netdev - Re: Network performance with small packets

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Message-Id: <201102091209.36546.rusty@rustcorp.com.au>
Date:	Wed, 9 Feb 2011 12:09:35 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Krishna Kumar2 <krkumar2@...ibm.com>,
	David Miller <davem@...emloft.net>, kvm@...r.kernel.org,
	Shirley Ma <mashirle@...ibm.com>, netdev@...r.kernel.org,
	steved@...ibm.com
Subject: Re: Network performance with small packets

On Wed, 9 Feb 2011 11:23:45 am Michael S. Tsirkin wrote:
> On Wed, Feb 09, 2011 at 11:07:20AM +1030, Rusty Russell wrote:
> > On Wed, 2 Feb 2011 03:12:22 pm Michael S. Tsirkin wrote:
> > > On Wed, Feb 02, 2011 at 10:09:18AM +0530, Krishna Kumar2 wrote:
> > > > > "Michael S. Tsirkin" <mst@...hat.com> 02/02/2011 03:11 AM
> > > > >
> > > > > On Tue, Feb 01, 2011 at 01:28:45PM -0800, Shirley Ma wrote:
> > > > > > On Tue, 2011-02-01 at 23:21 +0200, Michael S. Tsirkin wrote:
> > > > > > > Confused. We compare capacity to skb frags, no?
> > > > > > > That's sg I think ...
> > > > > >
> > > > > > Current guest kernel use indirect buffers, num_free returns how many
> > > > > > available descriptors not skb frags. So it's wrong here.
> > > > > >
> > > > > > Shirley
> > > > >
> > > > > I see. Good point. In other words when we complete the buffer
> > > > > it was indirect, but when we add a new one we
> > > > > can not allocate indirect so we consume.
> > > > > And then we start the queue and add will fail.
> > > > > I guess we need some kind of API to figure out
> > > > > whether the buf we complete was indirect?
> > 
> > I've finally read this thread... I think we need to get more serious
> > with our stats gathering to diagnose these kind of performance issues.
> > 
> > This is a start; it should tell us what is actually happening to the
> > virtio ring(s) without significant performance impact...
> > 
> > Subject: virtio: CONFIG_VIRTIO_STATS
> > 
> > For performance problems we'd like to know exactly what the ring looks
> > like.  This patch adds stats indexed by how-full-ring-is; we could extend
> > it to also record them by how-used-ring-is if we need.
> > 
> > Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
> 
> Not sure whether the intent is to merge this. If yes -
> would it make sense to use tracing for this instead?
> That's what kvm does.

Intent wasn't; I've not used tracepoints before, but maybe we should
consider a longer-term monitoring solution?

Patch welcome!

Cheers,
Rusty.
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