Understanding java memory usage

Question

I am trying to solve a memory issue I am having with my tomcat servers and I have some questions about memory usage. When I check my process memory usage with top I see its using 1Gb physical memory, after creating a core dump using gdb, the core file size is 2.5GB , and when analyzing the HPROF file created by jmap , it states that 240MB is used.

So if top shows 1GB why does the hprof file show only 240MB where did 760MB go ?

Is 240MB shown as Heap memory? If not, what is the Used Heap in it? Can you provide some details on the numbers provided by jmap — Kishore Bandi, Nov 15 '18 at 18:46
I think so, this is the total size shown in Eclipse Memory Analyzer — jacob, Nov 15 '18 at 18:48
If you only dump live objects this can be a fraction of the heap which is a fraction of the memory the process uses. — Peter Lawrey, Nov 15 '18 at 19:00

score 0 · Answer 1 · answered Nov 15 '18 at 19:15

Have you tried running Jmap using -heap:format option set? JVM usually runs a GC before taking a dump.

Also, JVM memory is not just Heap Memory. It contains Code, Stack, Native method, Direct memory, even Threads are not free to use. you could read more about it here. Just make sure to see if all these also add up to it.

I would suggest using VisualVM or yourkit and compare the memory. Also, which GC are you using?
Some GC's don't usually shrink the heap memory after increasing, but if GC got triggered during heapdump it might have freed up some memory(Try G1GC).

the heap dump was created from a core dump file ( created by gdb ) doesnt this mean that there was no GC before heap dump ? — jacob, Nov 15 '18 at 19:35

Understanding java memory usage

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