netdev - Re: Network latency regressions from 2.6.22 to 2.6.29

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Date:	Thu, 16 Apr 2009 15:47:52 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	cl@...ux.com
Cc:	bhutchings@...arflare.com, netdev@...r.kernel.org
Subject: Re: Network latency regressions from 2.6.22 to 2.6.29

From: Christoph Lameter <cl@...ux.com>
Date: Thu, 16 Apr 2009 15:02:36 -0400 (EDT)

> 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.

The driver is changing, and that can also be enough.
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