Re: Interesting scheduling times - NOT
Peter T. Breuer (ptb@it.uc3m.es)
Tue, 22 Sep 1998 11:18:53 +0200 (MET DST)
"A month of sundays ago Richard Gooch wrote:"
>
> > median. Even when I add about 1K interrupts/second, it still varies
> > less than 10%. Why does your benchmark vary so much by comparison?
> > Why doesn't lmbench vary the same amount?
>
> See above. The minimum time has a better chance of avoiding cache
> pollution/aliasing effects.
>
> Note: in my tests, I see substantial variance mainly with the process
> switching test, not the thread switching test. This is particularly
> the case now that Linus posted the FPU saving fix.
> On a PPro 180 I'm seeing minimum process switch times of 4.8 us to
> 8.5 us. That's a 77% increase. I think that variance is real, and not
> an artefact of my test code.
Eh? does anyone know the statistics of the MINIMUM result from N normally
distributed tests?
I don't, but since the minimum comes from the tail, it is a rare event
and hence its variance must be large.
Your result is what I would naively expect from a normal curve, but I don't
believe you have a normal curve. Negative times are impossible :-).
You must have a quantized distribution (nano seconds?) with a tail
at 0 and +oo. I suppose it's m*exp(-x/m) or something. Can you run
some monte carlo simulations with that to get an idea of the distribution of
the _minimum_?
Peter
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