netdev - Re: Network latency regressions from 2.6.22 to 2.6.29

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Message-ID: <1239919037.12883.15.camel@HP1>
Date:	Thu, 16 Apr 2009 14:57:17 -0700
From:	"Michael Chan" <mchan@...adcom.com>
To:	"Christoph Lameter" <cl@...ux.com>
cc:	"Eric Dumazet" <dada1@...mosbay.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Network latency regressions from 2.6.22 to 2.6.29


On Thu, 2009-04-16 at 12:55 -0700, Christoph Lameter wrote:
> On Thu, 16 Apr 2009, Eric Dumazet wrote:
> 
> > Is it a bnx2 or tg3 driver ?
> 
> bnx2.
> 
> > It would be sane to always set on both machines :
> > ethtool -C eth0 rx-usecs 0 rx-frames 1
> > prior to your tests...
> 
> rx-usecs is set to 18 by default by the driver and rx-frames is set to 6.
> We saw some funky behavior with rx-usecs 0 so we left the defaults.

We haven't changed these defaults since the beginning, but firmware
updates between 2.6.22 and 2.6.29 can affect latency.

Yes, Eric is right, using rx-frames 1 will give the best latency number.
(rx-usecs won't matter if rx-frames is set to 1).

> 
> But what does it matter? Just changing the kernel should not cause
> increases in latency.
> 

It will not matter if you use the same out-of-tree driver when switching
kernels.  If you use in-tree driver, the firmware may have changed
between kernel versions.



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