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SubjectRe: Netperf UDP_STREAM regression due to not sending IPIs in ttwu_queue()
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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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