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From "Caldarale, Charles R" <>
Subject RE: PermGen question
Date Wed, 20 Jan 2010 19:08:16 GMT
> From: Jeffrey Janner []
> Subject: PermGen question
> Instance 1:
>  PSPermGen       total 43712K, used 43613K [0x000000007fff0000,
> 0x0000000082aa0000, 0x00000000853f0000)
>   object space 43712K, 99% used
> [0x000000007fff0000,0x0000000082a875d8,0x0000000082aa0000)
> Instance 2:
>  PSPermGen       total 86016K, used 45979K [0x000000007fff0000,
> 0x00000000853f0000, 0x00000000853f0000)
>   object space 86016K, 53% used
> [0x000000007fff0000,0x0000000082cd6ef8,0x00000000853f0000)

> Obviously, I'm concerned about that 99% used value in the first
> instance, and will be going back to address it.

It is likely not a problem.  That's 99% used of what's allocated, not the maximum - 99% is
a good thing (high efficiency).  I'd be more concerned about Instance 2, where PermGen expanded
to 86MB at one point.

> However, I would have thought that it would have the same 
> size as instance 2.

Unless they have identical workloads with identical requests, there will be variance.

> Can someone explain what's happening here that I'm not seeing?

Nothing unusual is going on.

> I didn't think that the PermGen grew or shrunk over time.

You thought wrong.  There are parameters for both initial and maximum PermGen size; unless
they're explicitly set to the same value, PermGen will expand and contract, just like the
Java heap.

It's often more illuminating to use JConsole or JVisualVM to monitor heap and PermGen usage
over a significant time period, rather than looking at snapshots.

 - Chuck

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