after quiting java (Grails) app, “used” memory doesn't free up

Question

Using Linux 3.2.0-24-virtual #37-Ubuntu SMP Wed Apr 25 12:51:49 UTC 2012 i686 i686 i386 GNU/Linux in a Xen VPS.

Before I run a Grails java app, running free -m indicates:

             total       used       free     shared    buffers     cached
Mem:          4031        155       3875          0         30         87
-/+ buffers/cache:         37       3994
Swap:         8191          0       8191

After running the Grails app, performing some memory intensive processing, and quitting via Ctrl-C, it indicates:

             total       used       free     shared    buffers     cached
Mem:          4031       1031       2999          0         33        957
-/+ buffers/cache:         40       3991
Swap:         8191          0       8191

which doesn't seem to change until a reboot. top and ps aux don't indicate any heavy mem usage. The Grails app is the only app I run on this machine.

Perhaps I'm misinterpreting this, but I'm concerned that memory doesn't seem to be freeing post-quit. Normally I'd suspect a memory leak, but how is that possible once the java app has been terminated?

Answer 1

You are actually misinterpreting the output of the command.

The Mem line shows how much memory is currently allocated by the kernel; however, the kernel releases memory lazily. If no other process is requesting memory, it might keep allocated pages (maybe stuff read from disk).

The row where you see the "clean" used/free memory is the -/+ buffers/cache row, where you can see that your memory usage is approximately the same.

You can find some more detailed explanation on the web, e.g. here.