Operating system? 32 bit? 64 bit?

We run JBoss on multi-server configurations often, using SpecJAppServer benchmark as the test bed. But we also have some other hand-coded tests. One of the things you will find is that an app server will not scale beyond 4 CPUs. This applies to any app server, not just to JBoss. In fact, you should affinitize the app server to 2, 3 or 4 of your CPUs - the actual number depends on your application (try each one, pick the best), and preferrably CPUs that occupy a common bus (usually 4 CPUs share a bus).

Regarding Xmx/s, on a 32-bit system we have not been able to go over 1.7 GB of heap, on a 64-bit system only RAM is the limit (though with a really large heap you get really long pause times during a major collection.

Also see http://wiki.jboss.org/wiki/Wiki.jsp?page=SPECjAppServer2002Tuning for some heap tuning hints.

Specifically http://wiki.jboss.org/wiki/Wiki.jsp?page=SPECjAppServer2002TuningComponentsJVM

This wiki entry contains most of what we learned running on large-scale systems.


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