1 /*
2 * Copyright (c) 2007, Oracle and/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.
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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
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23 */
24
25 class G1CollectedHeap;
26
27 // This file contains the three classes that represent the memory
28 // pools of the G1 spaces: G1EdenPool, G1SurvivorPool, and
29 // G1OldGenPool. In G1, unlike our other GCs, we do not have a
30 // physical space for each of those spaces. Instead, we allocate
31 // regions for all three spaces out of a single pool of regions (that
32 // pool basically covers the entire heap). As a result, the eden,
33 // survivor, and old gen are considered logical spaces in G1, as each
34 // is a set of non-contiguous regions. This is also reflected in the
35 // way we map them to memory pools here. The easiest way to have done
36 // this would have been to map the entire G1 heap to a single memory
37 // pool. However, it's helpful to show how large the eden and survivor
38 // get, as this does affect the performance and behavior of G1. Which
39 // is why we introduce the three memory pools implemented here.
40 //
41 // The above approach inroduces a couple of challenging issues in the
42 // implementation of the three memory pools:
43 //
44 // 1) The used space calculation for a pool is not necessarily
45 // independent of the others. We can easily get from G1 the overall
46 // used space in the entire heap, the number of regions in the young
47 // generation (includes both eden and survivors), and the number of
48 // survivor regions. So, from that we calculate:
49 //
50 // survivor_used = survivor_num * region_size
51 // eden_used = young_region_num * region_size - survivor_used
52 // old_gen_used = overall_used - eden_used - survivor_used
53 //
54 // Note that survivor_used and eden_used are upper bounds. To get the
55 // actual value we would have to iterate over the regions and add up
56 // ->used(). But that'd be expensive. So, we'll accept some lack of
57 // accuracy for those two. But, we have to be careful when calculating
58 // old_gen_used, in case we subtract from overall_used more then the
59 // actual number and our result goes negative.
60 //
61 // 2) Calculating the used space is straightforward, as described
62 // above. However, how do we calculate the committed space, given that
63 // we allocate space for the eden, survivor, and old gen out of the
64 // same pool of regions? One way to do this is to use the used value
65 // as also the committed value for the eden and survivor spaces and
66 // then calculate the old gen committed space as follows:
67 //
68 // old_gen_committed = overall_committed - eden_committed - survivor_committed
69 //
70 // Maybe a better way to do that would be to calculate used for eden
71 // and survivor as a sum of ->used() over their regions and then
72 // calculate committed as region_num * region_size (i.e., what we use
73 // to calculate the used space now). This is something to consider
74 // in the future.
75 //
76 // 3) Another decision that is again not straightforward is what is
77 // the max size that each memory pool can grow to. One way to do this
78 // would be to use the committed size for the max for the eden and
79 // survivors and calculate the old gen max as follows (basically, it's
80 // a similar pattern to what we use for the committed space, as
81 // described above):
82 //
83 // old_gen_max = overall_max - eden_max - survivor_max
84 //
85 // Unfortunately, the above makes the max of each pool fluctuate over
86 // time and, even though this is allowed according to the spec, it
87 // broke several assumptions in the M&M framework (there were cases
88 // where used would reach a value greater than max). So, for max we
89 // use -1, which means "undefined" according to the spec.
90 //
91 // 4) Now, there is a very subtle issue with all the above. The
92 // framework will call get_memory_usage() on the three pools
93 // asynchronously. As a result, each call might get a different value
94 // for, say, survivor_num which will yield inconsistent values for
95 // eden_used, survivor_used, and old_gen_used (as survivor_num is used
96 // in the calculation of all three). This would normally be
97 // ok. However, it's possible that this might cause the sum of
98 // eden_used, survivor_used, and old_gen_used to go over the max heap
99 // size and this seems to sometimes cause JConsole (and maybe other
100 // clients) to get confused. There's not a really an easy / clean
101 // solution to this problem, due to the asynchrounous nature of the
102 // framework.
103
104
105 // This class is shared by the three G1 memory pool classes
106 // (G1EdenPool, G1SurvivorPool, G1OldGenPool). Given that the way we
107 // calculate used / committed bytes for these three pools is related
108 // (see comment above), we put the calculations in this class so that
109 // we can easily share them among the subclasses.
110 class G1MemoryPoolSuper : public CollectedMemoryPool {
111 private:
112 // It returns x - y if x > y, 0 otherwise.
113 // As described in the comment above, some of the inputs to the
114 // calculations we have to do are obtained concurrently and hence
115 // may be inconsistent with each other. So, this provides a
116 // defensive way of performing the subtraction and avoids the value
117 // going negative (which would mean a very large result, given that
118 // the parameter are size_t).
119 static size_t subtract_up_to_zero(size_t x, size_t y) {
120 if (x > y) {
121 return x - y;
122 } else {
123 return 0;
124 }
125 }
126
127 protected:
128 G1CollectedHeap* _g1h;
129
130 // Would only be called from subclasses.
131 G1MemoryPoolSuper(G1CollectedHeap* g1h,
132 const char* name,
133 size_t init_size,
134 bool support_usage_threshold);
135
136 // The reason why all the code is in static methods is so that it
137 // can be safely called from the constructors of the subclasses.
138
139 static size_t undefined_max() {
140 return (size_t) -1;
141 }
142
143 static size_t overall_committed(G1CollectedHeap* g1h) {
144 return g1h->capacity();
145 }
146 static size_t overall_used(G1CollectedHeap* g1h) {
147 return g1h->used_unlocked();
148 }
149
150 static size_t eden_space_committed(G1CollectedHeap* g1h);
151 static size_t eden_space_used(G1CollectedHeap* g1h);
152
153 static size_t survivor_space_committed(G1CollectedHeap* g1h);
154 static size_t survivor_space_used(G1CollectedHeap* g1h);
155
156 static size_t old_space_committed(G1CollectedHeap* g1h);
157 static size_t old_space_used(G1CollectedHeap* g1h);
158 };
159
160 // Memory pool that represents the G1 eden.
161 class G1EdenPool : public G1MemoryPoolSuper {
162 public:
163 G1EdenPool(G1CollectedHeap* g1h);
164
165 size_t used_in_bytes() {
166 return eden_space_used(_g1h);
167 }
168 size_t max_size() const {
169 return undefined_max();
170 }
171 MemoryUsage get_memory_usage();
172 };
173
174 // Memory pool that represents the G1 survivor.
175 class G1SurvivorPool : public G1MemoryPoolSuper {
176 public:
177 G1SurvivorPool(G1CollectedHeap* g1h);
178
179 size_t used_in_bytes() {
180 return survivor_space_used(_g1h);
181 }
182 size_t max_size() const {
183 return undefined_max();
184 }
185 MemoryUsage get_memory_usage();
186 };
187
188 // Memory pool that represents the G1 old gen.
189 class G1OldGenPool : public G1MemoryPoolSuper {
190 public:
191 G1OldGenPool(G1CollectedHeap* g1h);
192
193 size_t used_in_bytes() {
194 return old_space_used(_g1h);
195 }
196 size_t max_size() const {
197 return undefined_max();
198 }
199 MemoryUsage get_memory_usage();
200 };