On Tue, Oct 08, 2013 at 06:38:13PM +0100, Marc Zyngier 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. Also, using > CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT slightly improves the performance > when the VM is severely overcommited. > > Quick test to estimate the performance: hackbench 1 process 1000 > > 2xA15 host (baseline): 1.843s > > 2xA15 guest w/o patch: 2.083s > 4xA15 guest w/o patch: 80.212s > 8xA15 guest w/o patch: Could not be bothered to find out > > 2xA15 guest w/ patch: 2.102s > 4xA15 guest w/ patch: 3.205s > 8xA15 guest w/ patch: 6.887s > > So we go from a 40x degradation to 1.5x in the 2x overcommit case, > which is vaguely more acceptable. > Patch looks good, I can just apply it and add the other one I just send as a reply if there are no objections. Sorry for the long turn-around on this one. -Christoffer -- 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
