Re: [PATCH] Fix find busiest queue 2.6.0-test9

[Davide Libenzi]

On Sun, 9 Nov 2003, Nick Piggin wrote:

> Andrew Morton wrote:
> 
> >Con Kolivas <kernel@xxxxxxxxxxx> wrote:
> >
> >>Hi
> >>
> >>I believe this is a simple typo / variable name mixup between rq_src and 
> >>this_rq. So far all testing shows positive (if minor) improvements.
> >>
> >>
> >
> >Looks right to me.  I'll queue this up for the 2.6.1 deluge.
> >
> 
> prev_cpu_load[i] is nr_running of cpu i last time this operation was
> performed. Either it, or the current nr_running is taken, whichever
> is lower.
> 
> I guess its done this way for cache benefits, but it was correct as
> Ingo intended. For example, with Con's patch you can see
> rq_src->prev_cpu_load[i] will only ever use the ith position in the array.

Yes. The prev_cpu_load[] array takes a snapshot of the run queue lengths 
seen by the current rq (this_rq). The code is ok as is, and the reason is 
to avoid stealing tasks too fast from remote CPU (cache thing). Time ago I 
also tried to store an K-average (by varying K) rq length in 
prev_cpu_load[] instead of a simple min-of-two-values:

this_rq->prev_cpu_load[i] = (K * this_rq->prev_cpu_load[i] + rq_src->nr_running) / (K + 1);

I couldn't see any major improvements in my 2SMP (never tried on bigger SMP/NUMA).



- Davide



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