Re: performance of virtual functions compared to virtio

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 2011-04-25 at 13:49 -0600, David Ahern wrote:
> 
> On 04/25/11 13:29, Alex Williamson wrote:
> > So we're effectively getting host-host latency/throughput for the VF,
> > it's just that in the 82576 implementation of SR-IOV, the VF takes a
> > latency hit that puts it pretty close to virtio.  Unfortunate.  I think
> 
> For host-to-VM using VFs is worse than virtio which is counterintuitive.

On the same host, just think about the data path of one versus the
other.  On the guest side, there's virtio vs a physical NIC.  virtio is
designed to be virtualization friendly, so hopefully has less context
switches in setting up and processing transactions.  Once the packet
leaves the assigned physical NIC, it has to come back up the entire host
I/O stack, while the virtio device is connected to an internal bridge
and bypasses all but the upper level network routing.

> > you'll find that passing the PF to the guests should be pretty close to
> > that 185us latency.  I would assume (hope) the higher end NICs reduce
> 
> About that 185usec: do you know where the bottleneck is? It seems as if
> the packet is held in some queue waiting for an event/timeout before it
> is transmitted.

I don't know specifically, I don't do much network performance tuning.
Interrupt coalescing could be a factor, along with various offload
settings, and of course latency of the physical NIC hardware and
interconnects.  Thanks,

Alex