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From "Caldarale, Charles R" <Chuck.Caldar...@unisys.com>
Subject RE: Setting the Right Amount of Memory
Date Mon, 21 Jun 2010 04:14:11 GMT
> From: Robinson, Eric [mailto:eric.robinson@psmnv.com]
> Subject: RE: Setting the Right Amount of Memory
> 
> What qualifies as a tight heap and what qualifies as a 
> significant increase?

Both are entirely dependent on what's running inside the JVM.  Monitoring the GC actions will
tell you if you're close to being tight.

> Usually when we see OOMEs we increase the allocation by
> 32MB and they go away.

>From your descriptions, none of your webapps are memory-intensive, so 32 MB is likely
a "significant" increase in the heap size - in your case.

> is it generally better to custom-fit the RAM allocation 
> (as we currently do) or to unilaterally set all instances
> to a higher amount that we know will not generate OOMEs,
> such as 512MB?

Depends on how much time you've got to spend on the problem, and how much RAM you've got.
 If the RAM is available, it's certainly easier to set the heap size large for everyone and
let it rip.  If you are constrained such that doing so will result in an overcommitment of
RAM (or you like things to be just right), then you need to individually tune.  You might
actually have a situation where it would work to set -Xms small (16 or 32 MB) and -Xmx large
(512 MB), and let each JVM figure out what it really needs.

 - Chuck


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