Date: Wed, 10 Oct 2007 13:44:22 -0700
From: Mark Gross <mgross@linux.intel.com>
To: "Kok, Auke" <auke-jan.h.kok@intel.com>
Cc: Lennart Sorensen <lsorense@csclub.uwaterloo.ca>,
       Pavel Machek <pavel@ucw.cz>, Arjan van de Ven <arjan@infradead.org>,
       kernel list <linux-kernel@vger.kernel.org>
Subject: Re: gigabit ethernet power consumption
Message-ID: <20071010204422.GC5097@linux.intel.com>
Reply-To: mgross@linux.intel.com
References: <20071008220720.GF3484@elf.ucw.cz> <470AAFD7.9060002@intel.com> <20071009182857.GF4003@csclub.uwaterloo.ca> <470BCB4D.8090307@intel.com>
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On Tue, Oct 09, 2007 at 11:41:17AM -0700, Kok, Auke wrote:
> Lennart Sorensen wrote:
> > On Mon, Oct 08, 2007 at 03:31:51PM -0700, Kok, Auke wrote:
> >> you most certainly want to do this in userspace I think.
> >>
> >> One of the biggest problems is that link negotiation can take a significant amount
> >> of time, well over several seconds (1 to 3 seconds typical) with gigabit, and
> >> having your ethernet connection go offline for 3 seconds may not be the desired
> >> effect for when you want to get more bandwidth in the first place.
> >>
> >> However, when a laptop is in battery mode, switching down from gigabit to 100mbit
> >> makes a lot more sense, so this is something I would recommend. This can be as
> >> easy as changing the advertisement mask of the interface and renegotiating the
> >> link. Userspace could handle that very easily.
> > 
> > Now if you were trying to transfer a lot of data to the laptop, would it
> > be more power efficient to do it at gigabit speeds so you can finish
> > sooner and shut down the machine entirely, or to slow to 100mbit and
> > take longer to do it, and hence spend more time powering the cpu and
> > ram?
> 
> my suspicion is that the cost of switching is much higher than what you would
> consume running at 100mbit, even if the amount of data is quite large. going
> offline to renegotiate the link would already cost you 3W typically.
> 
> I definately think that userspace is the right field to solve this problem: let
> the users decide how to use the available power on their sytems through a decent
> power profile tool (perhaps gnome-power-manager or something like that). This way
> the user can choose.
> 

Auke, 
I was wondering if we could use PM-QOS to have the driver drop to the
100Mb speed, when requests for bandwidth and latency are not in effect?

--mgross
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