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From: Daniel Phillips <phillips@arcor.de>
To: torvalds@transmeta.com (Linus Torvalds), linux-kernel@vger.kernel.org
Subject: Re: HZ, preferably as small as possible
Date: Wed, 17 Jul 2002 21:33:07 +0200
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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
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