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. 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 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 };