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

["Michael S. Tsirkin" <mst@redhat.com>]

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
> stefanha@linux.vnet.ibm.com 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?

-- 
MST