Re: performance of virtual functions compared to virtio

[Avi Kivity <avi@redhat.com>]

On 04/21/2011 03:31 PM, Stefan Hajnoczi wrote:
> On Thu, Apr 21, 2011 at 9:07 AM, Avi Kivity<avi@redhat.com>  wrote:
> >  Note I think in both cases we can make significant improvements:
> >  - for VFs, steer device interrupts to the cpus which run the vcpus that will
> >  receive the interrupts eventually (ISTR some work about this, but not sure)
> >  - for virtio, use a DMA engine to copy data (I think there exists code in
> >  upstream which does this, but has this been enabled/tuned?)
>
> Which data copy in virtio?  Is this a vhost-net specific thing you're
> thinking about?

There are several copies.

qemu's virtio-net implementation incurs a copy on tx and on rx when 
calling the kernel; in addition there is also an internal copy:

         /* copy in packet.  ugh */
         len = iov_from_buf(sg, elem.in_num,
                            buf + offset, size - offset);

In principle vhost-net can avoid the tx copy, but I think now we have 1 
copy on rx and tx each.

If a host interface is dedicated to backing a vhost-net interface (say 
if you have an SR/IOV card) then you can in principle avoid the rx copy 
as well.

An alternative to avoiding the copies is to use a dma engine, like I 
mentioned.

-- 
error compiling committee.c: too many arguments to function