linux-kernel - Re: gigabit ethernet power consumption

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Date:	Thu, 11 Oct 2007 17:05:40 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	"Kok, Auke" <auke-jan.h.kok@...el.com>,
	"Lennart Sorensen" <lsorense@...lub.uwaterloo.ca>
Cc:	"Pavel Machek" <pavel@....cz>,
	"Arjan van de Ven" <arjan@...radead.org>,
	"kernel list" <linux-kernel@...r.kernel.org>
Subject: Re: gigabit ethernet power consumption

On Wed, 10 Oct 2007 00:11:17 +0530, Kok, Auke <auke-jan.h.kok@...el.com>  
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
> -

Perhaps interrupt moderation could be of help here (say - switch to lesser  
interrupts per unit of time when running on battery), which I find that  
the e1000 driver doesn't employ in the kernel presently. (For interrupt  
moderation, refer:  
http://download.intel.com/design/network/applnots/ap450.pdf.)

Without the side-effect of experiencing a link-flap when switching to a  
lower-speed (with its toll in terms of down-time for auto-negotiation,  
STP, etc), the Interrupt Moderation Algorithm dynamically adjusts the  
number of interrupts based on traffic - and presumably consume less power.  
For an "Optimise for Power" kind of profile - the driver can be loaded  
with a higher throttle rate during boot-time.

Thanks,
K.Prasad
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