Re: HZ, preferably as small as possible

Daniel Phillips
2002-07-17T20:36:08Z

On Wednesday 17 July 2002 22:31, Richard B. Johnson wrote:
> On Wed, 17 Jul 2002, Daniel Phillips wrote:
>
> > 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.
>
> It is hardly novel and I can't imagine how Bresenham or whomever
> could make such a claim to the obvious. Even the DOS writer(s) used
> this technique to get one-second time intervals from the 18.206
> ticks/per second. This is simply division by subtraction, but you
> don't throw away the remainder. Therefore, in the limit, there is
> no remainder. However, at any instant, the time can be off by as
> much as the divisor -1. FYI, you make digital filters using this
> same method, it's hardly novel.

It's novel for Linux then, because it seems not to have occured to
anyone here. I'll take your agressive response as a vote in favor.

--
Daniel