On 07.10.2013, at 17:40, Marc Zyngier <marc.zyngier@xxxxxxx> wrote: > On an (even slightly) oversubscribed system, spinlocks are quickly > becoming a bottleneck, as some vcpus are spinning, waiting for a > lock to be released, while the vcpu holding the lock may not be > running at all. > > This creates contention, and the observed slowdown is 40x for > hackbench. No, this isn't a typo. > > The solution is to trap blocking WFEs and tell KVM that we're > now spinning. This ensures that other vpus will get a scheduling > boost, allowing the lock to be released more quickly. > >> From a performance point of view: hackbench 1 process 1000 > > 2xA15 host (baseline): 1.843s > > 2xA15 guest w/o patch: 2.083s > 4xA15 guest w/o patch: 80.212s > > 2xA15 guest w/ patch: 2.072s > 4xA15 guest w/ patch: 3.202s I'm confused. You got from 2.083s when not exiting on spin locks to 2.072 when exiting on _every_ spin lock that didn't immediately succeed. I would've expected to second number to be worse rather than better. I assume it's within jitter, I'm still puzzled why you don't see any significant drop in performance. Alex -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
