netdev - Re: [v2 RFC PATCH 0/4] Implement multiqueue virtio-net

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Date:	Wed, 6 Oct 2010 19:50:14 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Krishna Kumar2 <krkumar2@...ibm.com>
Cc:	anthony@...emonkey.ws, avi@...hat.com, davem@...emloft.net,
	Ben Greear <greearb@...delatech.com>, kvm@...r.kernel.org,
	"Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org,
	rusty@...tcorp.com.au
Subject: Re: [v2 RFC PATCH 0/4] Implement multiqueue virtio-net

On Wednesday 06 October 2010 19:14:42 Krishna Kumar2 wrote:
> Arnd Bergmann <arnd@...db.de> wrote on 10/06/2010 05:49:00 PM:
> 
> > > I don't see any reasons mentioned above.  However, for higher
> > > number of netperf sessions, I see a big increase in retransmissions:
> > > _______________________________________
> > > #netperf      ORG           NEW
> > >             BW (#retr)    BW (#retr)
> > > _______________________________________
> > > 1          70244 (0)     64102 (0)
> > > 4          21421 (0)     36570 (416)
> > > 8          21746 (0)     38604 (148)
> > > 16         21783 (0)     40632 (464)
> > > 32         22677 (0)     37163 (1053)
> > > 64         23648 (4)     36449 (2197)
> > > 128        23251 (2)     31676 (3185)
> > > _______________________________________
> >
> >
> > This smells like it could be related to a problem that Ben Greear found
> > recently (see "macvlan:  Enable qdisc backoff logic"). When the hardware
> > is busy, used to just drop the packet. With Ben's patch, we return
> -EAGAIN
> > to qemu (or vhost-net) to trigger a resend.
> >
> > I suppose what we really should do is feed that condition back to the
> > guest network stack and implement the backoff in there.
> 
> Thanks for the pointer. I will take a look at this as I hadn't seen
> this patch earlier. Is there any way to figure out if this is the
> issue?

I think a good indication would be if this changes with/without the
patch, and if you see -EAGAIN in qemu with the patch applied.

	Arnd
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