linux-kernel - Re: [v2 PATCH 0/4] timers: framework for migration between CPU

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Message-ID: <20090306042333.GA9190@linux.vnet.ibm.com>
Date:	Fri, 6 Mar 2009 09:53:33 +0530
From:	Arun R Bharadwaj <arun@...ux.vnet.ibm.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org,
	a.p.zijlstra@...llo.nl, ego@...ibm.com, tglx@...utronix.de,
	mingo@...e.hu, andi@...stfloor.org, venkatesh.pallipadi@...el.com,
	vatsa@...ux.vnet.ibm.com, svaidy@...ux.vnet.ibm.com
Subject: Re: [v2 PATCH 0/4] timers: framework for migration between CPU

* Arjan van de Ven <arjan@...radead.org> [2009-03-05 16:14:51]:

> On Wed, 4 Mar 2009 17:42:49 +0530
> Arun R Bharadwaj <arun@...ux.vnet.ibm.com> wrote:
> > With the timers queued I measure the sleep state residency
> > for a period of 10s.
> > Next, I enable timer migration and measure the sleep state
> > residency period.
> 
> is your table in seconds? Because realistically, at least on PC like
> hardware, doing work to get sleep times over a 100 / 200 msecs or so
> isn't going to save you any amount of measurable power anymore...
>

Hi Arjan,

Yes, the table is in seconds.
Rather than looking at timer migration as a direct means of obtaining
power savings, I would like to see it as one of the mechanisms which
would aid in preventing an almost idle cpu from waking up
unnecessarily, thereby causing power penalty. Otherwise, there is a
chance of all the CPUs in a particular package being idle and the
entire package going into deep sleep.
So we cannot really get big power savings just by doing timer
migration alone.

--arun

> 
> 
> -- 
> 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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