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

[Martin J. Bligh]

>> 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).

I ran it on the 16-way - no difference in performance. If the code is 
correct as was before (and I agree, it seems it was), perhaps it's just
in need of a big fat comment to explain the confusion? ;-)

M.

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