RFC: powerd algorithms enhancements

Alexander Motin mav at FreeBSD.org
Fri Nov 7 01:02:27 PST 2008

Ian Smith wrote:
> Hi, sounds like sound's more or less under control, time on your hands? :)

There are many subsystems used in my laptop. :)

>  > I would like to propose the patch for powerd that fixes some issues, makes it
>  > more universal and on my opinion more usable. The main ideas of mine were:
>  > 
>  > 1. To make it more SMP polite. Previous version uses average CPU load that
>  > leads to the often load underestimation. It make powerd with default
>  > configuration unusable on systems with more then 2 CPUs. I propose to use
>  > summary load instead of average one. IMO this is the best we can do without
>  > specially tuned scheduler. Also as soon as measuring total load on SMP
>  > systems is more useful then total idle, I have switched to it.
> Ok, very interesting.  First, is this against CURRENT, or to what CVS 
> version, so I can read patched version in full?  Any change to .h files?

It's against the HEAD, but applies to 7-STABLE as well.

>  > 2. To make powerd's operation independent from number and size of frequency
>  > levels I have added internal frequency counter which translated into real
>  > frequencies only on a last stage and only as good as gone. Some systems may
>  > have only several power levels, while mine has 17 of them, so adaptation time
>  > in completely different. It would be good if algorithm was not depending on
>  > it.
> There were some XXX comments re longterm allowance for running different 
> cpus at different freqs .. I don't know if that's anything to consider?

I don't understand which comments do you mean. But I think that it is
now ineffective to run different CPUs ad different frequencies. To do it
we should have scheduler aware of CPUs speed to avoid using powered down
ones where it is possible. Now it will just lead to significant
performance degradation because of CPU load underestimation.

>  > 3. As part of previous I have changed adaptive mode to rise frequency on
>  > demand up to 2 times and fall on 1/8 per time internal.
> I'm wondering how the edge case with only 2 freqs would go?  Eg on my 
> T23, single cpu P3 Mobile at 1133 and 733MHz.  That is, I'm wondering if 
> your 1/8 factor might better be scaled to no. of cpus and/or no. of 
> freqs available?  I'd best say no more until studying your algorithm ..

I have not such case, but I think there should be no problem.

>  > 4. For desktop (AC-powered) systems I have added one more mode -
>  > "hiadaptive". It rises frequency twice faster, drops it 4 times slower,
>  > prefers twice lower CPU load and has additional delay before leaving the
>  > highest frequency after the period of maximum load. This mode was specially
>  > made to improve interactivity of the systems where operation capabilities are
>  > more significant then power consumption, but keeping maximum frequency all
>  > the time is not needed.
> Great idea.  And one (not so) small step towards some proper profiles, 
> where various degrees of performance vs responsiveness vs power use can 
> be setup by the user .. extending now binary AC/battery power_profile 
> choices (starting freq, lowest Cx state), later perhaps tying in with 
> the shutdown/wakeup stuff for both system and individual devices (eg D 
> states).  Sorry, just musing aloud .. this has needed a kick for ages :)

Move configuration from command line into configuration file will allow
more customized profiles to be written, so if somebody wants to - he may
do it. For trivial command line configuration this solution looks like

>  > 5. I have reduced polling interval from 1/2 to 1/4 of second. It is not
>  > important for algorithm math now, but gives better system interactivity.
> You mean the default polling interval I guess, as it's tuneable at least 
> on powerd startup, as are the loaded/idle points, which as someone else 
> mentioned, might be more dynamically modified while powerd is running?

It's possible, but I don't see real reason to do it. Increased polling
interval will lead to significant latency, while economy will be
minimal. I think 2KHz of timer interrupts per CPU consume much more
energy then powerd waking up 4 times per second.

> Then if you get really bored :) SMP suspend/resume and S4 suspend to 
> disk need a champion .. both of which have at least begun in acpi at .

If I do everything, there will nothing left to you. I don't want you to
become upset. ;)

Alexander Motin

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