g1MemoryPool.hpp revision 1879:f95d63e2154a
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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 33class 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. 118class G1MemoryPoolSuper : public CollectedMemoryPool { 119private: 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 135protected: 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. 169class G1EdenPool : public G1MemoryPoolSuper { 170public: 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. 183class G1SurvivorPool : public G1MemoryPoolSuper { 184public: 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. 197class G1OldGenPool : public G1MemoryPoolSuper { 198public: 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 211