Re: dynamic-hz

Pavel Machek
2004-12-12T23:49:37Z

Hi!

> >The overhead is a single l1 cacheline in the paths manipulating HZ
> >(rather than having an immediate value hardcoded in the asm, it reads it
> >from a memory location not in the icache). Plus there are some
> >conversion routines in the USER_HZ usages. It's not a measurable
> >difference.
>
> Just being devils advocate here...
>
> I had variable Hz in my tree for a while and found there was one
> solitary purpose to setting Hz to 100; to silence cheap capacitors.
>
> The rest of my users that were setting Hz to 100 for so-called
> performance gains were doing so under the false impression that cpu
> usage was lower simply because of the woefully inaccurate cpu usage
> calcuation at 100Hz.
>
> The performance benefit, if any, is often lost in noise during
> benchmarks and when there, is less than 1%. So I was wondering if you
> had some specific advantage in mind for this patch? Is there some
> arch-specific advantage? I can certainly envision disadvantages to lower Hz.

Actually, I measured about 1W power savings with HZ=100. That's about
as much as spindown of disk saves...
Pavel
--
People were complaining that M$ turns users into beta-testers...
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