Re: dynamic-hz

[Con Kolivas <kernel@kolivas.org>]

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Andrea Arcangeli wrote:
> On Sun, Dec 12, 2004 at 05:35:47PM +0100, Pavel Machek wrote:
> 
>>It certainly helps with singing capacitors... What is overhead of
> 
> 
> ;)
> 
> 
>>this?
> 
> 
> 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.

Cheers,
Con

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