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6815790: G1: Missing MemoryPoolMXBeans with -XX:+UseG1GC Summary: It introduces the necessary memory pools for G1. Reviewed-by: mchung, ysr
author tonyp
date Fri, 20 Nov 2009 14:47:01 -0500
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1088:3fc996d4edd2 1089:db0d5eba9d20
1 /*
2 * Copyright (c) 2007 Sun Microsystems, Inc. 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
20 * CA 95054 USA or visit www.sun.com if you need additional information or
21 * have any questions.
22 *
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. Right now, we set
78 // that the committed size for the eden and the survivors and
79 // calculate the old gen max as follows (basically, it's a similar
80 // pattern to what we use for the committed space, as described
81 // above):
82 //
83 // old_gen_max = overall_max - eden_max - survivor_max
84 //
85 // 4) Now, there is a very subtle issue with all the above. The
86 // framework will call get_memory_usage() on the three pools
87 // asynchronously. As a result, each call might get a different value
88 // for, say, survivor_num which will yield inconsistent values for
89 // eden_used, survivor_used, and old_gen_used (as survivor_num is used
90 // in the calculation of all three). This would normally be
91 // ok. However, it's possible that this might cause the sum of
92 // eden_used, survivor_used, and old_gen_used to go over the max heap
93 // size and this seems to sometimes cause JConsole (and maybe other
94 // clients) to get confused. There's not a really an easy / clean
95 // solution to this problem, due to the asynchrounous nature of the
96 // framework.
97
98
99 // This class is shared by the three G1 memory pool classes
100 // (G1EdenPool, G1SurvivorPool, G1OldGenPool). Given that the way we
101 // calculate used / committed bytes for these three pools is related
102 // (see comment above), we put the calculations in this class so that
103 // we can easily share them among the subclasses.
104 class G1MemoryPoolSuper : public CollectedMemoryPool {
105 private:
106 G1CollectedHeap* _g1h;
107
108 // It returns x - y if x > y, 0 otherwise.
109 // As described in the comment above, some of the inputs to the
110 // calculations we have to do are obtained concurrently and hence
111 // may be inconsistent with each other. So, this provides a
112 // defensive way of performing the subtraction and avoids the value
113 // going negative (which would mean a very large result, given that
114 // the parameter are size_t).
115 static size_t subtract_up_to_zero(size_t x, size_t y) {
116 if (x > y) {
117 return x - y;
118 } else {
119 return 0;
120 }
121 }
122
123 protected:
124 // Would only be called from subclasses.
125 G1MemoryPoolSuper(G1CollectedHeap* g1h,
126 const char* name,
127 size_t init_size,
128 size_t max_size,
129 bool support_usage_threshold);
130
131 // The reason why all the code is in static methods is so that it
132 // can be safely called from the constructors of the subclasses.
133
134 static size_t overall_committed(G1CollectedHeap* g1h) {
135 return g1h->capacity();
136 }
137 static size_t overall_used(G1CollectedHeap* g1h) {
138 return g1h->used_unlocked();
139 }
140
141 static size_t eden_space_committed(G1CollectedHeap* g1h);
142 static size_t eden_space_used(G1CollectedHeap* g1h);
143 static size_t eden_space_max(G1CollectedHeap* g1h);
144
145 static size_t survivor_space_committed(G1CollectedHeap* g1h);
146 static size_t survivor_space_used(G1CollectedHeap* g1h);
147 static size_t survivor_space_max(G1CollectedHeap* g1h);
148
149 static size_t old_space_committed(G1CollectedHeap* g1h);
150 static size_t old_space_used(G1CollectedHeap* g1h);
151 static size_t old_space_max(G1CollectedHeap* g1h);
152
153 // The non-static versions are included for convenience.
154
155 size_t eden_space_committed() {
156 return eden_space_committed(_g1h);
157 }
158 size_t eden_space_used() {
159 return eden_space_used(_g1h);
160 }
161 size_t eden_space_max() {
162 return eden_space_max(_g1h);
163 }
164
165 size_t survivor_space_committed() {
166 return survivor_space_committed(_g1h);
167 }
168 size_t survivor_space_used() {
169 return survivor_space_used(_g1h);
170 }
171 size_t survivor_space_max() {
172 return survivor_space_max(_g1h);
173 }
174
175 size_t old_space_committed() {
176 return old_space_committed(_g1h);
177 }
178 size_t old_space_used() {
179 return old_space_used(_g1h);
180 }
181 size_t old_space_max() {
182 return old_space_max(_g1h);
183 }
184 };
185
186 // Memory pool that represents the G1 eden.
187 class G1EdenPool : public G1MemoryPoolSuper {
188 public:
189 G1EdenPool(G1CollectedHeap* g1h);
190
191 size_t used_in_bytes() {
192 return eden_space_used();
193 }
194 size_t max_size() {
195 return eden_space_max();
196 }
197 MemoryUsage get_memory_usage();
198 };
199
200 // Memory pool that represents the G1 survivor.
201 class G1SurvivorPool : public G1MemoryPoolSuper {
202 public:
203 G1SurvivorPool(G1CollectedHeap* g1h);
204
205 size_t used_in_bytes() {
206 return survivor_space_used();
207 }
208 size_t max_size() {
209 return survivor_space_max();
210 }
211 MemoryUsage get_memory_usage();
212 };
213
214 // Memory pool that represents the G1 old gen.
215 class G1OldGenPool : public G1MemoryPoolSuper {
216 public:
217 G1OldGenPool(G1CollectedHeap* g1h);
218
219 size_t used_in_bytes() {
220 return old_space_used();
221 }
222 size_t max_size() {
223 return old_space_max();
224 }
225 MemoryUsage get_memory_usage();
226 };