Re: [PATCH] vhost: Add polling mode

Michael S. Tsirkin
2014-07-29T12:40:06Z

On Tue, Jul 29, 2014 at 03:23:59PM +0300, Razya Ladelsky wrote:
> >
> > Hmm there aren't a lot of numbers there :(. Speed increased by 33% but
> > by how much? E.g. maybe you are getting from 1Mbyte/sec to 1.3,
> > if so it's hard to get excited about it.
>
> Netperf 1 VM: 1516 MB/sec -> 2046 MB/sec
> and for 3 VMs: 4086 MB/sec -> 5545 MB/sec

What do you mean by 1 VM? Streaming TCP host to vm?
Also, your throughput is somewhat low, it's worth seeing
why you can't hit higher speeds.

> > Some questions that come to
> > mind: what was the message size? I would expect several measurements
> > with different values. How did host CPU utilization change?
> >
>
> message size was 64B in order to get the VM to be cpu saturated.
> so vhost had 99% cpu and vhost 38%, with the polling patch both had 99%.

Hmm so a net loss in throughput/CPU.

>
>
> > What about latency? As we are competing with guest for host CPU,
> > would worst-case or average latency suffer?
> >
>
> Polling indeed doesn't make a lot of sense if there aren't enough
> available cores.
> In these cases polling should not be used.
>
> Thank you,
> Razya

OK but scheduler might run vm and vhost on the same cpu
even if cores are available.
This needs to be detected somehow and polling disabled.

>
>
> > Thanks,
> >
> > --
> > MST
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