Re: performance of virtual functions compared to virtio

[Avi Kivity <avi@redhat.com>]

On 04/21/2011 05:35 AM, Alex Williamson wrote:
> Device assignment via a VF provides the lowest latency and most
> bandwidth for *getting data off the host system*, though virtio/vhost is
> getting better.  If all you care about is VM-VM on the same host or
> VM-host, then virtio is only limited by memory bandwidth/latency and
> host processor cycles.  Your processor has 25GB/s of memory bandwidth.
> On the other hand, the VF has to send data all the way out to the wire
> and all the way back up through the NIC to get to the other VM/host.
> You're using a 1Gb/s NIC.  Your results actually seem to indicate you're
> getting better than wire rate, so maybe you're only passing through an
> internal switch on the NIC, in any case, VFs are not optimal for
> communication within the same physical system.  They are optimal for off
> host communication.  Thanks,
>

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?)

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