linux-kernel - Re: CFS Bandwidth Control - Test results of cgroups tasks pinned vs unpinnede

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Message-ID: <4E730568.2030107@google.com>
Date:	Fri, 16 Sep 2011 01:14:32 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Pavel Emelianov <xemul@...allels.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: CFS Bandwidth Control - Test results of cgroups tasks pinned
 vs unpinnede

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@...llo.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%

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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