Re: Re: Centrino speedstep

Nate Lawson
2005-01-07T23:49:51Z

Fred Labrosse wrote:
> Dominik Brodowski writes:
> > (however, as the task now takes twice as long to complete, and the CPU still
> > consumes power in "t", the CPU does consume more energy than if it weren't
> > throttled. Only if the fan needs to run _because_ of the high CPU load and
> > it consumes more power than the CPU in throttling mode, throttling may save
> > energy.)
>
> I was missing something (brain power?). Very interesting. Thank you for
> that.

It's non-intuitive since power consumption is an integral (i.e., the sum
of individual power usage over time t). The area under the curve is
what you care about.

For interactive use, the best scheduling algorithm for CPU idling is to
run the CPU at 100% performance until there are no runnable tasks, then
slam it to the lowest Cx state until woken by the next timer tick or
interrupt. Since even slow C3 transitions on most platforms are 200
microseconds or less and given a HZ value of 1000, you still get 600 us
of sleeping at C3 (3x the latency of a transition.) The longer-latency
cpufreq transition is different, in that you should go to the lowest CPU
state once you have M wakeups over N amount of time but only a small
number of runnable tasks in that time period. Tune M and N based on the
latency of cpufreq transitions for a platform.

With even speedstep transitions being less than a millisecond, there is
no reason not to use it more.

--
Nate