Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt

[john stultz <johnstul@us.ibm.com>]

On Mon, 2005-07-11 at 17:38 -0700, George Anzinger wrote:
> Martin J. Bligh wrote:
> >>>Lots of people have switched from 2.4 to 2.6 (100 Hz to 1000 Hz) with no impact in
> >>>stability, AFAIK. (I only remember some weird warning about HZ with debian woody's
> >>>ps).
> >>>
> >>
> >>Yes, that's called "progress" so no one complained.  Going back is
> >>called a "regression".  People don't like those as much.
> > 
> > 
> > That's a very subjective viewpoint. Realize that this is a balancing
> > act between latency and overhead ... and you're firmly only looking
> > at one side of the argument, instead of taking a compromise in the
> > middle ...
> > 
> > If I start arguing for 100HZ on the grounds that it's much more efficient,
> > will that make 250/300 look much better to you? ;-)
> 
> I would like to interject an addition data point, and I will NOT be subjective. 
>   The nature of the PIT is that it can _hit_ some frequencies better than 
> others.  We have had complaints about repeating timers not keeping good time. 
> These are not jitter issues, but drift issues.  The standard says we may not 
> return early from a timer so any timer will either be on time or late.  The 
> amount of lateness depends very much on the HZ value.  Here is what the values 
> are for the standard CLOCK_TICK_RATE:
> 
> HZ  	TICK RATE	jiffie(ns)	second(ns)	 error (ppbillion)
>   100	 1193180	10000000	1000000000	       0
>   200	 1193180	 5000098	1000019600	   19600
>   250	 1193180	 4000250	1000062500	   62500
>   500	 1193180	 1999703	1001851203	 1851203
> 1000	 1193180	  999848	1000847848	  847848
> 
> The jiffie values here are exactly what the kernel uses and are based on the 
> best one can do with the PIT hardware.
> 
> I am not suggesting any given default HZ, but rather an argumentation of the 
> help text that goes with it.  For those who want timers to repeat at one second 
> (or multiples there of) this is useful info.
> 
> For you enjoyment I have attached the program used to print this.  It allows you 
> to try additional values...

If I recall, 1001 was a decent choice and is relatively close the the
expected frequency. Also I think the error is positive instead of
negative, so it avoids the "jiffies are shorter then I expected!"
issues.

>From your program's output:
HZ      TICK RATE       jiffie(ns)      second(ns)       error (ppbillion)
1001     1193180          999013        1000012013         12013

thanks
-john


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