linux-kernel - Re: [discussion]sched: a rough proposal to enable power saving in scheduler

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Message-ID: <502BE41B.4020508@linux.intel.com>
Date:	Wed, 15 Aug 2012 11:02:03 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Matthew Garrett <mjg59@...f.ucam.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Alex Shi <alex.shi@...el.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	vincent.guittot@...aro.org, svaidy@...ux.vnet.ibm.com,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [discussion]sched: a rough proposal to enable power saving in
 scheduler

On 8/15/2012 9:34 AM, Matthew Garrett wrote:
> On Wed, Aug 15, 2012 at 01:05:38PM +0200, Peter Zijlstra wrote:
>> On Mon, 2012-08-13 at 20:21 +0800, Alex Shi wrote:
>>> It bases on the following assumption:
>>> 1, If there are many task crowd in system, just let few domain cpus
>>> running and let other cpus idle can not save power. Let all cpu take the
>>> load, finish tasks early, and then get into idle. will save more power
>>> and have better user experience.
>>
>> I'm not sure this is a valid assumption. I've had it explained to me by
>> various people that race-to-idle isn't always the best thing. It has to
>> do with the cost of switching power states and the duration of execution
>> and other such things.
> 
> This is affected by Intel's implementation - if there's a single active 

not just intel.. also AMD
basically everyone who has the memory controller in the cpu package will end up with
a restriction very similar to this.

(this is because the exit-from-self-refresh latency is pretty high.. at least in DDR2/3)


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