netdev - Re: Network performance with small packets

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Date:	Wed, 2 Feb 2011 06:40:02 +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 02:59:57PM -0800, Shirley Ma wrote:
> On Tue, 2011-02-01 at 23:56 +0200, Michael S. Tsirkin wrote:
> > 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. 
> 
> The initial test results shows that the CPUs utilization has been
> reduced some, and BW has increased some with the default parameters,
> like 1K message size BW goes from 2.5Gb/s about 2.8Gb/s, CPU utilization
> down from 4x% to 38%, (Similar results from the patch I submitted a
> while ago to reduce signaling on vhost) but far away from dropping
> packet results.
> 
> I am going to change the code to use 1/2 ring size to wake the netif
> queue.
> 
> Shirley

Just tweak the parameters with sysfs, you do not have to edit the code:
echo 64 > /sys/module/vhost_net/parameters/tx_bufs_coalesce

Or in a similar way for tx_packets_coalesce (since we use indirect,
packets will typically use 1 buffer each).

-- 
MST
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