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

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Date:	Wed, 15 Aug 2012 21:14:27 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
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 08/15/2012 10:43 AM, Arjan van de Ven wrote:

> The easy cop-out is provide the sysadmin a slider.
> The slightly less easy one is to (and we're taking this approach
> in the new P state code we're working on) say "in the default
> setting, we're going to sacrifice up to 5% performance from peak
> to give you the best power savings within that performance loss budget"
> (with a slider that can give you 0%, 2 1/2%  5% and 10%)

On a related note, I am looking at the c-state menu governor.

We seem to have issues there, with Linux often going into a much
deeper C state than warranted, which can lead to a fairly steep
performance penalty for some workloads.

One of the issues we identified is that detect_repeating_patterns
would deal quite poorly with patterns that have a short pause,
followed by a long pause, followed by another short pause. For
example, pinging a virtual machine :)

The idea Matthew and I have is simply planning for a shorter
sleep period (discarding the outliers to the high end in the
function once known as detect_repeating_patterns), and going
to a deeper C state if we have significantly overslept.

The new estimation code is easy, but for the past days I have
been looking through the timer code to figure out how such a
timer could fire, and how we could recognize it without it
looking like a normal wakeup, if we do not end up accidentally
waking up another CPU, etc...

I guess I should probably post what I have and kick off a
debate between people who know that code much better than I do :)

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