为什么在linux&jdk1.6.0嫒32中jvm内存不能自动扩展到xmx3072m？
终身世代使用率为99%，使用频率非常高。正如top的输出所示，java进程只使用1g内存。
如果我们将jvm参数更改为-Xms3072m -Xmx3072m，那么它可以正常工作。但是如果我们使用-Xms256m -Xmx3072m，那么问题就会出现，fullgc会频繁出现。
这两个参数也是默认值：
MinHeapFreeRatio 40
MaxHeapFreeRatio 70
环境是：
OS:
Linux linux-wgvb 2.6.16.60-0.54.5-smp #1 SMP Fri Sep 4 01:28:03 UTC 2009 x86_64 x86_64 x86_64 GNU/Linux
JDK version:
java version "1.6.0_32"
Java(TM) SE Runtime Environment (build 1.6.0_32-b05)
Java HotSpot(TM) 64-Bit Server VM (build 20.7-b02, mixed mode)
JVM params:
-Xms256m -Xmx3072m -XX:+UseParallelOldGC -XX:MaxGCPauseMillis=1000
-XX:ParallelGCThreads=5 -Xrs -XX:PermSize=64m -XX:MaxPermSize=512m
-XX:+HeapDumpOnOutOfMemoryError
fullgc经常执行：
@linux> ./jstat -gcutil 18261 1000 500
S0 S1 E O P YGC YGCT FGC FGCT GCT
0.00 64.67 7.04 99.53 71.89 18293 510.574 1167 1616.594 2127.168
91.78 7.91 100.00 99.53 72.00 18299 510.821 1167 1616.594 2127.415
77.49 0.00 0.00 99.99 72.00 18300 510.947 1168 1616.594 2127.541
0.00 99.83 25.71 99.49 71.72 18305 511.178 1168 1617.915 2129.093
24.27 0.00 21.74 99.67 71.73 18310 511.342 1168 1617.915 2129.256
28.57 0.00 0.00 99.83 71.73 18314 511.427 1169 1617.915 2129.342
0.00 29.29 0.00 99.56 71.82 18317 511.465 1169 1618.919 2130.384
0.00 78.85 54.96 99.56 72.05 18326 511.777 1169 1618.919 2130.696
64.52 0.00 0.00 99.99 72.05 18328 511.920 1170 1618.919 2130.839
0.00 99.26 0.00 99.88 71.71 18333 512.079 1171 1620.014 2132.093
4.86 0.00 0.00 99.95 71.76 18338 512.255 1172 1620.957 2133.211
80.98 0.00 0.00 99.69 71.83 18350 512.453 1172 1621.906 2134.359
0.00 64.48 0.00 99.85 71.83 18355 512.677 1173 1621.906 2134.583
0.00 99.80 0.00 99.93 71.71 18359 512.876 1174 1623.264 2136.139
0.06 0.04 100.00 99.55 71.71 18360 512.876 1174 1624.193 2137.069
60.35 0.00 0.00 99.90 71.77 18376 513.429 1175 1624.193 2137.622
60.35 0.00 0.00 99.90 71.77 18376 513.429 1175 1624.193 2137.622
89.86 0.00 0.00 99.83 71.78 18384 513.839 1176 1625.476 2139.315
89.86 0.00 0.00 99.83 71.78 18384 513.839 1176 1625.476 2139.315
0.00 39.17 64.41 99.60 71.78 18394 514.202 1176 1626.880 2141.082
99.93 0.00 0.00 99.82 71.78 18396 514.326 1177 1626.880 2141.206
0.00 64.68 0.00 99.61 71.72 18401 514.409 1177 1627.945 2142.354
Java进程仅使用1G内存：
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
18261 ouser 15 0 4353m 1.0g 52m S 325 3.2 271:25.52 java
./jstat-总容量18261 1000 500
ngcmn ngcmx ngc s0c s1c ec ogcmn ogcmx ogc oc pgcmn pgcmx pgc pc ygc fgc
87360.0 1048576.0 136256.0 65984.0 68096.0 64.0 174784.0 2097152.0 785664.0 785664.0 65536.0 524288.0 65792.0 65792.0 141612 8100
87360.0 1048576.0 122816.0 61376.0 12992.0 64.0 174784.0 2097152.0 791360.0 791360.0 65536.0 524288.0 65792.0 65792.0 141616 8100
87360.0 1048576.0 115264.0 57600.0 7872.0 64.0 174784.0 2097152.0 791936.0 791936.0 65536.0 524288.0 65792.0 65792.0 141617 8101
87360.0 1048576.0 112576.0 54912.0 56256.0 64.0 174784.0 2097152.0 797952.0 797952.0 65536.0 524288.0 65792.0 65792.0 141618 8102
87360.0 1048576.0 112576.0 54912.0 56256.0 64.0 174784.0 2097152.0 797952.0 797952.0 65536.0 524288.0 65792.0 65792.0 141618 8102
87360.0 1048576.0 105408.0 52672.0 192.0 64.0 174784.0 2097152.0 804096.0 804096.0 65536.0 524288.0 65792.0 65792.0 141621 8103
87360.0 1048576.0 87360.0 35584.0 38464.0 8064.0 174784.0 2097152.0 805568.0 805568.0 65536.0 524288.0 65792.0 65792.0 141629 8103
87360.0 1048576.0 111168.0 39488.0 46080.0 12416.0 174784.0 2097152.0 805568.0 805568.0 65536.0 524288.0 65792.0 65792.0 141635 8103
87360.0 1048576.0 142848.0 69440.0 70592.0 1664.0 174784.0 2097152.0 805568.0 805568.0 65536.0 524288.0 65792.0 65792.0 141644 8104
87360.0 1048576.0 142848.0 69440.0 70592.0 1664.0 174784.0 2097152.0 805568.0 805568.0 65536.0 524288.0 65792.0 65792.0 141644 8104
87360.0 1048576.0 87360.0 35136.0 37376.0 10304.0 174784.0 2097152.0 769280.0 769280.0 65536.0 524288.0 65792.0 65792.0 141661 8105
最佳答案
RES的值不是进程使用了多少内存，而是它实际使用了多少内存。这不包括交换的页面和已分配但未映射的页面。确保系统本身有足够的可用内存，您可以运行free，甚至top应该在使用内存量之上打印。top的输出表明该进程映射了4g内存，但当前不在ram中（值为VIRT）。