powernowd and laptop mode

[Matthew Garrett]

On Mon, 2005-03-28 at 18:15 +0200, Luis Lopez Lopez wrote:

> - With powernowd running if I start some app powernowd puts CPU at max.
> speed with the consecuent power consume.

Case (a): I run a program that takes 100% of the CPU. I limit the CPU
speed to 600MHz. It takes 2 seconds.
Case (b): I run a program that takes 100% of the CPU. I let the CPU run
at 1200MHz. It takes 1 second.

At 1200MHz and full CPU load, my laptop draws 15 Watts. At 600MHz and
full CPU load, my laptop draws about 10 Watts. Therefore:

Case (a) consumes 20 Joules.
Case (b) consumes 15 Joules.

Case (a) will allow my laptop to run for longer, but I'll be able to get
more work done in that time. Now, this is somewhat skewed by the fact
that I'm using a low-voltage Pentium M. If you're using an amd64 laptop,
then processor power consumption is likely to outweigh the rest of the
system (though, to be fair, several amd64 laptops are actually unable to
run the CPU at full speed on battery anyway...).

So, yes, in some cases limiting the CPU speed to the lowest supported
will result in you being able to get more work done. In other cases,
it'll reduce it. I think the current behaviour is a reasonable default -
we could probably make it easier to configure.

-- 
Matthew Garrett | mjg59 at srcf.ucam.org