On Wed, Dec 1, 2010 at 12:30 PM, Avi Kivity <avi@xxxxxxxxxx> wrote: > On 12/01/2010 01:44 PM, Stefan Hajnoczi wrote: >> >> >> >> >> And, what about efficiency? As in bits/cycle? >> > >> > We are running benchmarks with this latest patch and will report >> > results. >> >> Full results here (thanks to Khoa Huynh): >> >> http://wiki.qemu.org/Features/VirtioIoeventfd >> >> The host CPU utilization is scaled to 16 CPUs so a 2-3% reduction is >> actually in the 32-48% range for a single CPU. >> >> The guest CPU utilization numbers include an efficiency metric: %vcpu >> per MB/sec. Here we see significant improvements too. Guests that >> previously couldn't get more CPU work done now have regained some >> breathing space. > > Thanks for those numbers. The guest improvements were expected, but the > host numbers surprised me. Do you have an explanation as to why total host > load should decrease? The first vcpu does virtqueue kick - it holds the guest driver vblk->lock across kick. Before this kick completes a second vcpu tries to acquire vblk->lock, finds it is contended, and spins. So we're burning CPU due to the long vblk->lock hold times. With virtio-ioeventfd those kick times are reduced an there is less contention on vblk->lock. Stefan -- 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
