We experienced several problems when we upgraded to Red Hat Enterprise Linux 6.2 from CentOS 5.4. A user of ours started reporting slowness on some of his larger HPC jobs. We looked at tons of things then started noticing that one or more nodes would start swapping for no reason. His job would only use about 60-70% of the memory on each node but some nodes would inexplicably swap (diagnosed with vmstat). I talked to people at other universities and HPC sites and verified that a similar problem was occurring on their RHEL 6.2 installations.