RE: linux-next network throughput performance regression

From: devel [mailto:driverdev-devel-bounces@linuxdriverproject.org] On Behalf
Of Eric Dumazet
Sent: Sunday, November 8, 2015 3:36
To: David Ahern <redacted>
Cc: netdev@vger.kernel.org; Haiyang Zhang <haiyangz@microsoft.com>; linux-
kernel@vger.kernel.org; devel@linuxdriverproject.org; David Miller
[off-list ref]
Subject: Re: linux-next network throughput performance regression

On Fri, 2015-11-06 at 14:30 -0700, David Ahern wrote:

quoted

On 11/6/15 2:18 PM, Simon Xiao wrote:

quoted

The .config file used to build linux-next kernel is attached to this mail.

Thanks.

Failed to notice this on the first response; my brain filled in. Why
linux-next tree? Can you try net-next which is more relevant for this
mailing list, post the top commit id and config file used?

Throughput on a single TCP flow for a 40G NIC can be tricky to tune.

Why is a single TCP flow trickier than multiple TCP flows?
IMO it should be easier to analyze the issue of a single TCP flow?

Here the perf drop in Simon's test is very obvious -- 50%, but it looks Eric
can't reproduce it, so I suppose some net-related kernel config options may
do the magic?

Maybe Simon can narrow the regression down by bisecting. :-)
 

Make sure IRQ are properly setup/balanced, as I know that IRQ names were
changed recently and your scripts might have not noticed...

Also "ethtool -c eth0" might show very different interrupt coalescing
params ?

I too have a Mellanox 40Gb in my lab and saw no difference in
performance with recent kernels.

Of course, a simple "perf record -a -g sleep 4 ; perf report" might
point to some obvious issue. Like unexpected segmentation in case of
forwarding...

Thanks,
-- Dexuan