Re: [MLUG] Power management app w/ CPU throttling for Ubuntu/Linux

[Jack Smith <EMAIL:PROTECTED>]

On Tue, 2007-07-10 at 12:46 -0500, Jim Locke wrote:
> What apps do people running Ubuntu (or other distros) recommend most
> for Power management (which include CPU throttling)?
> I just experienced the battery support on my toshiba laptopgo away
> after going to kernel 2.6.20.16 (I believe that's the correct number),
> which is a known bug, introduced in 2.6.20.16 (not existing in
> 2.6.20.15 as I understand).
> So, I figured out to correct this (did about 4 things that were
> suggested, probably 2 or more of which were actually needed for
> correction  :) 
> In doing so, I learned a bit more about gnome-power-manager, and also
> a KDE app (I forget it's name), that has more features, however it
> does not seem to support throttling of the CPU speed for power
> savings.
> 
> -- 
> - Jim Locke. 


Jim,

Power management support must be built into the kernel for it to work
properly. You must have the proper SpeedStep/Cool 'n Quiet/PowerNow!
module either compiled into the kernel or compiled as a module and
loaded for frequency scaling to work. You also must have the governors
that you want compiled as well. Generally "ondemand" is the one that
you'd want as it does automatic frequency scaling based on load.
"Powersave" simply locks the CPU at its lowest speed, "Performance" runs
it at full speed all the time, and "Userspace" allows for user programs
like cpufreqd or cpupw to scale the CPU's speed. If you're not running a
userspace governor, the kernel will automatically scale the frequency
without any user interaction. Programs like gnome-power-manager,
KPowersave (that's the KDE app) and the cpufreq-selector Gnome applet
simply echo things to the scaling kernel interface. 

You can also throttle an Intel CPU's throughput by making it perform
duty cycles. This will be represented by a throttling setting from 0 to
87.5% by 12.5%s. KPowersave can do this, but AFAIK the Gnome one will
not. I don't use it as it greatly increases latency but doesn't do much
to cut power usage unless the CPU is heavily loaded. If you're skipping
otherwise idle clock cycles, it won't do much for you. 

So if you are not getting any frequency scaling, first check to make
sure that the kernel cpufreq scaling is working. Go
to /sys/devices/system/cpu/cpu0 and see if there is a "cpufreq" folder
there. If the folder is missing, then you need to recompile your kernel
with the appropriate modules. If the folder is there, then the scaling
is working. The things inside the cpufreq folder show the current
scaling activity of the processor and you can also echo things to the
scaling_governor and scaling_setspeed to set the current governor and
speed. The list of available speeds and governors are in the
scaling_available_governors and scaling_available_frequencies. 

I hope this helps,

Jack


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