Re: dynamic-hz

Pavel Machek
2004-12-13T16:15:06Z

Hi!

> > > Just being devils advocate here...
> > >
> > > I had variable Hz in my tree for a while and found there was one
> > > solitary purpose to setting Hz to 100; to silence cheap capacitors.
> >
> > power savings? Having the cpu wake up 1000 times per second if the
> > machine is idle cannot be better than only waking it up 100 times.
> >
> > Yes, i am always on the quest for the 5 extra minutes on battery :-)
>
> This is an easy thing to grab hold of, but rather pointless in the
> overall scheme of things. Those of us who have done power usage
> measurements know this already.
>
> The only case where this really makes sense is where the CPU power
> usage outweighs the power consumption of all other peripherals by
> at least an order of magnitude such that the rest of the system is
> insignificant compared to the CPU power.

Why by order of magnitude? Anyway on PC machines, cpu in low-power
mode takes about as much as rest of system, and in high-power mode it
takes more than rest of system combined.

I measured 1W savings from HZ=100, and that was on system that takes
17W total (arima athlon64 notebook). That is > 5%.

> Lets take an example. Lets say that:
> * a CPU runs at about 245mA when active
> * 90mA when inactive
> * the timer interrupt takes 2us to execute 1000 times a second
> * no other processing is occuring

You assume that cpu goes to sleep immeidately. That is *very* far away
from reality on at least pentium 4. It takes half a milisecond to
sleep/wakeup the cpu, that basically means that low power mode is not
ever entered with HZ=1000...
Pavel

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