT ", Remaining evacuation budget: " PROPERFMT
228 ", Lost capacity: " PROPERFMT
229 ", Next candidate: " UINT32_FORMAT ", Last candidate: " UINT32_FORMAT,
230 PROPERFMTARGS(old_evacuation_reserve),
231 PROPERFMTARGS(remaining_old_evacuation_budget),
232 PROPERFMTARGS(lost_evacuation_capacity),
233 _next_old_collection_candidate, _last_old_collection_candidate);
234 }
235 }
236
237 return (included_old_regions > 0);
238 }
239
240 bool ShenandoahOldHeuristics::all_candidates_are_pinned() {
241 #ifdef ASSERT
242 if (uint(os::random()) % 100 < ShenandoahCoalesceChance) {
243 return true;
244 }
245 #endif
246
247 for (uint i = _next_old_collection_candidate; i < _last_old_collection_candidate; ++i) {
248 ShenandoahHeapRegion* region = _region_data[i].get_region();
249 if (!region->is_pinned()) {
250 return false;
251 }
252 }
253 return true;
254 }
255
256 void ShenandoahOldHeuristics::slide_pinned_regions_to_front() {
257 // Find the first unpinned region to the left of the next region that
258 // will be added to the collection set. These regions will have been
259 // added to the cset, so we can use them to hold pointers to regions
260 // that were pinned when the cset was chosen.
261 // [ r p r p p p r r ]
262 // ^ ^ ^
263 // | | | pointer to next region to add to a mixed collection is here.
264 // | | first r to the left should be in the collection set now.
265 // | first pinned region, we don't need to look past this
266 uint write_index = NOT_FOUND;
267 for (uint search = _next_old_collection_candidate - 1; search > _first_pinned_candidate; --search) {
268 ShenandoahHeapRegion* region = _region_data[search].get_region();
269 if (!region->is_pinned()) {
270 write_index = search;
271 assert(region->is_cset(), "Expected unpinned region to be added to the collection set.");
272 break;
273 }
274 }
275
276 // If we could not find an unpinned region, it means there are no slots available
277 // to move up the pinned regions. In this case, we just reset our next index in the
278 // hopes that some of these regions will become unpinned before the next mixed
279 // collection. We may want to bailout of here instead, as it should be quite
280 // rare to have so many pinned regions and may indicate something is wrong.
281 if (write_index == NOT_FOUND) {
282 assert(_first_pinned_candidate != NOT_FOUND, "Should only be here if there are pinned regions.");
283 _next_old_collection_candidate = _first_pinned_candidate;
284 return;
285 }
286
287 // Find pinned regions to the left and move their pointer into a slot
288 // that was pointing at a region that has been added to the cset (or was pointing
289 // to a pinned region that we've already moved up). We are done when the leftmost
290 // pinned region has been slid up.
291 // [ r p r x p p p r ]
292 // ^ ^
293 // | | next region for mixed collections
294 // | Write pointer is here. We know this region is already in the cset
295 // | so we can clobber it with the next pinned region we find.
296 for (int32_t search = (int32_t)write_index - 1; search >= (int32_t)_first_pinned_candidate; --search) {
297 RegionData& skipped = _region_data[search];
298 if (skipped.get_region()->is_pinned()) {
299 RegionData& available_slot = _region_data[write_index];
300 available_slot.set_region_and_livedata(skipped.get_region(), skipped.get_livedata());
301 --write_index;
302 }
303 }
304
305 // Update to read from the leftmost pinned region. Plus one here because we decremented
306 // the write index to hold the next found pinned region. We are just moving it back now
307 // to point to the first pinned region.
308 _next_old_collection_candidate = write_index + 1;
309 }
310
311 void ShenandoahOldHeuristics::prepare_for_old_collections() {
312 ShenandoahHeap* heap = ShenandoahHeap::heap();
313
314 const size_t num_regions = heap->num_regions();
315 size_t cand_idx = 0;
316 size_t immediate_garbage = 0;
317 size_t immediate_regions = 0;
318 size_t live_data = 0;
319
320 RegionData* candidates = _region_data;
321 for (size_t i = 0; i < num_regions; i++) {
322 ShenandoahHeapRegion* region = heap->get_region(i);
323 if (!region->is_old()) {
324 continue;
325 }
326
327 size_t garbage = region->garbage();
328 size_t live_bytes = region->get_live_data_bytes();
329 live_data += live_bytes;
330
331 if (region->is_regular() || region->is_regular_pinned()) {
332 // Only place regular or pinned regions with live data into the candidate set.
333 // Pinned regions cannot be evacuated, but we are not actually choosing candidates
334 // for the collection set here. That happens later during the next young GC cycle,
335 // by which time, the pinned region may no longer be pinned.
336 if (!region->has_live()) {
337 assert(!region->is_pinned(), "Pinned region should have live (pinned) objects.");
338 region->make_trash_immediate();
339 immediate_regions++;
340 immediate_garbage += garbage;
341 } else {
342 region->begin_preemptible_coalesce_and_fill();
343 candidates[cand_idx].set_region_and_livedata(region, 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 // Unlike young, we are more interested in efficiently packing OLD-gen than in reclaiming garbage first. We sort by live-data.
371 // Some regular regions may have been promoted in place with no garbage but also with very little live data. When we "compact"
372 // old-gen, we want to pack these underutilized regions together so we can have more unaffiliated (unfragmented) free regions
373 // in old-gen.
374
375 QuickSort::sort<RegionData>(candidates, cand_idx, compare_by_live);
376
377 const size_t region_size_bytes = ShenandoahHeapRegion::region_size_bytes();
378
379 // The convention is to collect regions that have more than this amount of garbage.
380 const size_t garbage_threshold = region_size_bytes * ShenandoahOldGarbageThreshold / 100;
381
382 // Enlightened interpretation: collect regions that have less than this amount of live.
383 const size_t live_threshold = region_size_bytes - garbage_threshold;
384
385 _last_old_region = (uint)cand_idx;
386 _last_old_collection_candidate = (uint)cand_idx;
387 _next_old_collection_candidate = 0;
388
389 size_t unfragmented = 0;
390 size_t candidates_garbage = 0;
391
392 for (size_t i = 0; i < cand_idx; i++) {
393 size_t live = candidates[i].get_livedata();
394 if (live > live_threshold) {
395 // Candidates are sorted in increasing order of live data, so no regions after this will be below the threshold.
396 _last_old_collection_candidate = (uint)i;
397 break;
398 }
399 ShenandoahHeapRegion* r = candidates[i].get_region();
400 size_t region_garbage = r->garbage();
401 size_t region_free = r->free();
402 candidates_garbage += region_garbage;
403 unfragmented += region_free;
404 }
405
406 // defrag_count represents regions that are placed into the old collection set in order to defragment the memory
407 // that we try to "reserve" for humongous allocations.
408 size_t defrag_count = 0;
409 size_t total_uncollected_old_regions = _last_old_region - _last_old_collection_candidate;
410
411 if (cand_idx > _last_old_collection_candidate) {
412 // Above, we have added into the set of mixed-evacuation candidates all old-gen regions for which the live memory
413 // that they contain is below a particular old-garbage threshold. Regions that were not selected for the collection
414 // set hold enough live memory that it is not considered efficient (by "garbage-first standards") to compact these
415 // at the current time.
416 //
417 // However, if any of these regions that were rejected from the collection set reside within areas of memory that
418 // might interfere with future humongous allocation requests, we will prioritize them for evacuation at this time.
419 // Humongous allocations target the bottom of the heap. We want old-gen regions to congregate at the top of the
420 // heap.
421 //
422 // Sort the regions that were initially rejected from the collection set in order of index. This allows us to
423 // focus our attention on the regions that have low index value (i.e. the old-gen regions at the bottom of the heap).
424 QuickSort::sort<RegionData>(candidates + _last_old_collection_candidate, cand_idx - _last_old_collection_candidate,
425 compare_by_index);
426
427 const size_t first_unselected_old_region = candidates[_last_old_collection_candidate].get_region()->index();
428 const size_t last_unselected_old_region = candidates[cand_idx - 1].get_region()->index();
429 size_t span_of_uncollected_regions = 1 + last_unselected_old_region - first_unselected_old_region;
430
431 // Add no more than 1/8 of the existing old-gen regions to the set of mixed evacuation candidates.
432 const int MAX_FRACTION_OF_HUMONGOUS_DEFRAG_REGIONS = 8;
433 const size_t bound_on_additional_regions = cand_idx / MAX_FRACTION_OF_HUMONGOUS_DEFRAG_REGIONS;
434
435 // The heuristic old_is_fragmented trigger may be seeking to achieve up to 75% density. Allow ourselves to overshoot
436 // that target (at 7/8) so we will not have to do another defragmenting old collection right away.
437 while ((defrag_count < bound_on_additional_regions) &&
438 (total_uncollected_old_regions < 7 * span_of_uncollected_regions / 8)) {
439 ShenandoahHeapRegion* r = candidates[_last_old_collection_candidate].get_region();
440 assert(r->is_regular() || r->is_regular_pinned(), "Region " SIZE_FORMAT " has wrong state for collection: %s",
441 r->index(), ShenandoahHeapRegion::region_state_to_string(r->state()));
442 const size_t region_garbage = r->garbage();
443 const size_t region_free = r->free();
444 candidates_garbage += region_garbage;
445 unfragmented += region_free;
446 defrag_count++;
447 _last_old_collection_candidate++;
448
449 // We now have one fewer uncollected regions, and our uncollected span shrinks because we have removed its first region.
450 total_uncollected_old_regions--;
451 span_of_uncollected_regions =
452 1 + last_unselected_old_region - candidates[_last_old_collection_candidate].get_region()->index();
453 }
454 }
455
456 // Note that we do not coalesce and fill occupied humongous regions
457 // HR: humongous regions, RR: regular regions, CF: coalesce and fill regions
458 const size_t collectable_garbage = immediate_garbage + candidates_garbage;
459 const size_t old_candidates = _last_old_collection_candidate;
460 const size_t mixed_evac_live = old_candidates * region_size_bytes - (candidates_garbage + unfragmented);
461 set_unprocessed_old_collection_candidates_live_memory(mixed_evac_live);
462
463 log_info(gc)("Old-Gen Collectable Garbage: " PROPERFMT " consolidated with free: " PROPERFMT ", over " SIZE_FORMAT " regions",
464 PROPERFMTARGS(collectable_garbage), PROPERFMTARGS(unfragmented), old_candidates);
465 log_info(gc)("Old-Gen Immediate Garbage: " PROPERFMT " over " SIZE_FORMAT " regions",
466 PROPERFMTARGS(immediate_garbage), immediate_regions);
467 log_info(gc)("Old regions selected for defragmentation: " SIZE_FORMAT, defrag_count);
468 log_info(gc)("Old regions not selected: " SIZE_FORMAT, total_uncollected_old_regions);
469
470 if (unprocessed_old_collection_candidates() > 0) {
471 _old_generation->transition_to(ShenandoahOldGeneration::EVACUATING);
472 } else if (has_coalesce_and_fill_candidates()) {
473 _old_generation->transition_to(ShenandoahOldGeneration::FILLING);
474 } else {
475 _old_generation->transition_to(ShenandoahOldGeneration::WAITING_FOR_BOOTSTRAP);
476 }
477 }
478
479 size_t ShenandoahOldHeuristics::unprocessed_old_collection_candidates_live_memory() const {
480 return _live_bytes_in_unprocessed_candidates;
481 }
482
483 void ShenandoahOldHeuristics::set_unprocessed_old_collection_candidates_live_memory(size_t initial_live) {
484 _live_bytes_in_unprocessed_candidates = initial_live;
485 }
486
487 void ShenandoahOldHeuristics::decrease_unprocessed_old_collection_candidates_live_memory(size_t evacuated_live) {
488 assert(evacuated_live <= _live_bytes_in_unprocessed_candidates, "Cannot evacuate more than was present");
489 _live_bytes_in_unprocessed_candidates -= evacuated_live;
490 }
491
492 // Used by unit test: test_shenandoahOldHeuristic.cpp
493 uint ShenandoahOldHeuristics::last_old_collection_candidate_index() const {
494 return _last_old_collection_candidate;
495 }
496
497 uint ShenandoahOldHeuristics::unprocessed_old_collection_candidates() const {
498 return _last_old_collection_candidate - _next_old_collection_candidate;
499 }
500
501 ShenandoahHeapRegion* ShenandoahOldHeuristics::next_old_collection_candidate() {
502 while (_next_old_collection_candidate < _last_old_collection_candidate) {
503 ShenandoahHeapRegion* next = _region_data[_next_old_collection_candidate].get_region();
504 if (!next->is_pinned()) {
505 return next;
506 } else {
507 assert(next->is_pinned(), "sanity");
508 if (_first_pinned_candidate == NOT_FOUND) {
509 _first_pinned_candidate = _next_old_collection_candidate;
510 }
511 }
512
513 _next_old_collection_candidate++;
514 }
515 return nullptr;
516 }
517
518 void ShenandoahOldHeuristics::consume_old_collection_candidate() {
519 _next_old_collection_candidate++;
520 }
521
522 unsigned int ShenandoahOldHeuristics::get_coalesce_and_fill_candidates(ShenandoahHeapRegion** buffer) {
523 uint end = _last_old_region;
524 uint index = _next_old_collection_candidate;
525 while (index < end) {
526 *buffer++ = _region_data[index++].get_region();
527 }
528 return (_last_old_region - _next_old_collection_candidate);
529 }
530
531 void ShenandoahOldHeuristics::abandon_collection_candidates() {
532 _last_old_collection_candidate = 0;
533 _next_old_collection_candidate = 0;
534 _last_old_region = 0;
535 }
536
537 void ShenandoahOldHeuristics::record_cycle_end() {
538 this->ShenandoahHeuristics::record_cycle_end();
539 clear_triggers();
540 }
541
542 void ShenandoahOldHeuristics::clear_triggers() {
543 // Clear any triggers that were set during mixed evacuations. Conditions may be different now that this phase has finished.
544 _cannot_expand_trigger = false;
545 _fragmentation_trigger = false;
546 _growth_trigger = false;
547 }
548
549 // This triggers old-gen collection if the number of regions "dedicated" to old generation is much larger than
550 // is required to represent the memory currently used within the old generation. This trigger looks specifically
551 // at density of the old-gen spanned region. A different mechanism triggers old-gen GC if the total number of
552 // old-gen regions (regardless of how close the regions are to one another) grows beyond an anticipated growth target.
553 void ShenandoahOldHeuristics::set_trigger_if_old_is_fragmented(size_t first_old_region, size_t last_old_region,
554 size_t old_region_count, size_t num_regions) {
555 if (ShenandoahGenerationalHumongousReserve > 0) {
556 // Our intent is to pack old-gen memory into the highest-numbered regions of the heap. Count all memory
557 // above first_old_region as the "span" of old generation.
558 size_t old_region_span = (first_old_region <= last_old_region)? (num_regions - first_old_region): 0;
559 // Given that memory at the bottom of the heap is reserved to represent humongous objects, the number of
560 // regions that old_gen is "allowed" to consume is less than the total heap size. The restriction on allowed
561 // span is not strictly enforced. This is a heuristic designed to reduce the likelihood that a humongous
562 // allocation request will require a STW full GC.
563 size_t allowed_old_gen_span = num_regions - (ShenandoahGenerationalHumongousReserve * num_regions) / 100;
564
565 size_t old_available = _old_gen->available() / HeapWordSize;
566 size_t region_size_words = ShenandoahHeapRegion::region_size_words();
567 size_t old_unaffiliated_available = _old_gen->free_unaffiliated_regions() * region_size_words;
568 assert(old_available >= old_unaffiliated_available, "sanity");
569 size_t old_fragmented_available = old_available - old_unaffiliated_available;
570
571 size_t old_words_consumed = old_region_count * region_size_words - old_fragmented_available;
572 size_t old_words_spanned = old_region_span * region_size_words;
573 double old_density = ((double) old_words_consumed) / old_words_spanned;
574
575 double old_span_percent = ((double) old_region_span) / allowed_old_gen_span;
576 if (old_span_percent > 0.50) {
577 // Squaring old_span_percent in the denominator below allows more aggressive triggering when we are
578 // above desired maximum span and less aggressive triggering when we are far below the desired maximum span.
579 double old_span_percent_squared = old_span_percent * old_span_percent;
580 if (old_density / old_span_percent_squared < 0.75) {
581 // We trigger old defragmentation, for example, if:
582 // old_span_percent is 110% and old_density is below 90.8%, or
583 // old_span_percent is 100% and old_density is below 75.0%, or
584 // old_span_percent is 90% and old_density is below 60.8%, or
585 // old_span_percent is 80% and old_density is below 48.0%, or
586 // old_span_percent is 70% and old_density is below 36.8%, or
587 // old_span_percent is 60% and old_density is below 27.0%, or
588 // old_span_percent is 50% and old_density is below 18.8%.
589
590 // Set the fragmentation trigger and related attributes
591 _fragmentation_trigger = true;
592 _fragmentation_density = old_density;
593 _fragmentation_first_old_region = first_old_region;
594 _fragmentation_last_old_region = last_old_region;
595 }
596 }
597 }
598 }
599
600 void ShenandoahOldHeuristics::set_trigger_if_old_is_overgrown() {
601 size_t old_used = _old_gen->used() + _old_gen->get_humongous_waste();
602 size_t trigger_threshold = _old_gen->usage_trigger_threshold();
603 // Detects unsigned arithmetic underflow
604 assert(old_used <= _heap->capacity(),
605 "Old used (" SIZE_FORMAT ", " SIZE_FORMAT") must not be more than heap capacity (" SIZE_FORMAT ")",
606 _old_gen->used(), _old_gen->get_humongous_waste(), _heap->capacity());
607 if (old_used > trigger_threshold) {
608 _growth_trigger = true;
609 }
610 }
611
612 void ShenandoahOldHeuristics::evaluate_triggers(size_t first_old_region, size_t last_old_region,
613 size_t old_region_count, size_t num_regions) {
614 set_trigger_if_old_is_fragmented(first_old_region, last_old_region, old_region_count, num_regions);
615 set_trigger_if_old_is_overgrown();
616 }
617
618 bool ShenandoahOldHeuristics::should_start_gc() {
619 // Cannot start a new old-gen GC until previous one has finished.
620 //
621 // Future refinement: under certain circumstances, we might be more sophisticated about this choice.
622 // For example, we could choose to abandon the previous old collection before it has completed evacuations.
623 ShenandoahHeap* heap = ShenandoahHeap::heap();
624 if (!_old_generation->can_start_gc() || heap->collection_set()->has_old_regions()) {
625 return false;
626 }
627
628 if (_cannot_expand_trigger) {
629 const size_t old_gen_capacity = _old_generation->max_capacity();
630 const size_t heap_capacity = heap->capacity();
631 const double percent = percent_of(old_gen_capacity, heap_capacity);
632 log_info(gc)("Trigger (OLD): Expansion failure, current size: " SIZE_FORMAT "%s which is %.1f%% of total heap size",
633 byte_size_in_proper_unit(old_gen_capacity), proper_unit_for_byte_size(old_gen_capacity), percent);
634 return true;
635 }
636
637 if (_fragmentation_trigger) {
638 const size_t used = _old_generation->used();
639 const size_t used_regions_size = _old_generation->used_regions_size();
640
641 // used_regions includes humongous regions
642 const size_t used_regions = _old_generation->used_regions();
643 assert(used_regions_size > used_regions, "Cannot have more used than used regions");
644
645 size_t first_old_region, last_old_region;
646 double density;
647 get_fragmentation_trigger_reason_for_log_message(density, first_old_region, last_old_region);
648 const size_t span_of_old_regions = (last_old_region >= first_old_region)? last_old_region + 1 - first_old_region: 0;
649 const size_t fragmented_free = used_regions_size - used;
650
651 log_info(gc)("Trigger (OLD): Old has become fragmented: "
652 SIZE_FORMAT "%s available bytes spread between range spanned from "
653 SIZE_FORMAT " to " SIZE_FORMAT " (" SIZE_FORMAT "), density: %.1f%%",
654 byte_size_in_proper_unit(fragmented_free), proper_unit_for_byte_size(fragmented_free),
655 first_old_region, last_old_region, span_of_old_regions, density * 100);
656 return true;
657 }
658
659 if (_growth_trigger) {
660 // Growth may be falsely triggered during mixed evacuations, before the mixed-evacuation candidates have been
661 // evacuated. Before acting on a false trigger, we check to confirm the trigger condition is still satisfied.
662 const size_t current_usage = _old_generation->used() + _old_generation->get_humongous_waste();
663 const size_t trigger_threshold = _old_generation->usage_trigger_threshold();
664 const size_t heap_size = heap->capacity();
665 const size_t ignore_threshold = (ShenandoahIgnoreOldGrowthBelowPercentage * heap_size) / 100;
666 size_t consecutive_young_cycles;
667 if ((current_usage < ignore_threshold) &&
668 ((consecutive_young_cycles = heap->shenandoah_policy()->consecutive_young_gc_count())
669 < ShenandoahDoNotIgnoreGrowthAfterYoungCycles)) {
670 log_debug(gc)("Ignoring Trigger (OLD): Old has overgrown: usage (" SIZE_FORMAT "%s) is below threshold ("
671 SIZE_FORMAT "%s) after " SIZE_FORMAT " consecutive completed young GCs",
672 byte_size_in_proper_unit(current_usage), proper_unit_for_byte_size(current_usage),
673 byte_size_in_proper_unit(ignore_threshold), proper_unit_for_byte_size(ignore_threshold),
674 consecutive_young_cycles);
675 _growth_trigger = false;
676 } else if (current_usage > trigger_threshold) {
677 const size_t live_at_previous_old = _old_generation->get_live_bytes_after_last_mark();
678 const double percent_growth = percent_of(current_usage - live_at_previous_old, live_at_previous_old);
679 log_info(gc)("Trigger (OLD): Old has overgrown, live at end of previous OLD marking: "
680 SIZE_FORMAT "%s, current usage: " SIZE_FORMAT "%s, percent growth: %.1f%%",
681 byte_size_in_proper_unit(live_at_previous_old), proper_unit_for_byte_size(live_at_previous_old),
682 byte_size_in_proper_unit(current_usage), proper_unit_for_byte_size(current_usage), percent_growth);
683 return true;
684 } else {
685 // Mixed evacuations have decreased current_usage such that old-growth trigger is no longer relevant.
686 _growth_trigger = false;
687 }
688 }
689
690 // Otherwise, defer to inherited heuristic for gc trigger.
691 return this->ShenandoahHeuristics::should_start_gc();
692 }
693
694 void ShenandoahOldHeuristics::record_success_concurrent() {
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_concurrent();
698 }
699
700 void ShenandoahOldHeuristics::record_success_degenerated() {
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_degenerated();
704 }
705
706 void ShenandoahOldHeuristics::record_success_full() {
707 // Forget any triggers that occurred while OLD GC was ongoing. If we really need to start another, it will retrigger.
708 clear_triggers();
709 this->ShenandoahHeuristics::record_success_full();
710 }
711
712 const char* ShenandoahOldHeuristics::name() {
713 return "Old";
714 }
715
716 bool ShenandoahOldHeuristics::is_diagnostic() {
717 return false;
718 }
719
720 bool ShenandoahOldHeuristics::is_experimental() {
721 return true;
722 }
723
724 void ShenandoahOldHeuristics::choose_collection_set_from_regiondata(ShenandoahCollectionSet* set,
725 ShenandoahHeuristics::RegionData* data,
726 size_t data_size, size_t free) {
727 ShouldNotReachHere();
728 }