Linux下Java进程RES是1.6G,但是jmap里用到的才五百多M，剩下的1.1G左右是去哪了？

Question

具体信息如下：
RES是1.6G

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND  
28527 root      20   0 12.1g 1.6g  13m S  0.0  5.0   0:52.19 java 

但是used:堆用了262M， concurrent mark-sweep generation用了48M，interned String占了250多M

Attaching to process ID 28527, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.51-b03

using parallel threads in the new generation.
using thread-local object allocation.
Concurrent Mark-Sweep GC

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 8589934592 (8192.0MB)
   NewSize                  = 4294967296 (4096.0MB)
   MaxNewSize               = 4294967296 (4096.0MB)
   OldSize                  = 4294967296 (4096.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
New Generation (Eden + 1 Survivor Space):
   capacity = 3865509888 (3686.4375MB)
   used     = 274884512 (262.1502990722656MB)
   free     = 3590625376 (3424.2872009277344MB)
   7.1112096454169% used
Eden Space:
   capacity = 3436052480 (3276.875MB)
   used     = 274884512 (262.1502990722656MB)
   free     = 3161167968 (3014.7247009277344MB)
   8.000009126752337% used
From Space:
   capacity = 429457408 (409.5625MB)
   used     = 0 (0.0MB)
   free     = 429457408 (409.5625MB)
   0.0% used
To Space:
   capacity = 429457408 (409.5625MB)
   used     = 0 (0.0MB)
   free     = 429457408 (409.5625MB)
   0.0% used
concurrent mark-sweep generation:
   capacity = 4294967296 (4096.0MB)
   used     = 50954384 (48.59388732910156MB)
   free     = 4244012912 (4047.4061126708984MB)
   1.1863742023706436% used

22283 interned Strings occupying 2737728 bytes.

Answer 1

用top中查看RES是操作系统角度看jvm的内存占用。

用jmap查看的堆内存，是用jvm的角度看jvm内部程序的内存占用。

存在差异是因为jvm有一些共享库和共享内存，被操作系统计入RES中，但未被jvm计入。