1 /*
2 * Copyright (c) 2018, Red Hat, Inc. All rights reserved.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation.
7 *
8 * This code is distributed in the hope that it will be useful, but WITHOUT
9 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
10 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
11 * version 2 for more details (a copy is included in the LICENSE file that
12 * accompanied this code).
13 *
14 * You should have received a copy of the GNU General Public License version
15 * 2 along with this work; if not, write to the Free Software Foundation,
16 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
17 *
18 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
19 * or visit www.oracle.com if you need additional information or have any
20 * questions.
21 *
22 */
23
24 #include "precompiled.hpp"
25
26 #include "gc/shenandoah/heuristics/shenandoahTraversalHeuristics.hpp"
27 #include "gc/shenandoah/shenandoahFreeSet.hpp"
28 #include "gc/shenandoah/shenandoahHeuristics.hpp"
29 #include "gc/shenandoah/shenandoahTraversalGC.hpp"
30 #include "logging/log.hpp"
31 #include "logging/logTag.hpp"
32 #include "utilities/quickSort.hpp"
33
34 ShenandoahTraversalHeuristics::ShenandoahTraversalHeuristics() : ShenandoahHeuristics(),
35 _last_cset_select(0)
36 {
37 FLAG_SET_DEFAULT(ShenandoahSATBBarrier, false);
38 FLAG_SET_DEFAULT(ShenandoahStoreValReadBarrier, false);
39 FLAG_SET_DEFAULT(ShenandoahStoreValEnqueueBarrier, true);
40 FLAG_SET_DEFAULT(ShenandoahKeepAliveBarrier, false);
41 FLAG_SET_DEFAULT(ShenandoahAllowMixedAllocs, false);
42
43 SHENANDOAH_ERGO_OVERRIDE_DEFAULT(ShenandoahRefProcFrequency, 1);
44
45 // Adjust class unloading settings only if globally enabled.
46 if (ClassUnloadingWithConcurrentMark) {
47 SHENANDOAH_ERGO_OVERRIDE_DEFAULT(ShenandoahUnloadClassesFrequency, 1);
48 }
49
50 SHENANDOAH_ERGO_ENABLE_FLAG(ExplicitGCInvokesConcurrent);
51 SHENANDOAH_ERGO_ENABLE_FLAG(ShenandoahImplicitGCInvokesConcurrent);
52 }
53
54 bool ShenandoahTraversalHeuristics::should_start_normal_gc() const {
55 return false;
56 }
57
58 bool ShenandoahTraversalHeuristics::is_experimental() {
59 return true;
60 }
61
62 bool ShenandoahTraversalHeuristics::is_diagnostic() {
63 return false;
64 }
65
66 bool ShenandoahTraversalHeuristics::can_do_traversal_gc() {
67 return true;
68 }
69
70 const char* ShenandoahTraversalHeuristics::name() {
71 return "traversal";
72 }
73
74 void ShenandoahTraversalHeuristics::choose_collection_set(ShenandoahCollectionSet* collection_set) {
75 ShenandoahHeap* heap = ShenandoahHeap::heap();
76
77 ShenandoahTraversalGC* traversal_gc = heap->traversal_gc();
78
79 ShenandoahHeapRegionSet* traversal_set = traversal_gc->traversal_set();
80 traversal_set->clear();
81
82 RegionData *data = get_region_data_cache(heap->num_regions());
83 size_t cnt = 0;
84
85 // Step 0. Prepare all regions
86
87 for (size_t i = 0; i < heap->num_regions(); i++) {
88 ShenandoahHeapRegion* r = heap->get_region(i);
89 if (r->used() > 0) {
90 if (r->is_regular()) {
91 data[cnt]._region = r;
92 data[cnt]._garbage = r->garbage();
93 data[cnt]._seqnum_last_alloc = r->seqnum_last_alloc_mutator();
94 cnt++;
95 }
96 traversal_set->add_region(r);
97 }
98 }
99
100 // The logic for cset selection is similar to that of adaptive:
101 //
102 // 1. We cannot get cset larger than available free space. Otherwise we guarantee OOME
103 // during evacuation, and thus guarantee full GC. In practice, we also want to let
104 // application to allocate something. This is why we limit CSet to some fraction of
105 // available space. In non-overloaded heap, max_cset would contain all plausible candidates
106 // over garbage threshold.
107 //
108 // 2. We should not get cset too low so that free threshold would not be met right
109 // after the cycle. Otherwise we get back-to-back cycles for no reason if heap is
110 // too fragmented. In non-overloaded non-fragmented heap min_garbage would be around zero.
111 //
112 // Therefore, we start by sorting the regions by garbage. Then we unconditionally add the best candidates
113 // before we meet min_garbage. Then we add all candidates that fit with a garbage threshold before
114 // we hit max_cset. When max_cset is hit, we terminate the cset selection. Note that in this scheme,
115 // ShenandoahGarbageThreshold is the soft threshold which would be ignored until min_garbage is hit.
116 //
117 // The significant complication is that liveness data was collected at the previous cycle, and only
118 // for those regions that were allocated before previous cycle started.
119
120 size_t capacity = heap->capacity();
121 size_t actual_free = heap->free_set()->available();
122 size_t free_target = ShenandoahMinFreeThreshold * capacity / 100;
123 size_t min_garbage = free_target > actual_free ? (free_target - actual_free) : 0;
124 size_t max_cset = (size_t)(1.0 * ShenandoahEvacReserve * capacity / 100 / ShenandoahEvacWaste);
125
126 log_info(gc, ergo)("Adaptive CSet Selection. Target Free: " SIZE_FORMAT "M, Actual Free: "
127 SIZE_FORMAT "M, Max CSet: " SIZE_FORMAT "M, Min Garbage: " SIZE_FORMAT "M",
128 free_target / M, actual_free / M, max_cset / M, min_garbage / M);
129
130 // Better select garbage-first regions, and then older ones
131 QuickSort::sort<RegionData>(data, (int) cnt, compare_by_garbage_then_alloc_seq_ascending, false);
132
133 size_t cur_cset = 0;
134 size_t cur_garbage = 0;
135
136 size_t garbage_threshold = ShenandoahHeapRegion::region_size_bytes() / 100 * ShenandoahGarbageThreshold;
137
138 // Step 1. Add trustworthy regions to collection set.
139 //
140 // We can trust live/garbage data from regions that were fully traversed during
141 // previous cycle. Even if actual liveness is different now, we can only have _less_
142 // live objects, because dead objects are not resurrected. Which means we can undershoot
143 // the collection set, but not overshoot it.
144
145 for (size_t i = 0; i < cnt; i++) {
146 if (data[i]._seqnum_last_alloc > _last_cset_select) continue;
147
148 ShenandoahHeapRegion* r = data[i]._region;
149 assert (r->is_regular(), "should have been filtered before");
150
151 size_t new_garbage = cur_garbage + r->garbage();
152 size_t new_cset = cur_cset + r->get_live_data_bytes();
153
154 if (new_cset > max_cset) {
155 break;
156 }
157
158 if ((new_garbage < min_garbage) || (r->garbage() > garbage_threshold)) {
159 assert(!collection_set->is_in(r), "must not yet be in cset");
160 collection_set->add_region(r);
161 cur_cset = new_cset;
162 cur_garbage = new_garbage;
163 }
164 }
165
166 // Step 2. Try to catch some recently allocated regions for evacuation ride.
167 //
168 // Pessimistically assume we are going to evacuate the entire region. While this
169 // is very pessimistic and in most cases undershoots the collection set when regions
170 // are mostly dead, it also provides more safety against running into allocation
171 // failure when newly allocated regions are fully live.
172
173 for (size_t i = 0; i < cnt; i++) {
174 if (data[i]._seqnum_last_alloc <= _last_cset_select) continue;
175
176 ShenandoahHeapRegion* r = data[i]._region;
177 assert (r->is_regular(), "should have been filtered before");
178
179 // size_t new_garbage = cur_garbage + 0; (implied)
180 size_t new_cset = cur_cset + r->used();
181
182 if (new_cset > max_cset) {
183 break;
184 }
185
186 assert(!collection_set->is_in(r), "must not yet be in cset");
187 collection_set->add_region(r);
188 cur_cset = new_cset;
189 }
190
191 // Step 3. Clear liveness data
192 // TODO: Merge it with step 0, but save live data in RegionData before.
193 for (size_t i = 0; i < heap->num_regions(); i++) {
194 ShenandoahHeapRegion* r = heap->get_region(i);
195 if (r->used() > 0) {
196 r->clear_live_data();
197 }
198 }
199
200 collection_set->update_region_status();
201
202 _last_cset_select = ShenandoahHeapRegion::seqnum_current_alloc();
203 }
204
205 bool ShenandoahTraversalHeuristics::should_start_traversal_gc() {
206 ShenandoahHeap* heap = ShenandoahHeap::heap();
207 assert(!heap->has_forwarded_objects(), "no forwarded objects here");
208
209 size_t capacity = heap->capacity();
210 size_t available = heap->free_set()->available();
211
212 // Check if we are falling below the worst limit, time to trigger the GC, regardless of
213 // anything else.
214 size_t min_threshold = ShenandoahMinFreeThreshold * heap->capacity() / 100;
215 if (available < min_threshold) {
216 log_info(gc)("Trigger: Free (" SIZE_FORMAT "M) is below minimum threshold (" SIZE_FORMAT "M)",
217 available / M, min_threshold / M);
218 return true;
219 }
220
221 // Check if are need to learn a bit about the application
222 const size_t max_learn = ShenandoahLearningSteps;
223 if (_gc_times_learned < max_learn) {
224 size_t init_threshold = ShenandoahInitFreeThreshold * heap->capacity() / 100;
225 if (available < init_threshold) {
226 log_info(gc)("Trigger: Learning " SIZE_FORMAT " of " SIZE_FORMAT ". Free (" SIZE_FORMAT "M) is below initial threshold (" SIZE_FORMAT "M)",
227 _gc_times_learned + 1, max_learn, available / M, init_threshold / M);
228 return true;
229 }
230 }
231
232 // Check if allocation headroom is still okay. This also factors in:
233 // 1. Some space to absorb allocation spikes
234 // 2. Accumulated penalties from Degenerated and Full GC
235
236 size_t allocation_headroom = available;
237
238 size_t spike_headroom = ShenandoahAllocSpikeFactor * capacity / 100;
239 size_t penalties = _gc_time_penalties * capacity / 100;
240
241 allocation_headroom -= MIN2(allocation_headroom, spike_headroom);
242 allocation_headroom -= MIN2(allocation_headroom, penalties);
243
244 double average_gc = _gc_time_history->avg();
245 double time_since_last = time_since_last_gc();
246 double allocation_rate = heap->bytes_allocated_since_gc_start() / time_since_last;
247
248 if (average_gc > allocation_headroom / allocation_rate) {
249 log_info(gc)("Trigger: Average GC time (%.2f ms) is above the time for allocation rate (%.2f MB/s) to deplete free headroom (" SIZE_FORMAT "M)",
250 average_gc * 1000, allocation_rate / M, allocation_headroom / M);
251 log_info(gc, ergo)("Free headroom: " SIZE_FORMAT "M (free) - " SIZE_FORMAT "M (spike) - " SIZE_FORMAT "M (penalties) = " SIZE_FORMAT "M",
252 available / M, spike_headroom / M, penalties / M, allocation_headroom / M);
253 return true;
254 } else if (ShenandoahHeuristics::should_start_normal_gc()) {
255 return true;
256 }
257
258 return false;
259 }
260
261 void ShenandoahTraversalHeuristics::choose_collection_set_from_regiondata(ShenandoahCollectionSet* set,
262 RegionData* data, size_t data_size,
263 size_t free) {
264 ShouldNotReachHere();
265 }