RE: linux-next network throughput performance regression

[Dexuan Cui]

> -----Original Message-----
> From: David Miller [mailto:davem@xxxxxxxxxxxxx]
> Sent: Monday, November 9, 2015 10:53
> To: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Cc: eric.dumazet@xxxxxxxxx; dsa@xxxxxxxxxxxxxxxxxxx; Simon Xiao
> <sixiao@xxxxxxxxxxxxx>; netdev@xxxxxxxxxxxxxxx; Haiyang Zhang
> <haiyangz@xxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
> devel@xxxxxxxxxxxxxxxxxxxxxx
> Subject: Re: linux-next network throughput performance regression
> 
> From: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Date: Mon, 9 Nov 2015 02:39:24 +0000
> 
> >> 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?
> 
> Because a single TCP flow can only use one of the many TX queues
> that such modern NICs have.
> 
> The single TX queue becomes the bottleneck.
> 
> Whereas if you have several TCP flows, all of them can use independant
> TX queues on the NIC in parallel to fill the link with traffic.
> 
> That's why.

Thanks, David!
I understand 1 TX queue is the bottleneck (however in Simon's
test, TX=1 => 36.7Gb/s, TX=8 => 37.7 Gb/s, so it looks the TX=1 bottleneck
is not so obvious).
I'm just wondering how the bottleneck became much narrower with
recent linux-next in Simon's result (36.7 Gb/s vs. 18.2 Gb/s). IMO there
must be some latency somewhere.

Thanks,
-- Dexuan
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