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

Bill Davidsen
2005-07-15T02:03:53Z

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