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From "Jeffrey Janner" <Jeffrey.Jan...@PolyDyne.com>
Subject RE: PermGen question
Date Wed, 20 Jan 2010 20:24:47 GMT
Thanks Charles.  I found that later in a link I got from Google.  I'd
found lots of references to how to set MaxPermGen, but I already knew
that.  I also found lots of references that the default was 64Mb, which
didn't jive with the 86Mb that one instance had.  Perhaps the 64bit
server default is a good bit higher than the 32bit version.  Plus I was
reading it, erroneously, as the starting value.  
I think I'll stick to searching the SUN docs on this stuff like I used
to.  I googled thinking it would probably get me to some Sun docs
quickly, but it's returning a lot of other links.

Yea, if I'm really worried about it, I watch it with jconsole. I had
just found it interesting on reviewing the thread dumps - it looked odd.
Yes, the two loads are vastly different, different # users, different
dataset sizes, different usage patterns.  From my support folks, I hear
that instance 1 has probably been lightly used the last 2 days, but will
probably be having more usage today/tomorrow - and heavy data pulls at
that.

The numbers will probably be vastly different at the end of the day
tomorrow.

Thanks again.

-----Original Message-----
From: Caldarale, Charles R [mailto:Chuck.Caldarale@unisys.com] 
Sent: Wednesday, January 20, 2010 1:08 PM
To: Tomcat Users List
Subject: RE: PermGen question

> From: Jeffrey Janner [mailto:Jeffrey.Janner@PolyDyne.com]
> 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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