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When a cpu has lots of steal time, while the other cpus does not, the guest scheduler, not knowing about this fact, will still try to equalize the load in guests cpus. Upstream now has a patch that includes steal time information in scheduler decisions. The best and easy way to demonstrate this problem, is by resorting to pinning: * For N cpus, start N cpu hogs in the host ( The command "yes" in background will do). Pin each process to a different cpu. * In the guest, with N vcpus, start at least 5 * N cpu hogs. * Pin each vcpu to a different cpu. * increase the priority of one of the host cpu hogs * At this point, the non-patched kernel will still see an equal distribution of processes inside the guest. The patched kernel, will not.
This is more of a feature enhancement, which we were unsure whether we would need it or not when steal time was introduced in RHEL 6 KVM. We have never seen a customer issue that suggests we need this, so the risk of adding the feature is not worthwhile.