RE: linux-next network throughput performance regression

[Dexuan Cui]

> From: devel [mailto:driverdev-devel-bounces@xxxxxxxxxxxxxxxxxxxxxx] On Behalf
> Of Eric Dumazet
> Sent: Sunday, November 8, 2015 3:36
> To: David Ahern <dsa@xxxxxxxxxxxxxxxxxxx>
> Cc: netdev@xxxxxxxxxxxxxxx; Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>; linux-
> kernel@xxxxxxxxxxxxxxx; devel@xxxxxxxxxxxxxxxxxxxxxx; David Miller
> <davem@xxxxxxxxxxxxx>
> Subject: Re: linux-next network throughput performance regression
> 
> On Fri, 2015-11-06 at 14:30 -0700, David Ahern wrote:
> > On 11/6/15 2:18 PM, Simon Xiao wrote:
> > > 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
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