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From "Shapira, Yoav" <Yoav.Shap...@mpi.com>
Subject RE: OutOfMemoryError
Date Wed, 06 Aug 2003 16:51:53 GMT

Howdy,

>Does any one know of a neat way of monitoring memory usage, so that our
>servlet can decide to refuse a request if it senses that memory is
about
>to run out?

You have the two calls in the Runtime class (totalMemory() and
freeMemory()), and that's pretty much it.  This of course reflects only
the heap.  So you can have a background thread that checks freeMemory()
every so often and if the free percentage falls below a certain
configured threshold, sets some global flag variable that servlets (or
better yet a simple filter) consults when deciding whether to allow
servlet requests.

>free Memory = 172544, totalMemory = 6.6650112E7, maxMemory =
>1.34217728E8

How were the above obtained?

>and I don't understand why the JVM didn't try to grab more memory when
>it could - is it because one of tomcat's threads unexpectedly ran out
of
>memory that it should have been allocated?

The JVM is constricted by how you configure it, e.g. the -Xmx parameter.
If not specified, the JVM will allocate up to 64MB to the heap.

>thanks for any suggestions,

This is a fragile situation with or without the above solution.
Benchmark your system to know how many max concurrent requests you can
support given your hardware, and make sure your customers/users know
this number.

You may also want to consider a script to monitor your log for
OutOfMemoryError messages and restart tomcat or notify admins in this
event.

Yoav Shapira





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