Re: About p4-clockmod breakage/removal

Matthew Garrett
2008-05-14T23:11:54Z

On Wed, May 14, 2008 at 11:18:37AM -0400, Len Brown wrote:
> On Wednesday 14 May 2008, Matthew Garrett wrote:
> > Check processor_thermal.c. It explicitly interfaces with cpufreq in
> > order to perform P state management.
>
> The hook above is so that ACPI thermal throttling
> does not invoke T-states until the more efficient
> P-states have already been exhausted.
>
> It doesn't imply the presence of thermal awareness
> anywhere in the cpufreq sub-system.

The cpufreq sub-system itself isn't aware of thermal issues, but the
ACPI system is. I'm not sure that the distinction's especially
important.

> It also doesn't imply that running cpufreq on top
> of T-states is a good idea. Cpufreq as implemented,
> say by the ondemand governor, is based
> on the assumption that it is running on top of
> P-states. Deeper P-states result in
> more efficient operation (less energy/instruction)
> due to their lower voltage, because energy varies with voltage^2.
> T-states violate that assumpion and provide
> all the performance cost with none of the efficiency
> gains of P-states, because energy varies directly
> with frequencey -- but so does performance cost.

Right. I'm not implying that this is a desirable situation, but the fact
remains that some machines will reach critical shutdown temperature
unless something is done to reduce the speed of the processor. If you
don't have P states or ACPI-exposed T states, then p4-clockmod is about
the best you can manage.

> > > There is already a well known thermal throttling interface
> > > available via ACPI and it does not need p4_clockmod to run.
> > > Passive trip points work automatically even without cpufreq being present.
> > > If they do not, then we need to fix them.
> >
> > You're assuming that the throttling interface is always exposed via
> > ACPI. I've seen machines where this isn't the case.
>
> Please show me those machines.

http://www.google.co.uk/search?hl=en&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&hs=R8P&q=acpi+throttling+"<not+supported>"+-C3&btnG=Search&meta=

(a lot of false positives, but many of these are genuine)
--
Matthew Garrett | mjg59@srcf.ucam.org