According to your heap settings, the JVM will use at least 150MB for the heap, but can use as much as 600MB. Apparently, the JVM has at this time increased the heap size to 295MB, of which only 103MB is in use. The non-heap memory usage is for the thread stacks, the JVM's C/C++ data structures and other odds and ends. By the way, your new size setting (-Xmn) practically guarantees full GCs every time, and full GCs are about way more expensive (10 - 100 times) than minor GCs. You should set the new size to at most 1/3 the size of the heap. See my presentation on the Java heap: http://www.cecmg.de/doc/tagung_2007/agenda07/24-mai/2b3-peter-johnson/ind... View the original post : http://www.jboss.org/index.html?module=bb&op=viewtopic&p=4221057#... Reply to the post : http://www.jboss.org/index.html?module=bb&op=posting&mode=reply&a...