Re: HZ, preferably as small as possible

Daniel Phillips
2002-07-17T19:29:42Z

On Monday 15 July 2002 07:06, Linus Torvalds wrote:
> There is, of course, the option to do variable frequency (and make it
> integer multiples of the exposed "constant HZ" so that kernel code
> doesn't actually need to _care_ about the variability). There are
> patches to play with things like that.

We don't have to feel restricted to integer multiples. I'll paste in my
earlier post, for your convenience:

> ...If somebody wants a cruder scheduling interval than the raw timer
> interrupt, that's child's play, just step the interval down.  The
> only slightly challenging thing is do that without restricting
> choice of rate for the raw timer and scheduler, respectively.  Here,
> a novel application of Bresenham's algorithm (the line drawing
> algorithm) works nicely: at each raw interrupt, subtract the period
> of the raw interrupt from an accumulator; if the result is less
> than zero, add the period of the scheduler to the accumlator and
> drop into the scheduler's part of the timer interrupt.

[which just increments the timer variable I believe]

> This Bresenham trick works for arbitrary collections of interrupt
> rates, all with different periods.  It has the property that,
> over time, the total number of invocations at each rate remains
> *exactly* correct, and so long as the raw interrupt runs at a
> reasonably high rate, displacement isn't that bad either.

This technique is scarcely less efficient than the cruder method.

--
Daniel