netdev - Re: Network performance with small packets

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Date:	Tue, 01 Feb 2011 23:14:51 -0800
From:	Shirley Ma <mashirle@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Sridhar Samudrala <sri@...ibm.com>,
	Steve Dobbelstein <steved@...ibm.com>,
	David Miller <davem@...emloft.net>, kvm@...r.kernel.org,
	mashirle@...ux.vnet.ibm.com, netdev@...r.kernel.org
Subject: Re: Network performance with small packets

On Wed, 2011-02-02 at 08:29 +0200, Michael S. Tsirkin wrote:
> On Tue, Feb 01, 2011 at 10:19:09PM -0800, Shirley Ma wrote:
> > On Tue, 2011-02-01 at 22:05 -0800, Shirley Ma wrote:
> > > 
> > > The way I am changing is only when netif queue has stopped, then
> we
> > > start to count num_free descriptors to send the signal to wake
> netif
> > > queue. 
> > 
> > I forgot to mention, the code change I am making is in guest kernel,
> in
> > xmit call back only wake up the queue when it's stopped && num_free
> >=
> > 1/2 *vq->num, I add a new API in virtio_ring.
> 
> Interesting. Yes, I agree an API extension would be helpful. However,
> wouldn't just the signaling reduction be enough, without guest
> changes?

w/i guest change, I played around the parameters,for example: I could
get 3.7Gb/s with 42% CPU BW increasing from 2.5Gb/s for 1K message size,
w/i dropping packet, I was able to get up to 6.2Gb/s with similar CPU
usage.

> > However vhost signaling reduction is needed as well. The patch I
> > submitted a while ago showed both CPUs and BW improvement.
> > 
> > Thanks
> > Shirley
> 
> Which patch was that? 

The patch was called "vhost: TX used buffer guest signal accumulation".
You suggested to split add_used_bufs and signal. I am still thinking
what's the best approach to cooperate guest (virtio_kick) and
vhost(handle_tx), vhost(signaling) and guest (xmit callback) to reduce
the overheads, so I haven't submit the new patch yet.

Thanks
Shirley

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