found this : http://jroller.com/rreyelts/date/20040909 After running some tests with the JVM, I've come to the conclusion that Sun's JDK 1.4 allocates about 256K of address space per thread. I also ran some tests with JDK 1.5, which seem to indicate that it was allocating about 1M of address space per thread. Those numbers come from an assumption of a 2G address space per process under Windows, and the following statistics for the maximum number of threads I could create: JDK1.4 -Xmx750 = 4580 threads. -Xmx1000 = 3608 threads. -Xmx1500M = 1663 threads JDK1.5 -Xmx750M = 1129 threads -Xmx1000M = 880 threads -Xmx1500M = 384 threads As you can see, the maximum number of threads I can create decreases as the heap size gets larger. This is because the JVM immediately reserves the address space specified for the maximum heap size - That's probably because it needs that memory to be contiguous. Hence the reaon why when I reduce the heap I get more done ... So I have restarted my JBoss 5 with 1024 Heap, 256 Permsize and limited the Thread size to 128K (-Xss128k). I have run my 5 users scenario (which didn't work previously) and I'm currently running the 20. My 2 cents : it could just be that JBoss 5 creates far more threads than JBoss 4.2.3 used to. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4185691#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...