On Wed, Dec 15, 2010 at 11:42:12AM +0000, Stefan Hajnoczi wrote: > On Mon, Dec 13, 2010 at 6:52 PM, Michael S. Tsirkin <m...@redhat.com> wrote: > > On Mon, Dec 13, 2010 at 05:57:28PM +0000, Stefan Hajnoczi wrote: > >> On Mon, Dec 13, 2010 at 4:28 PM, Stefan Hajnoczi <stefa...@gmail.com> > >> wrote: > >> > On Mon, Dec 13, 2010 at 4:12 PM, Michael S. Tsirkin <m...@redhat.com> > >> > wrote: > >> >> On Mon, Dec 13, 2010 at 03:27:06PM +0000, Stefan Hajnoczi wrote: > >> >>> On Mon, Dec 13, 2010 at 1:36 PM, Michael S. Tsirkin <m...@redhat.com> > >> >>> wrote: > >> >>> > On Mon, Dec 13, 2010 at 03:35:38PM +0200, Michael S. Tsirkin wrote: > >> >>> >> On Mon, Dec 13, 2010 at 01:11:27PM +0000, Stefan Hajnoczi wrote: > >> >>> >> > Fresh results: > >> >>> >> > > >> >>> >> > 192.168.0.1 - host (runs netperf) > >> >>> >> > 192.168.0.2 - guest (runs netserver) > >> >>> >> > > >> >>> >> > host$ src/netperf -H 192.168.0.2 -- -m 200 > >> >>> >> > > >> >>> >> > ioeventfd=on > >> >>> >> > TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to > >> >>> >> > 192.168.0.2 > >> >>> >> > (192.168.0.2) port 0 AF_INET > >> >>> >> > Recv Send Send > >> >>> >> > Socket Socket Message Elapsed > >> >>> >> > Size Size Size Time Throughput > >> >>> >> > bytes bytes bytes secs. 10^6bits/sec > >> >>> >> > 87380 16384 200 10.00 1759.25 > >> >>> >> > > >> >>> >> > ioeventfd=off > >> >>> >> > TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to > >> >>> >> > 192.168.0.2 > >> >>> >> > (192.168.0.2) port 0 AF_INET > >> >>> >> > Recv Send Send > >> >>> >> > Socket Socket Message Elapsed > >> >>> >> > Size Size Size Time Throughput > >> >>> >> > bytes bytes bytes secs. 10^6bits/sec > >> >>> >> > > >> >>> >> > 87380 16384 200 10.00 1757.15 > >> >>> >> > > >> >>> >> > The results vary approx +/- 3% between runs. > >> >>> >> > > >> >>> >> > Invocation: > >> >>> >> > $ x86_64-softmmu/qemu-system-x86_64 -m 4096 -enable-kvm -netdev > >> >>> >> > type=tap,id=net0,ifname=tap0,script=no,downscript=no -device > >> >>> >> > virtio-net-pci,netdev=net0,ioeventfd=on|off -vnc :0 -drive > >> >>> >> > if=virtio,cache=none,file=$HOME/rhel6-autobench-raw.img > >> >>> >> > > >> >>> >> > I am running qemu.git with v5 patches, based off > >> >>> >> > 36888c6335422f07bbc50bf3443a39f24b90c7c6. > >> >>> >> > > >> >>> >> > Host: > >> >>> >> > 1 Quad-Core AMD Opteron(tm) Processor 2350 @ 2 GHz > >> >>> >> > 8 GB RAM > >> >>> >> > RHEL 6 host > >> >>> >> > > >> >>> >> > Next I will try the patches on latest qemu-kvm.git > >> >>> >> > > >> >>> >> > Stefan > >> >>> >> > >> >>> >> One interesting thing is that I put virtio-net earlier on > >> >>> >> command line. > >> >>> > > >> >>> > Sorry I mean I put it after disk, you put it before. > >> >>> > >> >>> I can't find a measurable difference when swapping -drive and -netdev. > >> >> > >> >> One other concern I have is that we are apparently using > >> >> ioeventfd for all VQs. E.g. for virtio-net we probably should not > >> >> use it for the control VQ - it's a waste of resources. > >> > > >> > One option is a per-device (block, net, etc) bitmap that masks out > >> > virtqueues. Is that something you'd like to see? > >> > > >> > I'm tempted to mask out the RX vq too and see how that affects the > >> > qemu-kvm.git specific issue. > >> > >> As expected, the rx virtqueue is involved in the degradation. I > >> enabled ioeventfd only for the TX virtqueue and got the same good > >> results as userspace virtio-net. > >> > >> When I enable only the rx virtqueue, performs decreases as we've seen > >> above. > >> > >> Stefan > > > > Interesting. In particular this implies something's wrong with the > > queue: we should not normally be getting notifications from rx queue > > at all. Is it running low on buffers? Does it help to increase the vq > > size? Any other explanation? > > I made a mistake, it is the *tx* vq that causes reduced performance on > short packets with ioeventfd. I double-checked the results and the rx > vq doesn't affect performance. > > Initially I thought the fix would be to adjust the tx mitigation > mechanism since ioeventfd does its own mitigation of sorts. Multiple > eventfd signals will be coalesced into one qemu-kvm event handler call > if qemu-kvm didn't have a chance to handle the first event before the > eventfd was signalled again. > > I added -device virtio-net-pci tx=immediate to flush the TX queue > immediately instead of scheduling a BH or timer. Unfortunately this > had little measurable effect and performance stayed the same. This > suggests most of the latency is between the guest's pio write and > qemu-kvm getting around to handling the event. > > You mentioned that vhost-net has the same performance issue on this > benchmark. I guess a solution for vhost-net may help virtio-ioeventfd > and vice versa. > > Are you happy with this patchset if I remove virtio-net-pci > ioeventfd=on|off so only virtio-blk-pci has ioeventfd=on|off (with > default on)? For block we've found it to be a win and the initial > results looked good for net too. > > Stefan
I'm concerned that the tests were done on qemu.git. Could you check block with qemu-kvm too please?