Windows Server 2012 on KVM on Ubuntu 14.04. Runs well for a while and then slows down
Solution 1:
It seems that the issue was a result of a kernel bug in the 3.13 kernel that causes KSM to be a bit over eager when identifying memory to share. (https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1341195)
We fixed this by setting KSM_ENABLED in /etc/default/qemu-kvm to 0. Alternatively, we could have upgraded to the 3.14 kernel.