Re: CFS Bandwidth Control - Test results of cgroups tasks pinnedvs unpinnede

[Paul Turner]

On 09/13/11 11:23, Peter Zijlstra wrote:
> On Tue, 2011-09-13 at 23:31 +0530, Srivatsa Vaddagiri wrote:
> > * Peter Zijlstra<a.p.zijlstra@xxxxxxxxx>  [2011-09-13 16:19:39]:
> >
> > > > Booting with "nohz=off" also helps significantly.
> > > >
> > > > With nohz=on, average idle time (over 1 min) is 10.3%
> > > > With nohz=off, average idle time (over 1 min) is 3.9%

I think more compelling here is that it looks like nohz load-balance 
needs more love.

> > > So we should put the cpufreq/idle governor into the nohz/idle path, it
> > > already tries to predict the idle duration in order to pick a C state,
> > > that same prediction should be used to determine if stopping the tick is
> > > worth it.
> >
> > Hmm ..I tried performance governor and found that it slightly increases
> > idle time.
> >
> >    With nohz=off&&  ondemand governor, idle time = 4%
> >    With nohz=off&&  performance governor on all cpus, idle time = 6%
> >
> > I can't see obvious reasons for that ..afaict bandwidth capping should
> > be independent of frequency (i.e task gets capped by "used" time,
> > irrespective of frequency at which it was "using" the cpu)?
>
> That's not what I said.. what I said is that the nohz code should also
> use the idle time prognosis.. disabling the tick is a costly operation,
> doing it only to have to undo it costs time, and will be accounted to
> idle time, hence your improvement with nohz=off.

Enabling Venki's CONFIG_IRQ_TIME_ACCOUNTING=y would discount to provide 
a definitive answer here yes?

- Paul

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