Messages in this thread | | | Date | Mon, 13 Dec 2004 17:06:05 +0100 | From | Pavel Machek <> | Subject | Re: dynamic-hz |
| |
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
-- Boycott Kodak -- for their patent abuse against Java. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |