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

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Message-ID: <OFB5315194.76E80A8A-ON652577B4.005DBA77-652577B4.005E77C6@in.ibm.com>
Date:	Wed, 6 Oct 2010 22:44:42 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	Arnd Bergmann <arnd@...db.de>
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

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?

Thanks,

- KK

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