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    DateMon, 13 Dec 2004 00:42:56 +0100
    FromPavel Machek <>
    SubjectRe: dynamic-hz
    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...
    ...jr ghea gurz vagb qrirybcref, naq gurl frrz gb yvxr vg gung jnl!
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