Re: performance of virtual functions compared to virtio

[Andrew Theurer <habanero@linux.vnet.ibm.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.
> 
> > 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.

you might want to check the VF driver.  I know versions of the ixgbevf
driver have a throttled interrupt option which will increase latency
with some settings.  I don't remember if the igbvf driver has the same
feature.  If it does, you will want to turn this option off for best
latency.

> 
> David
> 
> 
> > this, but it seems to be a hardware limitation, so it's hard to predict.
> > Thanks,
> > 
> > Alex

-Andrew