NFS server high CPU utilization after upgrade on Ubuntu 12.04

view story

http://serverfault.com – We have a university cluster consisting of 1 NFS server, and 40 client nodes. We are having problem recently where the NFS processes on the server shoot to 100% CPU utilization, and the whole cluster becomes unusable. We have found that a majority of the client nodes have a kernel thread [] that are CPU bound, talking to the NFS server. There isn't any userspace process putting demand on the nfs mount. We used wireshark to see the conversation: The clients are repeatedly sending SETCLIENTID_CONFIRM, and the server responds every time immediately with NFS4ERR_STALE_CLIENTID. ( (HowTos)