linux-kernel - Re: sched: deep power-saving states

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Date:	Wed, 22 Oct 2008 07:07:01 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: sched: deep power-saving states

On Wed, 22 Oct 2008 10:05:21 -0400
Gregory Haskins <ghaskins@...ell.com> wrote:

> Arjan van de Ven wrote:
> > On Wed, 22 Oct 2008 09:42:52 -0400
> > Gregory Haskins <gregory.haskins.ml@...il.com> wrote:
> >
> >   
> >> What I was thinking is that a simple mechanism to quantify the
> >> power-state penalty would be to add those states as priority
> >> levels in the cpupri namespace.  E.g. We could substitute
> >> IDLE-RUNNING for IDLE, and add IDLE-PS1, IDLE-PS2, .. IDLE-PSn,
> >> OTHER, RT1, .. RT99.  This means the scheduler would favor waking
> >> an IDLE-RUNNING core over an IDLE-PS1-PSn, etc.  The question in
> >> my mind is: can the power-states be determined in a static fashion
> >> such that we know what value to quantify the idle state before we
> >> enter it?  Or is it more dynamic (e.g. the longer it is in an
> >> MWAIT, the deeper the sleep gets). 
> >
> > it's a little dynamic, but just assuming the worst will be a very
> > good approximation of reality. And we know what we're getting into
> > in that sense.
> >   
> 
> Ok, but if we just assume the worst case always, how do I
> differentiate between, say, IDLE-RUNNING and IDLE-PSn?  If I assign
> them all to IDLE-PSn apriori its no better than the basic single IDLE
> state we support today.  Or am I misunderstanding you?

eh yes I wasn't very clear; it's pre-coffee time here ;)

we know *for each C state* we go in, what its maximum latency is.
Now, that is the *maximum*; there are times where it'll be less
(there are several steps for going into a C-state hardware wise, and if
an interrupt comes in before they're all completed, getting out of it
means not having to undo ALL the steps, so it'll be faster)



-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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