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    SubjectRe: dynamic-hz
    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
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