Subject: Re: Network latency regressions from 2.6.22 to 2.6.29

From: Eric Dumazet <[email-masked]>
Date: Thu, 16 Apr 2009 21:43:53 +0200

Christoph Lameter a écrit :
> 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.
> 
> The tests were done over 1G ethernet using
> 09:00.0 Ethernet controller: Broadcom Corporation NetXtreme BCM5754
> Gigabit Ethernet PCI Express (rev 02)
> 
> Results:
> 
>            2.6.22 2.6.23 2.6.24 2.6.25 2.6.26 2.6.27 2.6.28 2.6.29
> 40 Bytes    89.50  90.75  89.61  91.51  91.89  99.17  99.80  99.34
> 400 Bytes   98.58 101.44  97.85  99.61 100.36 117.96 118.10 126.79
> 1400 Bytes 152.76 153.75 153.85	156.22 156.66 163.92 165.54 166.04
> 
> Compared to 2.6.22 2.6.23 2.6.24 2.6.25 2.6.26 2.6.27 2.6.28 2.6.29
> 40 Bytes            -1.4%  -0.1%  -2.2%  -2.6%  -9.8%  -10.3% -9.9%
> 400 Bytes           -2.8%   0.7%  -1.0%  -1.8% -16.4%  -16.5%-22.2%
> 1400 Bytes          -0.6%  -0.7%  -2.2%  -2.5%  -6.8%  -7.7%  -8.0%
> 
> I presented these numbers with some nice graphs at the Linux Collab Summit
> last week.
> 
> See
> http://www.kernel.org/pub/linux/kernel/people/christoph/collab-spring-2009/Collab-summit-2009-sf.pdf
> 

Is it a bnx2 or tg3 driver ?

copybreak feature could explain a regression for small packets, since
an extra copy is done.

It would be sane to always set on both machines :
ethtool -C eth0 rx-usecs 0 rx-frames 1
prior to your tests...