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

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Message-ID: <5034DD35.9090504@linux.intel.com>
Date:	Wed, 22 Aug 2012 06:23:01 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Matthew Garrett <mjg59@...f.ucam.org>
CC:	Mike Galbraith <efault@....de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ingo Molnar <mingo@...nel.org>,
	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,
	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 8/22/2012 6:21 AM, Matthew Garrett wrote:
> On Wed, Aug 22, 2012 at 06:02:48AM -0700, Arjan van de Ven wrote:
>> On 8/21/2012 10:41 PM, Mike Galbraith wrote:
>>> For my dinky dual core laptop, I suspect you're right, but for a more
>>> powerful laptop, I'd expect spread/don't to be noticeable.
>>
>> yeah if you don't spread, you will waste some power.
>> but.. current linux behavior is to spread.
>> so we can only make it worse.
> 
> Right. For a single socket system the only thing you can do is use two 
> threads in preference to using two cores. That'll keep an extra core in 
> a deep C state for longer, at the cost of keeping the package out of a 
> deep C state for longer. There might be a win if the two processes 
> benefit from improved L1 cache locality, or if you're talking about 

basically "if HT sharing would be good for performance" ;-)

(btw this is good news, it means this is not an actual power/performance tradeoff, but a "get it right" tradeoff)

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