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?
									Pavel 
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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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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Hi Auke,


If something does that, it must *only* be in userspace so that we can
easily disable it. It's amazing how many laptops consider that you
don't want any performance when you run off batteries. I've seen a
2GHz laptop falling back to 600 MHz when running on batteries, which
was very inconvenient when the laptop in question was used to go
sniffing gigabit traffic in datacenters... I would even go as far
as to say that my notebook runs lowpower only when it's plugged into
the wall because it's when I'm typing or doing low activity things.

In my opinion, battery != low power, battery == mobility. It's user's
choice which must imply low power, so that must be done with a dedicated
daemon.

Regards,
Willy

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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?

--
Len Sorensen
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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
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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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sure, that would probably work just fine. Technically this is not that hard anyway

I suppose that HAL integration is lacking a bit and missing ethtool ioctl code for
this which would be another solution - and would work without any driver changes.

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

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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Not true, interrupt moderation has always been part of the e1000 driver in one
form or another. In the basic form, you have always been able to set a manual
rate, which might help somewhat because it reduces CPU wakeups on mostly-idle
systems. Refer to the InterruptThrottleRate parameter of the e1000 module.


We're changing this to be run-time adjustable in newer drivers.

However, the power consumed by your nic staying in gigabit mode is much greater in
the long run then what you can save by trying to scrounge for milliwatts reducing
interrupts generated by the nic. By default it already moderates them somewhat.
Practically this feature is really not useful for powersaving, it just won't add
up to actual benefits in a real life situtation I think.

Auke
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There's an amount of data which would be the cutoff point.

But I don't think you can by default make machines slower. So as soon as
the link is saturated you have to switch to the higher speed. To avoid a
ping-pong effect I'd use a timer for bringing the link speed down again.

	Regards
		Oliver



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I believe you misspelled "ethtool".

While you're at it, why stop at 100Mb?  I believe you save even more power at 
10Mb, which is why WOL puts the card in 10Mb mode.  In my experience, you 
generally want either the maximum setting or the minimum setting when going for 
power savings, because of the race-to-idle effect.  Workloads that have a 
sustained fractional utilization are rare.  Right now I'm at home, hooked up to 
a cable modem, so anything over 4Mb is wasted, unless I'm talking to the box 
across the room, which is rare.

Talk to the NetworkManager folks.  This is right up their alley.

	-- Chris
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Do you only want to affect true ethernet devices this way? It seems
to me that the savings for wireless devices could be larger and we don't
want a separate mechanism for each type of network device. So I think
you need to tell the kernel why you want to reduce the link speed.

	Regards
		Oliver

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