Re: dynamic-hz

From: Alan Cox
Date: Mon Dec 13 2004 - 11:05:43 EST

On Llu, 2004-12-13 at 13:58, Russell King wrote:
> 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

Now take a real laptop and the numbers are in the 20W (15A) range.

> 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)

On a PC it makes huge sense, the deeply embedded folks who do turn the
thing off for 30secs at a time (Eg cellphone) also want it as do
virtualisation people where it trashes your scaling. API wise it isn't
too hard, its just a matter of time to convert the jiffies users away
and to do relative versions of add_timer with accuracy info included.

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