Re: [PATCH] 2.4: variable HZ

[Richard B. Johnson]

On 18 Oct 2002, Robert Love wrote:

> On Fri, 2002-10-18 at 07:51, Neil Conway wrote:
> 
> > I was looking at your jiffies_to_clock_t() macro, and I notice that it
> > will screw up badly if the user chooses a HZ value that isn't a multiple
> > of the normal value (e.g. 1000 is OK, 512 isn't).
> 
> OK, sure, but why specify a power-of-two HZ?  There is absolutely no
> reason to, at least on x86.
> 
> Want 512?  500 will do just as well and has the benefit of (a) being a
> multiple of the previous HZ and (b) evenly dividing into our concept of
> time.
> 
> 	Robert Love
> 

At least on ix86, HZ needs to be something that CLOCK_TICK_RATE/LATCH
comes out fairly close. Remember, LATCH is the divisor for the PIT
and that PIT gets CLOCK_TICK_RATE for its input. If this number isn't
fairly 'exact' there will be much jumping of time in the sawtooth
corrector.

If you are not using ELAN, CLOCK_TICK_RATE is 1193180. If your HZ is
100, you have 1193180/100 = 1193.18, not too exact. if you use
500, you get 1193180/500 = 2386.36 which has twice as much round-off.
If you use 1193180/512 = 2330.43, even a higher fractional part.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
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