Re: gigabit ethernet power consumption

["K.Prasad" <prasad@linux.vnet.ibm.com>]

On Wed, 10 Oct 2007 00:11:17 +0530, Kok, Auke <auke-jan.h.kok@intel.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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/