netdev - Re: Network performance with small packets

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Date:	Wed, 02 Feb 2011 10:11:51 -0800
From:	Shirley Ma <mashirle@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.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, 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. 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.

> > 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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