We have a large Xen deployment running on both RHEL and CentOS and have recently started looking at KVM since this is where it looks like the future of VM's are on Linux.

We can load the server and get everything running without an issue. However when we load up a new one with JBoss (4.2 Community edition, Sun JDK 6) and load a large EAR the server goes a little crazy. The %sy will jump to 80-99% and just hang for large periods of time we see a similar jump in %us on the host machine. We though at first this might be I/O as it seems to happen at start of JBoss but then would "cool down" after everything got loaded. We did some tests by extracting some large tar.gz files and using jar -xvf on the ear but could not re-create.

Then we starting thinking this might be some type of memory access issues. We loaded a c-program that would generate a lot of memory activity and sure enough we saw the spikes again. Not as high mind you but we did see it jump. We then wrote a small java program to do the same thing and sure enough we saw it jump again.

Any thoughts on what might be causing this? Is this just the way KVM works?

As a side note we do NOT see this behavior on any other setup. Xen, VMWare and/or native iron. The system does seem a bit slower than our Xen / VMware ones.