1 /*
2 * Copyright (c) 2007, 2010, 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.
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 #ifndef SHARE_VM_SERVICES_G1MEMORYPOOL_HPP
26 #define SHARE_VM_SERVICES_G1MEMORYPOOL_HPP
27
28 #ifndef SERIALGC
29 #include "services/memoryPool.hpp"
30 #include "services/memoryUsage.hpp"
31 #endif
32
33 class G1CollectedHeap;
34
35 // This file contains the three classes that represent the memory
36 // pools of the G1 spaces: G1EdenPool, G1SurvivorPool, and
37 // G1OldGenPool. In G1, unlike our other GCs, we do not have a
38 // physical space for each of those spaces. Instead, we allocate
39 // regions for all three spaces out of a single pool of regions (that
40 // pool basically covers the entire heap). As a result, the eden,
41 // survivor, and old gen are considered logical spaces in G1, as each
42 // is a set of non-contiguous regions. This is also reflected in the
43 // way we map them to memory pools here. The easiest way to have done
44 // this would have been to map the entire G1 heap to a single memory
45 // pool. However, it's helpful to show how large the eden and survivor
46 // get, as this does affect the performance and behavior of G1. Which
47 // is why we introduce the three memory pools implemented here.
48 //
49 // The above approach inroduces a couple of challenging issues in the
50 // implementation of the three memory pools:
51 //
52 // 1) The used space calculation for a pool is not necessarily
53 // independent of the others. We can easily get from G1 the overall
54 // used space in the entire heap, the number of regions in the young
55 // generation (includes both eden and survivors), and the number of
56 // survivor regions. So, from that we calculate:
57 //
58 // survivor_used = survivor_num * region_size
59 // eden_used = young_region_num * region_size - survivor_used
60 // old_gen_used = overall_used - eden_used - survivor_used
61 //
62 // Note that survivor_used and eden_used are upper bounds. To get the
63 // actual value we would have to iterate over the regions and add up
64 // ->used(). But that'd be expensive. So, we'll accept some lack of
65 // accuracy for those two. But, we have to be careful when calculating
66 // old_gen_used, in case we subtract from overall_used more then the
67 // actual number and our result goes negative.
68 //
69 // 2) Calculating the used space is straightforward, as described
70 // above. However, how do we calculate the committed space, given that
71 // we allocate space for the eden, survivor, and old gen out of the
72 // same pool of regions? One way to do this is to use the used value
73 // as also the committed value for the eden and survivor spaces and
74 // then calculate the old gen committed space as follows:
75 //
76 // old_gen_committed = overall_committed - eden_committed - survivor_committed
77 //
78 // Maybe a better way to do that would be to calculate used for eden
79 // and survivor as a sum of ->used() over their regions and then
80 // calculate committed as region_num * region_size (i.e., what we use
81 // to calculate the used space now). This is something to consider
82 // in the future.
83 //
84 // 3) Another decision that is again not straightforward is what is
85 // the max size that each memory pool can grow to. One way to do this
86 // would be to use the committed size for the max for the eden and
87 // survivors and calculate the old gen max as follows (basically, it's
88 // a similar pattern to what we use for the committed space, as
89 // described above):
90 //
91 // old_gen_max = overall_max - eden_max - survivor_max
92 //
93 // Unfortunately, the above makes the max of each pool fluctuate over
94 // time and, even though this is allowed according to the spec, it
95 // broke several assumptions in the M&M framework (there were cases
96 // where used would reach a value greater than max). So, for max we
97 // use -1, which means "undefined" according to the spec.
98 //
99 // 4) Now, there is a very subtle issue with all the above. The
100 // framework will call get_memory_usage() on the three pools
101 // asynchronously. As a result, each call might get a different value
102 // for, say, survivor_num which will yield inconsistent values for
103 // eden_used, survivor_used, and old_gen_used (as survivor_num is used
104 // in the calculation of all three). This would normally be
105 // ok. However, it's possible that this might cause the sum of
106 // eden_used, survivor_used, and old_gen_used to go over the max heap
107 // size and this seems to sometimes cause JConsole (and maybe other
108 // clients) to get confused. There's not a really an easy / clean
109 // solution to this problem, due to the asynchrounous nature of the
110 // framework.
111
112
113 // This class is shared by the three G1 memory pool classes
114 // (G1EdenPool, G1SurvivorPool, G1OldGenPool). Given that the way we
115 // calculate used / committed bytes for these three pools is related
116 // (see comment above), we put the calculations in this class so that
117 // we can easily share them among the subclasses.
118 class G1MemoryPoolSuper : public CollectedMemoryPool {
119 private:
120 // It returns x - y if x > y, 0 otherwise.
121 // As described in the comment above, some of the inputs to the
122 // calculations we have to do are obtained concurrently and hence
123 // may be inconsistent with each other. So, this provides a
124 // defensive way of performing the subtraction and avoids the value
125 // going negative (which would mean a very large result, given that
126 // the parameter are size_t).
127 static size_t subtract_up_to_zero(size_t x, size_t y) {
128 if (x > y) {
129 return x - y;
130 } else {
131 return 0;
132 }
133 }
134
135 protected:
136 G1CollectedHeap* _g1h;
137
138 // Would only be called from subclasses.
139 G1MemoryPoolSuper(G1CollectedHeap* g1h,
140 const char* name,
141 size_t init_size,
142 bool support_usage_threshold);
143
144 // The reason why all the code is in static methods is so that it
145 // can be safely called from the constructors of the subclasses.
146
147 static size_t undefined_max() {
148 return (size_t) -1;
149 }
150
151 static size_t overall_committed(G1CollectedHeap* g1h) {
152 return g1h->capacity();
153 }
154 static size_t overall_used(G1CollectedHeap* g1h) {
155 return g1h->used_unlocked();
156 }
157
158 static size_t eden_space_committed(G1CollectedHeap* g1h);
159 static size_t eden_space_used(G1CollectedHeap* g1h);
160
161 static size_t survivor_space_committed(G1CollectedHeap* g1h);
162 static size_t survivor_space_used(G1CollectedHeap* g1h);
163
164 static size_t old_space_committed(G1CollectedHeap* g1h);
165 static size_t old_space_used(G1CollectedHeap* g1h);
166 };
167
168 // Memory pool that represents the G1 eden.
169 class G1EdenPool : public G1MemoryPoolSuper {
170 public:
171 G1EdenPool(G1CollectedHeap* g1h);
172
173 size_t used_in_bytes() {
174 return eden_space_used(_g1h);
175 }
176 size_t max_size() const {
177 return undefined_max();
178 }
179 MemoryUsage get_memory_usage();
180 };
181
182 // Memory pool that represents the G1 survivor.
183 class G1SurvivorPool : public G1MemoryPoolSuper {
184 public:
185 G1SurvivorPool(G1CollectedHeap* g1h);
186
187 size_t used_in_bytes() {
188 return survivor_space_used(_g1h);
189 }
190 size_t max_size() const {
191 return undefined_max();
192 }
193 MemoryUsage get_memory_usage();
194 };
195
196 // Memory pool that represents the G1 old gen.
197 class G1OldGenPool : public G1MemoryPoolSuper {
198 public:
199 G1OldGenPool(G1CollectedHeap* g1h);
200
201 size_t used_in_bytes() {
202 return old_space_used(_g1h);
203 }
204 size_t max_size() const {
205 return undefined_max();
206 }
207 MemoryUsage get_memory_usage();
208 };
209
210 #endif // SHARE_VM_SERVICES_G1MEMORYPOOL_HPP