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Re: CPU power management

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Eddy Petrișor wrote:
> Florian Reitmeir wrote:
>> Hi,
>> On Mon, 05 Feb 2007, Eddy Petri??or wrote:
>>> On a side note, I have found out that FreeBSD is able to bring down the
>>> processor speed of a Pentium M processor on a laptop of a friend a LOT
>>> lower than what Linux is able to do (200MHz as opposed to 800MHz in Linux).
>>> I am wondering if the information is correct. I thought of installing
>>> Debian GNU/kFreeBSD but I failed miserably and I don't know when and if
>>> I'll try again.
>> I know these patches. The 200mhz are only shown in the kernel (in my case
>> linux) but the hardware is not capable of that, look for your self into the
>> intel datasheets.
>> But indeed its possible to "undervolt" the cpu to save power, approx. 10-20%,
>> search for the linux-phc patches.
>> https://www.dedigentoo.org/trac/linux-phc/
> This seems to apply only to older cores. I have a Intel Core 2 Duo
> processor. Anybody knows if this is supported? i browsed the forums on
> gentoo and found that someone provided baz some info on the Core 2 Duo,
> but I don't know what is the current status.

I have installed Debian GNU/kFreeBSD and indeed, the levels are many
more than on Linux:

sysctl dev.cpu.0.freq_levels

Results in a list of frequencies down to 31 MHz. I tried some script I
found on the freebsd lists[1] to test the speed of blowfish at various
speeds and observed that on my Core 2 Duo that:

- - speed does not go linearly down with the scaling of the frequency -
some of the frequencies like 750, 500, 250 were actually more slower
than the previous ones, but the interim values were faster.
- - starting downwards from (I think, but I am not sure) 750 MHz, there
were some ugly debugging messages that seem to originate from libc or
something similar (sorry, I don't have the output due to the following)
- - after running a while (2-3 min) at a low speed the system freezes,
probably the cpu goes mad :-) - I have tested this and it happens on my
system (starting from low speeds upwards) up to 500MHz, including that

So I am thinking that maybe, just maybe, 750MHz would be stable enough,
but I haven't found the time to test. This would be uber-cool in long
trips during which I read stuff or hack on stuff (without compiling) in
case of mock-ups or similar situations.

In conclusion, it would be nice if this code would be ported to Linux, too.

BTW, I like the sysctl aproach they use in kFreeBSD instead of proc/sys,
it seems more intuitive.

[1] http://lists.freebsd.org/pipermail/freebsd-acpi/2005-July/001695.html
- --
"Imagination is more important than knowledge" A.Einstein

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