Messages in this thread | | | Date | Mon, 13 Dec 2004 14:14:20 +0000 | From | Russell King <> | Subject | Re: dynamic-hz |
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On Mon, Dec 13, 2004 at 01:58:20PM +0000, Russell King wrote: > On Mon, Dec 13, 2004 at 08:43:55AM +0100, Stefan Seyfried wrote: > > Con Kolivas wrote: > > > 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. > > 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 > > This means that the average current consumption is about: > 245mA * 2 * 10^-6 + 90mA * (1 - 2 * 10^-6) = 90.00031mA
Sorry, missed out the 1000 times a second. Grumble.
245mA * 1000 * 2 * 10^-6 + 90mA * (1 - 2 * 10^-6 * 1000) = 90.31mA
> This means that the timer interrupt has increased CPU power by > 0.00034%.
0.34%
> Now, lets factor in the rest of a system. Lets the rest of the > system takes 84mA. Recalculating (by increasing each figure by > 84mA) gives us 174.00031mA, or an increase in overall system
174.31mA
> power by about 0.00018%.
0.18%
> Assuming your battery normally lasts exactly 24 hours on a current > drain of 174.00031mA, completely eliminating the tick gives you
174.31mA
> an extra 0.15 seconds battery life.
2mins 30secs
> Note: the above CPU power consumption figures were taken from > the Intel PXA255 processor electrical specifications, and the > "rest of the system" current consumption taken from a real life > device. The timer interrupt taking 2us is probably an over- > estimation. Only the battery lifetime of 24 hours is ficticious. > > And yes, from time to time I keep thinking that it would be nice > to eliminate the timer tick to save some power. However, I've > never been able to justify the extra code complexity against the > power savings. It really only makes sense if you can essentially > _power off_ your system until the next timer interrupt (thereby, > in the above example, reducing the power consumption by some 174mA)
-- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: 2.6 PCMCIA - http://pcmcia.arm.linux.org.uk/ 2.6 Serial core - 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/
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