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

[Bill Davidsen <davidsen@tmr.com>]

Linus Torvalds wrote:

> On Thu, 14 Jul 2005, Lee Revell wrote:
>  
>
> > I don't think this will fly because we take a big performance hit by
> > calculating HZ at runtime.
> >    
>
> I think it might be an acceptable solution for a distribution that really 
> needed it, since it should be fairly simple. However, it's definitely not 
> the right solution.
>
> HOWEVER. I bet that somebody who really really cares (hint hint) could
> easily make HZ be 1000, and then dynamically tweak the divisor at bootup
> to be either 1000, 250, or 100, and then increment "jiffies" by 1, 4 or
> 10.
>
> My wild guess is that this is 20 lines of code, plus another 20 for 
> "setup", so that you can choose between 100/250/1000 Hz with a kernel 
> command line.
>
> It wouldn't be "dynamic" at first - you'd just set it up at bootup, and 
> set a "jiffies_increment" variable, and change the
>
> 	jiffies_64++;
>
> into
>
> 	jiffies_64 += jiffies_increment;
>
> and you'd be done. 
>
> Really. I dare you guys. First one to send me a tested patch gets a gold 
> star.
>
> Then, a year from now, people will realize how _easy_ it is to change the
> jiffies_increment on the fly, and add a /sys/kernel/timer_frequency file, 
> and then you can switch it around at run-time.
>
> Trust me. When I say that the right thing to do is to just have a fixed 
> (but high) HZ value, and just changing the timer rate, I'm -right-.
>
> I'm always right. This time I'm just even more right than usual.

And humble, too ;-)

Do you actually have something against tickless, or just don't think it 
can be done in reasonable time? I can see this needing very careful 
thought on SMP.

--
bill davidsen <davidsen@tmr.com>
 CTO TMR Associates, Inc
 Doing interesting things with small computers since 1979

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