为什么JVM消耗的内存比-Xms指定的少?

Why does the JVM consume less memory than -Xms specified?(为什么JVM消耗的内存比-Xms指定的少?)

问题描述

我的问题如题，通过搜索了解了一些知识:

My question is as the title, and I got some knowledge by searching:

1. linux 有共享内存如何测量一个应用程序的实际内存使用情况申请或流程?

JVM会保留Xms中设置的内存量-Xms JVM 是什么意思堆内存?

JVM will reserve the amount of memory setted in Xms What does the -Xms JVM mean in reference to heap memory?

但还是不知道为什么，有的可以帮忙吗?

But still don't know why, some on can offer some help?

这是我在 Ubuntu12.04(64bit) 上运行的测试 |JDK 1.7.0_04.顶部显示如下:

Here is my test run on Ubuntu12.04(64bit) | JDK 1.7.0_04. and top shows below:

  PID USER      PR  NI  VIRT  RES       SHR S %CPU %MEM    TIME+  COMMAND
 4067 brian     20   0 5316m **262m** 7496 S    0  3.3   0:00.30 java -**Xms4096m** -Xmx4096m Test
 4066 brian     20   0 3182m **256m** 7496 S    0  3.3   0:00.30 java -**Xms2048m** -Xmx2048m Test
 4065 brian     20   0 2114m **252m** 7492 S    0  3.2   0:00.30 java -**Xms1024m** -Xmx1024m Test
 4064 brian     20   0 1314m  **76m** 7584 S    0  1.0   0:00.20 java -**Xms256m** -Xmx256m Test
 4063 brian     20   0 1180m  **51m** 7608 S    0  0.7   0:00.21 java -**Xms128m** -Xmx128m Test

推荐答案

您正在查看驻留内存 - 即消耗的物理 RAM.请参阅此处了解更多信息.

You're looking at the resident memory - that is, the physical RAM consumed. See here for more info.

虚拟内存是应用程序消耗的内存，包括换出(到磁盘)的内存.您会看到与虚拟内存和您的 -Xms 设置有更密切的对应关系.

The virtual memory, however, is the memory consumed by your application, including the memory swapped out (to disk). You'll see there's a closer correspondance with the virtual memory and your -Xms settings.

有关详细信息，请参阅 此 ServerFault 答案.

See this ServerFault answer for more info.

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