[Qemu-devel] Re: [PATCH v5 0/4] virtio: Use ioeventfd for virtqueue notify

[Michael S. Tsirkin]

On Sun, Dec 12, 2010 at 10:42:28PM +0200, Michael S. Tsirkin wrote:
> On Sun, Dec 12, 2010 at 10:41:28PM +0200, Michael S. Tsirkin wrote:
> > On Sun, Dec 12, 2010 at 03:02:04PM +0000, Stefan Hajnoczi wrote:
> > > See below for the v5 changelog.
> > > 
> > > Due to lack of connectivity I am sending from GMail.  Git should retain my
> > > address@hidden From address.
> > > 
> > > Virtqueue notify is currently handled synchronously in userspace virtio.  
> > > This
> > > prevents the vcpu from executing guest code while hardware emulation code
> > > handles the notify.
> > > 
> > > On systems that support KVM, the ioeventfd mechanism can be used to make
> > > virtqueue notify a lightweight exit by deferring hardware emulation to the
> > > iothread and allowing the VM to continue execution.  This model is 
> > > similar to
> > > how vhost receives virtqueue notifies.
> > > 
> > > The result of this change is improved performance for userspace virtio 
> > > devices.
> > > Virtio-blk throughput increases especially for multithreaded scenarios and
> > > virtio-net transmit throughput increases substantially.
> > 
> > Interestingly, I see decreased throughput for small message
> > host to get netperf runs.
> > 
> > The command that I used was:
> > netperf -H $vguest -- -m 200
> > 
> > And the results are:
> > - with ioeventfd=off
> > TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 11.0.0.104 
> > (11.0.0.104) port 0 AF_INET : demo
> > Recv   Send    Send                          Utilization       Service 
> > Demand
> > Socket Socket  Message  Elapsed              Send     Recv     Send    Recv
> > Size   Size    Size     Time     Throughput  local    remote   local   
> > remote
> > bytes  bytes   bytes    secs.    10^6bits/s  % S      % S      us/KB   us/KB
> > 
> >  87380  16384    200    10.00      3035.48   15.50    99.30    6.695   
> > 2.680  
> > 
> > - with ioeventfd=on
> > TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 11.0.0.104 
> > (11.0.0.104) port 0 AF_INET : demo
> > Recv   Send    Send                          Utilization       Service 
> > Demand
> > Socket Socket  Message  Elapsed              Send     Recv     Send    Recv
> > Size   Size    Size     Time     Throughput  local    remote   local   
> > remote
> > bytes  bytes   bytes    secs.    10^6bits/s  % S      % S      us/KB   us/KB
> > 
> >  87380  16384    200    10.00      1770.95   18.16    51.65    13.442  
> > 2.389  
> > 
> > 
> > Do you see this behaviour too?
> 
> Just a note: this is with the patchset ported to qemu-kvm.

And just another note: the trend is reversed for larged messages,
e.g. with 1.5k messages ioeventfd=on outputforms ioeventfd=off.

> > -- 
> > MST