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From Peter Crowther <>
Subject Re: GC(JVM Heap usage) tool
Date Mon, 01 Mar 2010 09:56:49 GMT
On 1 March 2010 08:30, Paulwintech <> wrote:
>  I need to check how many threads are running and its memory utilization in
> tomcat6, I used jstack command to get thread details - but not able to get
> memory usage details.
>  Recently i get free memory critical for the particular time from tomcat6
> and automatically get resolve after some time.I am not able to troubleshoot
> the exact issue causing free memory critical - So to troubleshoot and to
> find the exact reason - i wanted to know how to check the number of threads
> are currently running and its memory usage.
>   After knowing the exact thread - i can check why that particular thread
> occupies more memory. If im wrong please let me know any other ways to check
> the issue.

Much of the memory use is not thread-specific.  For example, Session
objects that are held for sessions that don't currently have a request
are not associated with any thread.  So I think you will have problems
checking why particular threads occupy more memory.

Also... how do you know your monitoring system isn't giving false
alarms?  A garbage collector should only run when it needs to - when
the JVM doesn't have enough free memory to satisfy the memory it
expects to use before the GC completes.  So I would expect free memory
to decrease to a very low amount, then the GC will run and free memory
will increase again.  Are you sure you understand what your monitoring
system is telling you, and once you know you understand it are you
sure you want to respond to that particular alert?

- Peter

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