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 ?
Have you tried running Jmap using
-heap:formatoption 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).