steved@xxxxxxxxxx wrote on 01/28/2011 12:29:37 PM: > > On Thu, 2011-01-27 at 22:05 +0200, Michael S. Tsirkin wrote: > > > One simple theory is that guest net stack became faster > > > and so the host can't keep up. > > > > Yes, that's what I think here. Some qdisc code has been changed > > recently. > > I ran a test with txqueuelen set to 128, instead of the default of 1000, in > the guest in an attempt to slow down the guest transmits. The change had > no effect on the throughput nor on the CPU usage. > > On the other hand, I ran some tests with different CPU pinnings and > with/without hyperthreading enabled. Here is a summary of the results. > > Pinning configuration 1: pin the VCPUs and pin the vhost thread to one of > the VCPU CPUs > Pinning configuration 2: pin the VCPUs and pin the vhost thread to a > separate CPU on the same socket > Pinning configuration 3: pin the VCPUs and pin the vhost thread to a > separate CPU a different socket > > HT Pinning Throughput CPU > Yes config 1 - 40% - 40% > Yes config 2 - 37% - 35% > Yes config 3 - 37% - 36% > No none 0% - 5% > No config 1 - 41% - 43% > No config 2 + 32% - 4% > No config 3 + 34% + 9% > > Pinning the vhost thread to the same CPU as a guest VCPU hurts performance. > Turning off hyperthreading and pinning the VPUS and vhost thread to > separate CPUs significantly improves performance, getting it into the > competitive range with other hypervisors. > > Steve D. Those results for configs 2 and 3 with hyperthreading off are a little strange. Digging into the cause I found that my automation script for pinning the vhost thread failed and pinned it to CPU 1, the same as config 1, giving results similar to config 1. I reran the tests making sure the pinning script did the right thing. The results are more consistent. HT Pinning Throughput CPU Yes config 1 - 40% - 40% Yes config 2 + 33% - 8% Yes config 3 + 34% + 9% No none 0% - 5% No config 1 - 41% - 43% No config 2 + 32% - 4% No config 3 + 34% + 9% It appears that we have a scheduling problem. If the processes are pinned we can get good performance. We also se that hyperthreading makes little difference. Sorry for the initial misleading data. Steve D. -- 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
