Re: CFS Bandwidth Control - Test results of cgroups tasks pinned vsunpinnede

[Peter Zijlstra]

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%
> >
> > 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.


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