netdev - Re: Network performance with small packets

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Date:	Wed, 2 Feb 2011 13:07:54 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	David Miller <davem@...emloft.net>, kvm@...r.kernel.org,
	mashirle@...ux.vnet.ibm.com, "Michael S. Tsirkin" <mst@...hat.com>,
	netdev@...r.kernel.org, netdev-owner@...r.kernel.org,
	Sridhar Samudrala <sri@...ibm.com>,
	Steve Dobbelstein <steved@...ibm.com>
Subject: Re: Network performance with small packets

> Shirley Ma <mashirle@...ibm.com> wrote:
>
> > I have tried this before. There are a couple of issues:
> >
> > 1. the free count will not reduce until you run free_old_xmit_skbs,
> >    which will not run anymore since the tx queue is stopped.
> > 2. You cannot call free_old_xmit_skbs directly as it races with a
> >    queue that was just awakened (current cb was due to the delay
> >    in disabling cb's).
> >
> > You have to call free_old_xmit_skbs() under netif_queue_stopped()
> > check to avoid the race.
>
> Yes, that' what I did, when the netif queue stop, don't enable the
> queue, just free_old_xmit_skbs(), if not enough freed, then enabling
> callback until half of the ring size are freed, then wake the netif
> queue. But somehow I didn't reach the performance compared to drop
> packets, need to think about it more. :)

Did you check if the number of vmexits increased with this
patch? This is possible if the device was keeping up (and
not going into a stop, start, xmit 1 packet, stop, start
loop). Also maybe you should try for 1/4th instead of 1/2?

MST's delayed signalling should avoid this issue, I haven't
tried both together.

Thanks,

- KK

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