linux-kernel - Re: CONFIG_SUSPEND and power consumption

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Message-Id: <200708192242.43819.rjw@sisk.pl>
Date:	Sun, 19 Aug 2007 22:42:43 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Jean Delvare <khali@...ux-fr.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: CONFIG_SUSPEND and power consumption

Hi,

On Sunday, 19 August 2007 15:32, Jean Delvare wrote:
> Hi all, hi Rafael,
> 
> Running kernel 2.6.23-rc3-git1, I noticed yesterday that my CPU (AMD
> Sempron 2600+) was running at a much lower temperature when
> CONFIG_SUSPEND was enabled.

Hm, interesting.

> The temperature difference was quite significant, about 6 degrees Celsius at
> idle. Measuring the power consumption of my system confirmed that the energy
> savings were real: with CONFIG_SUSPEND=n, the system consumes 80 W of power
> (idle), while with CONFIG_SUSPEND=y, the system consumes only 69 W (idle)!
> Can anyone explain how this works?

I can't.

> I didn't expect CONFIG_SUSPEND to make any difference before actually
> switching the system to standby or suspend state.

Yes, that's the expected behavior.

> I tried the same trick on two Intel motherboards I use for testing, but
> this option didn't seem to make any difference in the power consumption
> for these.

Do you have CONFIG_HIBERNATION set?  If not, please see if setting it instead
of CONFIG_SUSPEND leads to the same result on the affected box (ie. running
at lower temperatures).

Greetings,
Rafael
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