netdev - Re: Network latency regressions from 2.6.22 to 2.6.29

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Message-ID: <alpine.DEB.1.10.0904161500020.15835@qirst.com>
Date:	Thu, 16 Apr 2009 15:02:36 -0400 (EDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
cc:	netdev@...r.kernel.org
Subject: Re: Network latency regressions from 2.6.22 to 2.6.29

On Thu, 16 Apr 2009, Ben Hutchings wrote:

> On Thu, 2009-04-16 at 12:10 -0400, Christoph Lameter wrote:
> > The following are results of lantency measurements using udpping
> > (available from http://gentwo.org/ll). It shows that significant latencies
> > were added since 2.6.27. I surely wish we could get back to times below 90
> > microseconds.
> [...]
>
> This "90 microseconds" figure is specific to a particular driver and
> hardware.  Have you verified that it applies to others?  The variation
> you've reported is tiny compared to the improvements that can be made or
> lost by hardware tuning.

The RX delay can influence this of course and so can the driver. But the
measurements are with the same RX delay and the same driver. Variations
are up to 20% which is not tiny and its only due to different kernel
versions.

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