netdev - Re: Network performance with small packets

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Date:	Tue, 1 Feb 2011 23:56:03 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Shirley Ma <mashirle@...ibm.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 Tue, Feb 01, 2011 at 01:53:05PM -0800, Shirley Ma wrote:
> On Tue, 2011-02-01 at 23:42 +0200, Michael S. Tsirkin wrote:
> > On Tue, Feb 01, 2011 at 01:32:35PM -0800, Shirley Ma wrote:
> > > On Tue, 2011-02-01 at 23:24 +0200, Michael S. Tsirkin wrote:
> > > > My theory is that the issue is not signalling.
> > > > Rather, our queue fills up, then host handles
> > > > one packet and sends an interrupt, and we
> > > > immediately wake the queue. So the vq
> > > > once it gets full, stays full.
> > > 
> > > >From the printk debugging output, it might not be exactly the case.
> > The
> > > ring gets full, run a bit, then gets full, then run a bit, then
> > full...
> > 
> > Yes, but does it get even half empty in between?
> 
> Sometimes, most of them not half of empty in between. But printk slow
> down the traffics, so it's not accurate. I think your patch will improve
> the performance if it signals guest when half of the ring size is
> empty. 
> 
> But you manage signal by using TX bytes,

There are flags for bytes, buffers and packets.
Try playing with any one of them :)
Just be sure to use v2.


>I would like to change it to
> half of the ring size instead for signaling. Is that OK?
> 
> Shirley
> 
> 

Sure that is why I made it a parameter so you can experiment.

-- 
MST
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