linux-kernel - Re: gigabit ethernet power consumption

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Date:	Mon, 08 Oct 2007 15:31:51 -0700
From:	"Kok, Auke" <auke-jan.h.kok@...el.com>
To:	Pavel Machek <pavel@....cz>
CC:	Arjan van de Ven <arjan@...radead.org>,
	kernel list <linux-kernel@...r.kernel.org>
Subject: Re: gigabit ethernet power consumption

Pavel Machek wrote:
> Hi!
> 
> I've found that gbit vs. 100mbit power consumption difference is about
> 1W -- pretty significant. (Maybe powertop should include it in the
> tips section? :).
> 
> Energy Star people insist that machines should switch down to 100mbit
> when network is idle, and I guess that makes a lot of sense -- you
> save 1W locally and 1W on the router.
> 
> Question is, how to implement it correctly? Daemon that would watch
> data rates and switch speeds using mii-tool would be simple, but is
> that enough?

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.

Auke
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