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

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Date:	Wed, 22 Aug 2012 14:21:05 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Arjan van de Ven <arjan@...ux.intel.com>
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 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 
short periodic work, but for the majority of cases I'd expect Arjan to 
be completely correct here. Things get more interesting with 
multi-socket systems, but that's beyond the laptop use case.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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