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From "Caldarale, Charles R" <>
Subject RE: Tomcat memory allocation
Date Fri, 09 Dec 2011 14:45:33 GMT
> From: Martin O'Shea [] 
> Subject: Tomcat memory allocation

> Following advice found elsewhere on the internet

Always to be taken with large chunks of salt.

> set JAVA_OPTS=%JAVA_OPTS% -Xms128m -Xmx512m -XX:MaxPermSize=128m

You would be better off using CATALINA_OPTS, since setting JAVA_OPTS pointlessly affects the
shutdown script as well as the startup one.

> I know that settings:
> Xms128m -Xmx512m

> Control the initial heap size and what it can expand to.

In a server environment, you normally want Xms and Xmx set to the same value to avoid heap
thrashing.  The exact size is completely dependent on what your webapps need.

> But what exactly is:
> -XX:MaxPermSize=128m

It's the amount of space to which the so-called permanent generation can expand.  PermGen
holds primarily instances of java.lang.Class, so it only needs to be specified if you have
a large number of classes in your environment.

> Should it be set to an addition of the other settings,
> or the other settings to an addition of it?

What does that question mean?  PermGen size is completely independent of the heap size.

Make sure you have enough RAM available on the system to support the Xmx + PermGen + a_lot_of_other_stuff.
 Monitor the system to make sure you're not getting into paging.

 - Chuck

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