I have a quad-core and for my testing I used a 1GB heap (I did not specify a young gen size, but I believe the JVM never set it to more than 100M). When using multiple tenured GC threads the JVM splits the tenured generation into sections and lets each thread clean its own section to minimize contention. So I had 4 thread cleaning about 200MB each. You, or course, had 8 threads so your lock-contention is higher. But I read a very interesting paper the other day regarding cache coherency between L2 caches in the CPUs that caused a significant performance drop when running a multi-threaded app, so I'm wondering if that could be a reason. Of course, I'd need VTune to track that down. View the original post : http://www.jboss.org/index.html?module=bb&op=viewtopic&p=4240093#... Reply to the post : http://www.jboss.org/index.html?module=bb&op=posting&mode=reply&a...