netdev - Re: Network performance with small packets

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Date:	Wed, 2 Feb 2011 20:27:20 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	Krishna Kumar2 <krkumar2@...ibm.com>,
	David Miller <davem@...emloft.net>, kvm@...r.kernel.org,
	mashirle@...ux.vnet.ibm.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

On Wed, Feb 02, 2011 at 10:11:51AM -0800, Shirley Ma wrote:
> On Wed, 2011-02-02 at 19:32 +0200, Michael S. Tsirkin wrote:
> > OK, but this should have no effect with a vhost patch
> > which should ensure that we don't get an interrupt
> > until the queue is at least half empty.
> > Right?
> 
> There should be some coordination between guest and vhost.

What kind of coordination? With a patched vhost, and a full ring.
you should get an interrupt per 100 packets.
Is this what you see? And if yes, isn't the guest patch
doing nothing then?

> We shouldn't
> count the TX packets when netif queue is enabled since next guest TX
> xmit will free any used buffers in vhost. We need to be careful here in
> case we miss the interrupts when netif queue has stopped.
> 
> However we can't change old guest so we can test the patches separately
> for guest only, vhost only, and the combination.
> 
> > > > 
> > > > Yes, it seems unrelated to tx interrupts. 
> > > 
> > > The issue is more likely related to latency.
> > 
> > Could be. Why do you think so?
> 
> Since I played with latency hack, I can see performance difference for
> different latency.

Which hack was that?

> > > Do you have anything in
> > > mind on how to reduce vhost latency?
> > > 
> > > Thanks
> > > Shirley
> > 
> > Hmm, bypassing the bridge might help a bit.
> > Are you using tap+bridge or macvtap? 
> 
> I am using tap+bridge for TCP_RR test, I think Steven tested macvtap
> before. He might have some data from his workload performance
> measurement.
> 
> Shirley
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