RFC: powerd algorithms enhancements

[Kevin Oberman]

> Date: Sat, 08 Nov 2008 07:19:20 +0200
> From: Alexander Motin <mav at FreeBSD.org>
> Sender: owner-freebsd-mobile at freebsd.org
> 
> Ian Smith wrote:
> > We're now seeing cpus that can vary freq, with absolute and relative 
> > cpufreq drivers enabled, in ratios up to 32:1 or so, so the advice, 
> > apart from 'disable powerd' :), seems to be to at least try setting 
> > cpufreq.lowest to some reasonable speed for workload, maybe 300MHz?
> 
> I surely should not be the default, but it is reasonable if systems 
> should have some guarantied minimal performance.
> 
> PS: At any modern SMP/HTT system, even if scheduler is unable to manage 
> this IRQ situation, powerd running on different CPU will rise clock to 
> required level just in second. It is hard to lock-out all CPUs same 
> time. It's surely not solution, but still...

While I am hardly an expert on CPU power management, I have done a lot
of testing and benchmarking and I really think that FreeBSD uses CPU
throttling and/or P4TCC lin a manner not intended by Intel and not as
Windows does and I think that is why we have the problem.

TCC was designed to be used for thermal control and I suspect, though I
have no documentation to support it, that throttling was, as well. They
were to allow the temperature of the CPU to be kept at a safe level
when, due to inadequate heat transfer or some other problem, a CPU could
be getting hot enough to damage itself. 

While both result in steps of 12.5% reduction in speed, for their
designed purpose, they probably will only be used when the CPU is being
clocked (or over-clocked) at its highest speed and will only be slowed
by, perhaps, 25 or 37.5%, not 75 or 87.5%. Those long pauses simply
don't work well.

SpeedStep, EST, Cool 'n' Quiet, and the like are designed for power
management and my tests have shown them to be far more effective for
this than throttling. I simply disable both TCC and throttling and let
EST/Cool 'n' Quiet do the job. It seem almost as effective in reducing
power consumption and provides better responsiveness in my systems. It
does mean that, instead of 32 speeds, I have only 5, ranging from 2 GHz
down to .8 Ghz, but that is quite adequate for my needs.

Note that this applies to newer systems. SpeedStep was not very good and
TCC/throttling was probably needed on old systems. (It was on my old 1
GHz P4, at least.) But modern systems seem happiest and most efficient
when just using the tools designed for power management.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751
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