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Subject: Re: CFS Bandwidth Control - Test results of cgroups tasks pinned vs
 unpinnede
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Paul Turner <pjt@google.com>,
        Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
        Vladimir Davydov <vdavydov@parallels.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Bharata B Rao <bharata@linux.vnet.ibm.com>,
        Dhaval Giani <dhaval.giani@gmail.com>,
        Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@elte.hu>, Pavel Emelianov <xemul@parallels.com>,
        Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 13 Sep 2011 20:23:46 +0200
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On Tue, 2011-09-13 at 23:31 +0530, Srivatsa Vaddagiri wrote:
> * Peter Zijlstra <a.p.zijlstra@chello.nl> [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.