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From "Caldarale, Charles R" <Chuck.Caldar...@unisys.com>
Subject RE: tomcat memory usage
Date Wed, 27 Jan 2010 16:28:15 GMT
> From: Christopher Schultz [mailto:chris@christopherschultz.net]
> Subject: Re: tomcat memory usage
> 
> All Java objects and their associated data are stored 
> within the heap.

Well... no.  The OP's original question was about what was outside the -Xmx heap setting,
and Class objects and the associated static fields are /not/ included in that value.  Instances
of java.lang.Class and the static fields are kept in PermGen, which is separate from the regular
Java heap, and sized independently of -Xmx.

> Unless you are writing JNI code, then everything you are
> effecting is happening in the heap.

Also not strictly true, since a major chunk of the JVM and JRE are native code, and consume
considerable amounts of process space to implement the Java operations.

> Static variables and references (which are the same thing,
> depending on how you see things) are not special in any way
> when it comes to memory use.

Other than their location outside of the -Xmx-controlled heap.

> It's the JVM itself which is allocating memory outside that heap.

And the OS and the various libraries used by the JVM.

> Many DLLs loaded into a process's memory space are, in fact, shared
> with many other processes.

They're shared in real memory, but not process space (virtual memory).

> the fact that WTM shows you only a single number for "memory" 
> means that you're not getting the whole story.

The Task Manager can actually show several memory-related values, including virtual memory
and various working set sizes.  What's available to display depends heavily on the version
of Windows one's using.

> So, maybe WTM is giving you a decent measure of what memory is actually
> being used by that process, specifically.

Yes, it's pretty decent these days.  Still has nothing to do with the Java heap, of course.

 - Chuck


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