linux-kernel - Re: Netperf UDP_STREAM regression due to not sending IPIs in ttwu

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Date:	Wed, 10 Oct 2012 15:02:01 +0200
From:	Mike Galbraith <efault@....de>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Netperf UDP_STREAM regression due to not sending IPIs in
 ttwu_queue()

On Wed, 2012-10-10 at 13:29 +0100, Mel Gorman wrote: 
> On Wed, Oct 03, 2012 at 03:30:01PM +0200, Mike Galbraith wrote:

> > nohz=off, pipe-test with one half pinned to CPU0, the other to CPU1.
> > 
> > procs -----------memory---------- ---swap-- -----io---- -system-- -----cpu------
> >  r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
> > TTW_QUEUE
> >  1  0      0 3039488  50948 444720    0    0     0     0 539724 1013417  1 15 84  0  0
> >  1  0      0 3039488  50956 444720    0    0     0     1 540853 1015679  1 15 84  0  0
> >  1  0      0 3039364  50956 444720    0    0     0     0 541630 1017239  1 16 83  0  0
> >  2  0      0 3038992  50956 444720    0    0     0     0 335550 1096569  4 20 76  0  0
> > NO_TTWU_QUEUE
> >  1  0      0 3038992  50956 444720    0    0     0     0 33100 1318984  1 27 71  0  0
> >  1  0      0 3038868  50956 444720    0    0     0     0 33100 1319126  2 27 71  0  0
> >  1  0      0 3038868  50956 444720    0    0     0     0 33097 1317968  1 27 72  0  0
> >  2  0      0 3038868  50964 444720    0    0     0     1 33104 1318558  2 27 71  0  0
> > 
> > We can switch faster with NO_TTWU_QUEUE, so we switch more, and that
> > hurts netperf UDP_STREAM throughput.. somehow.  Fatter is better is not
> > the way context switch happy benchmarks usually work.
> > 
> 
> Do we really switch more though?

Yup, pipe-test measures the full round trip, and agrees with vmstat.
netperf TCP_RR agrees, tbench agrees... this STREAM thingy is the only
high frequency switcher I've seen go all weird like this.

(hm, wonder how fast box can blast IPIs.. 1MHz?)

-Mike

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