Re: Ondemand governor by default in etch
Anthony DeRobertis <anthony@derobert.net> wrote:
> Matthew Garrett wrote:
>> p4-clockmod is entirely useless. It's high-latency and doesn't drop the
>> core voltage.
>
> Nice. Is there a good alternative for P4 machines? Is the ACPI one any
> better (assuming a semi-sane BIOS)?
It really depends on the hardware - not all P4s support voltage scaling,
and without that you'll see no useful effects. The acpi driver is
certainly worth a go, as is speedstep-ich (which /might/ work, but I
wouldn't be optimistic).
Just to elaborate on why p4-clockmod isn't terribly useful: modern CPUs
expose multiple degrees of chip-level power management (C states in ACPI
speak). C1 is equivilent to having the idle loop use the hlt
instruction, and results in lower power usage. C2 is similar to the old
APM idle modes, which allowed the processor power down even more of
itself. Modern chips tend to support C3 and C4 states, in which the CPU
actually disconnects itself from the bus and is pretty much unclocked.
At that point, the frequency of the CPU is fairly unimportant. Simply
throttling down the processor won't cause any significant reduction in
power consumption, since most of what's left switched on would be
drawing that much power anyway. Dropping the core voltage, however, is
an obvious win - since power consumption is v^2/r, you're saving the
square of the amount you've reduced it by.
So p4-clockmod doesn't really help you in the case of an entirely idle
CPU. And in the more common case of a CPU that /isn't/ entire idle, it
can be more harm than good. Halving the speed of the chip means that
you're awake for twice as long as you would otherwise be, which may mean
that you don't get long enough to drop into the deeper sleep states.
Intel's presented figures (at last year's OLS, I think) that suggest
that p4-clockmod and straightforward throttling aren't useful approaches
to dealing with power consumption. The main reason for the methods being
implemented is to deal with cases where a loaded CPU is becoming too hot
- the hardware has no way to prevent the OS from scheduling more work,
so instead it slows down to reduce heat output. Instantaneous power
consumption is reduced, but it now takes twice as long for your job to
complete and your hard drive hasn't halved the amount of power it's
consuming. If you own a machine that doesn't implement the ACPI
interface to throttling (/proc/acpi/processor/*/throttling), then
p4-clockmod may be required in order to allow the OS to respond to high
temperatures.
On the other hand, if you're not in that situation, it's probably more
harm than good. There's a few laptops around that don't implement C3 or
C4 states, and it might be beneficial there as well. Otherwise, just
skip it.
--
Matthew Garrett | mjg59-chiark.mail.debian.devel@srcf.ucam.org
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