How can a Java process with -Xmx1024m occupy 3GB resident memory?

Question

It's a Java web application on Websphere6.1, Solaris 10, JDK 1.5.0_13. We set the maximum heap size to 1024m. jmap shows the heap status is healthy. The heap memory usage is only 57%. No OutOfMemory at all.

But we saw very high RSS (3GB) for this java process from ps. pmap shows a block of 1.9G private memory.

3785:   /dmwdkpmmkg/was/610/java/bin/java -server -Dwas.status.socket=65370 -X
 Address  Kbytes     RSS    Anon  Locked Pgsz Mode   Mapped File
...
0020A000    2008    2008    2008       -   8K rwx--    [ heap ]
00400000 1957888 1957888 1957888       -   4M rwx--    [ heap ]
8D076000      40      40      40       -   8K rw--R    [ stack tid=10786 ]
...

Is it a C heap memory leak in native code? What approach is recommended to find out the root cause?

Answer 1

Are you using JNI libraries? I'm not sure how native code allocates RAM but that's where I'd start looking.

Answer 2

The heap size os the Java heap size, there is still the VM and other libraries which are part of the process.

Try running Hello World with a 1024m heap size and a "for(;;)" in it and see how much it takes up. That should give you a baseline for overall memory usage.

Answer 3

This Troubleshooting Memory Leaks document from Sun may help you with finding the problem why your high RSS, specially in section 3.4.

As you are running Websphere, maybe you can use the -memorycheck on your VM. For details see here.

It's not necessarily a leak in native code. If you look here, on Solaris there might be an issue with files being kept open.

It's just a bunch of links and hints, but maybe helpful to track down your problem.