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1 /*
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2 * Copyright (c) 2007 Sun Microsystems, Inc. All Rights Reserved.
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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4 *
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5 * This code is free software; you can redistribute it and/or modify it
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6 * under the terms of the GNU General Public License version 2 only, as
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7 * published by the Free Software Foundation.
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8 *
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9 * This code is distributed in the hope that it will be useful, but WITHOUT
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10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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12 * version 2 for more details (a copy is included in the LICENSE file that
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13 * accompanied this code).
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14 *
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15 * You should have received a copy of the GNU General Public License version
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16 * 2 along with this work; if not, write to the Free Software Foundation,
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17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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18 *
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19 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
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20 * CA 95054 USA or visit www.sun.com if you need additional information or
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21 * have any questions.
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22 *
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23 */
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24
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25 class G1CollectedHeap;
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26
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27 // This file contains the three classes that represent the memory
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28 // pools of the G1 spaces: G1EdenPool, G1SurvivorPool, and
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29 // G1OldGenPool. In G1, unlike our other GCs, we do not have a
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30 // physical space for each of those spaces. Instead, we allocate
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31 // regions for all three spaces out of a single pool of regions (that
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32 // pool basically covers the entire heap). As a result, the eden,
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33 // survivor, and old gen are considered logical spaces in G1, as each
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34 // is a set of non-contiguous regions. This is also reflected in the
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35 // way we map them to memory pools here. The easiest way to have done
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36 // this would have been to map the entire G1 heap to a single memory
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37 // pool. However, it's helpful to show how large the eden and survivor
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38 // get, as this does affect the performance and behavior of G1. Which
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39 // is why we introduce the three memory pools implemented here.
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40 //
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41 // The above approach inroduces a couple of challenging issues in the
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42 // implementation of the three memory pools:
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43 //
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44 // 1) The used space calculation for a pool is not necessarily
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45 // independent of the others. We can easily get from G1 the overall
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46 // used space in the entire heap, the number of regions in the young
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47 // generation (includes both eden and survivors), and the number of
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48 // survivor regions. So, from that we calculate:
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49 //
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50 // survivor_used = survivor_num * region_size
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51 // eden_used = young_region_num * region_size - survivor_used
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52 // old_gen_used = overall_used - eden_used - survivor_used
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53 //
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54 // Note that survivor_used and eden_used are upper bounds. To get the
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55 // actual value we would have to iterate over the regions and add up
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56 // ->used(). But that'd be expensive. So, we'll accept some lack of
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57 // accuracy for those two. But, we have to be careful when calculating
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58 // old_gen_used, in case we subtract from overall_used more then the
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59 // actual number and our result goes negative.
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60 //
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61 // 2) Calculating the used space is straightforward, as described
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62 // above. However, how do we calculate the committed space, given that
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63 // we allocate space for the eden, survivor, and old gen out of the
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64 // same pool of regions? One way to do this is to use the used value
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65 // as also the committed value for the eden and survivor spaces and
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66 // then calculate the old gen committed space as follows:
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67 //
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68 // old_gen_committed = overall_committed - eden_committed - survivor_committed
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69 //
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70 // Maybe a better way to do that would be to calculate used for eden
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71 // and survivor as a sum of ->used() over their regions and then
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72 // calculate committed as region_num * region_size (i.e., what we use
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73 // to calculate the used space now). This is something to consider
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74 // in the future.
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75 //
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76 // 3) Another decision that is again not straightforward is what is
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77 // the max size that each memory pool can grow to. Right now, we set
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78 // that the committed size for the eden and the survivors and
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79 // calculate the old gen max as follows (basically, it's a similar
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80 // pattern to what we use for the committed space, as described
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81 // above):
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82 //
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83 // old_gen_max = overall_max - eden_max - survivor_max
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84 //
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85 // 4) Now, there is a very subtle issue with all the above. The
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86 // framework will call get_memory_usage() on the three pools
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87 // asynchronously. As a result, each call might get a different value
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88 // for, say, survivor_num which will yield inconsistent values for
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89 // eden_used, survivor_used, and old_gen_used (as survivor_num is used
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90 // in the calculation of all three). This would normally be
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91 // ok. However, it's possible that this might cause the sum of
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92 // eden_used, survivor_used, and old_gen_used to go over the max heap
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93 // size and this seems to sometimes cause JConsole (and maybe other
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94 // clients) to get confused. There's not a really an easy / clean
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95 // solution to this problem, due to the asynchrounous nature of the
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96 // framework.
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97
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98
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99 // This class is shared by the three G1 memory pool classes
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100 // (G1EdenPool, G1SurvivorPool, G1OldGenPool). Given that the way we
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101 // calculate used / committed bytes for these three pools is related
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102 // (see comment above), we put the calculations in this class so that
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103 // we can easily share them among the subclasses.
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104 class G1MemoryPoolSuper : public CollectedMemoryPool {
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105 private:
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106 G1CollectedHeap* _g1h;
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107
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108 // It returns x - y if x > y, 0 otherwise.
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109 // As described in the comment above, some of the inputs to the
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110 // calculations we have to do are obtained concurrently and hence
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111 // may be inconsistent with each other. So, this provides a
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112 // defensive way of performing the subtraction and avoids the value
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113 // going negative (which would mean a very large result, given that
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114 // the parameter are size_t).
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115 static size_t subtract_up_to_zero(size_t x, size_t y) {
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116 if (x > y) {
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117 return x - y;
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118 } else {
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119 return 0;
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120 }
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121 }
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122
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123 protected:
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124 // Would only be called from subclasses.
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125 G1MemoryPoolSuper(G1CollectedHeap* g1h,
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126 const char* name,
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127 size_t init_size,
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128 size_t max_size,
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129 bool support_usage_threshold);
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130
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131 // The reason why all the code is in static methods is so that it
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132 // can be safely called from the constructors of the subclasses.
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133
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134 static size_t overall_committed(G1CollectedHeap* g1h) {
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135 return g1h->capacity();
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136 }
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137 static size_t overall_used(G1CollectedHeap* g1h) {
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138 return g1h->used_unlocked();
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139 }
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140
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141 static size_t eden_space_committed(G1CollectedHeap* g1h);
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142 static size_t eden_space_used(G1CollectedHeap* g1h);
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143 static size_t eden_space_max(G1CollectedHeap* g1h);
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144
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145 static size_t survivor_space_committed(G1CollectedHeap* g1h);
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146 static size_t survivor_space_used(G1CollectedHeap* g1h);
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147 static size_t survivor_space_max(G1CollectedHeap* g1h);
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148
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149 static size_t old_space_committed(G1CollectedHeap* g1h);
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150 static size_t old_space_used(G1CollectedHeap* g1h);
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151 static size_t old_space_max(G1CollectedHeap* g1h);
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152
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153 // The non-static versions are included for convenience.
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154
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155 size_t eden_space_committed() {
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156 return eden_space_committed(_g1h);
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157 }
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158 size_t eden_space_used() {
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159 return eden_space_used(_g1h);
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160 }
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161 size_t eden_space_max() {
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162 return eden_space_max(_g1h);
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163 }
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164
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165 size_t survivor_space_committed() {
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166 return survivor_space_committed(_g1h);
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167 }
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168 size_t survivor_space_used() {
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169 return survivor_space_used(_g1h);
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170 }
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171 size_t survivor_space_max() {
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172 return survivor_space_max(_g1h);
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173 }
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174
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175 size_t old_space_committed() {
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176 return old_space_committed(_g1h);
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177 }
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178 size_t old_space_used() {
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179 return old_space_used(_g1h);
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180 }
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181 size_t old_space_max() {
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182 return old_space_max(_g1h);
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183 }
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184 };
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185
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186 // Memory pool that represents the G1 eden.
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187 class G1EdenPool : public G1MemoryPoolSuper {
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188 public:
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189 G1EdenPool(G1CollectedHeap* g1h);
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190
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191 size_t used_in_bytes() {
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192 return eden_space_used();
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193 }
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194 size_t max_size() {
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195 return eden_space_max();
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196 }
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197 MemoryUsage get_memory_usage();
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198 };
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199
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200 // Memory pool that represents the G1 survivor.
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201 class G1SurvivorPool : public G1MemoryPoolSuper {
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202 public:
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203 G1SurvivorPool(G1CollectedHeap* g1h);
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204
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205 size_t used_in_bytes() {
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206 return survivor_space_used();
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207 }
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208 size_t max_size() {
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209 return survivor_space_max();
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210 }
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211 MemoryUsage get_memory_usage();
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212 };
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213
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214 // Memory pool that represents the G1 old gen.
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215 class G1OldGenPool : public G1MemoryPoolSuper {
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216 public:
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217 G1OldGenPool(G1CollectedHeap* g1h);
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218
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219 size_t used_in_bytes() {
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220 return old_space_used();
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221 }
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222 size_t max_size() {
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223 return old_space_max();
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224 }
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225 MemoryUsage get_memory_usage();
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226 };
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