Weird behavior with Tomcat

I think I've mentioned this specific customer-box situation before: really slow performance with our webapp.

This happened with Tomcat running in a subsystem, with a 4096M private pool, and the JVM params including -Xms512m and -Xmx2048m (heap size). They're running under V7R1, with a 1.6 JVM.

On investigation, I found that the GC was going to town like I'd never seen before, e.g:

Number Start End Freed
779 2017.8M 1197.1M 779.9M
778 2001.6M 1277.7M 723.9M
777 2009.4M 1181.0M 828.3M
776 1993.4M 1248.8M 744.6M
775 1184.7M 1172.1M 12.6M
774 1215.2M 1172.4M 42.8M
773 1177.8M 1171.8M 5.9M
772 1738.0M 1173.0M 565.0M

So I tried changing the -Xmx to 3072m.

It crashed with out-of-memory errors, on two separate, consecutive Tomcat restarts. It wasn't doing that with -Xmx at 2048m.

I also noticed that regardless of the heap size setting, it seems to be throwing lots of exceptions, filling the catalina.out full of stack traces. Could that be part of what's driving the excessive GC?

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JHHL