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POHow can a 1Gb Java heap on a 64bit machine use 3Gb of VIRT space?
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copied!<p>I run the same process on a 32bit machine as on a 64bit machine with the same memory VM settings (-Xms1024m -Xmx1024m) and similar VM version (1.6.0_05 vs 1.6.0_16). However the virtual space used by the 64bit machine (as shown in top under "VIRT") is almost three times as big as that in 32bit!</p> <p>I know 64bit VMs will use a little more memory for the larger references, but how can it be three times as big? Am I reading VIRT in top incorrectly?</p> <p>Full data shown below, showing top and then the result of jmap -heap, first for 64bit, then for 32bit. Note the VIRT for 64bit is 3319m for 32bit is 1220m.</p> <p><strong>* 64bit *</strong></p> <pre> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 22534 agent 20 0 3319m 163m 14m S 4.7 2.0 0:04.28 java $ jmap -heap 22534 Attaching to process ID 22534, please wait... Debugger attached successfully. Server compiler detected. JVM version is 10.0-b19 using thread-local object allocation. Parallel GC with 4 thread(s) Heap Configuration: MinHeapFreeRatio = 40 MaxHeapFreeRatio = 70 MaxHeapSize = 1073741824 (1024.0MB) NewSize = 2686976 (2.5625MB) MaxNewSize = -65536 (-0.0625MB) OldSize = 5439488 (5.1875MB) NewRatio = 2 SurvivorRatio = 8 PermSize = 21757952 (20.75MB) MaxPermSize = 88080384 (84.0MB) Heap Usage: PS Young Generation Eden Space: capacity = 268500992 (256.0625MB) used = 247066968 (235.62142181396484MB) free = 21434024 (20.441078186035156MB) 92.01715277089181% used From Space: capacity = 44695552 (42.625MB) used = 0 (0.0MB) free = 44695552 (42.625MB) 0.0% used To Space: capacity = 44695552 (42.625MB) used = 0 (0.0MB) free = 44695552 (42.625MB) 0.0% used PS Old Generation capacity = 715849728 (682.6875MB) used = 0 (0.0MB) free = 715849728 (682.6875MB) 0.0% used PS Perm Generation capacity = 21757952 (20.75MB) used = 16153928 (15.405586242675781MB) free = 5604024 (5.344413757324219MB) 74.24378912132907% used </pre> <p><strong>* 32bit *</strong></p> <pre> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 30168 agent 20 0 1220m 175m 12m S 0.0 2.2 0:13.43 java $ jmap -heap 30168 Attaching to process ID 30168, please wait... Debugger attached successfully. Server compiler detected. JVM version is 14.2-b01 using thread-local object allocation. Parallel GC with 8 thread(s) Heap Configuration: MinHeapFreeRatio = 40 MaxHeapFreeRatio = 70 MaxHeapSize = 1073741824 (1024.0MB) NewSize = 1048576 (1.0MB) MaxNewSize = 4294901760 (4095.9375MB) OldSize = 4194304 (4.0MB) NewRatio = 8 SurvivorRatio = 8 PermSize = 16777216 (16.0MB) MaxPermSize = 67108864 (64.0MB) Heap Usage: PS Young Generation Eden Space: capacity = 89522176 (85.375MB) used = 80626352 (76.89128112792969MB) free = 8895824 (8.483718872070312MB) 90.0629940005033% used From Space: capacity = 14876672 (14.1875MB) used = 14876216 (14.187065124511719MB) free = 456 (4.3487548828125E-4MB) 99.99693479832048% used To Space: capacity = 14876672 (14.1875MB) used = 0 (0.0MB) free = 14876672 (14.1875MB) 0.0% used PS Old Generation capacity = 954466304 (910.25MB) used = 10598496 (10.107513427734375MB) free = 943867808 (900.1424865722656MB) 1.1104107034039412% used PS Perm Generation capacity = 16777216 (16.0MB) used = 11366448 (10.839889526367188MB) free = 5410768 (5.1601104736328125MB) 67.74930953979492% used </pre>

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