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SubjectRe: -mm seems significanty slower than mainline on kernbench
Peter Williams wrote:
> Martin Bligh wrote:
>
>>
>>>>
>>>> But I was thinking more about the code that (in the original)
>>>> handled the case where the number of tasks to be moved was less than
>>>> 1 but more than 0 (i.e. the cases where "imbalance" would have been
>>>> reduced to zero when divided by SCHED_LOAD_SCALE). I think that I
>>>> got that part wrong and you can end up with a bias load to be moved
>>>> which is less than any of the bias_prio values for any queued tasks
>>>> (in circumstances where the original code would have rounded up to 1
>>>> and caused a move). I think that the way to handle this problem is
>>>> to replace 1 with "average bias prio" within that logic. This would
>>>> guarantee at least one task with a bias_prio small enough to be moved.
>>>>
>>>> I think that this analysis is a strong argument for my original
>>>> patch being the cause of the problem so I'll go ahead and generate a
>>>> fix. I'll try to have a patch available later this morning.
>>>
>>>
>>>
>>>
>>> Attached is a patch that addresses this problem. Unlike the
>>> description above it does not use "average bias prio" as that
>>> solution would be very complicated. Instead it makes the assumption
>>> that NICE_TO_BIAS_PRIO(0) is a "good enough" for this purpose as this
>>> is highly likely to be the median bias prio and the median is
>>> probably better for this purpose than the average.
>>>
>>> Signed-off-by: Peter Williams <pwil3058@bigpond.com.au>
>>
>>
>>
>> Doesn't fix the perf issue.
>
>
> OK, thanks. I think there's a few more places where SCHED_LOAD_SCALE
> needs to be multiplied by NICE_TO_BIAS_PRIO(0). Basically, anywhere
> that it's added to, subtracted from or compared to a load. In those
> cases it's being used as a scaled version of 1 and we need a scaled

This would have been better said as "the load generated by 1 task"
rather than just "a scaled version of 1". Numerically, they're the same
but one is clearer than the other and makes it more obvious why we need
NICE_TO_BIAS_PRIO(0) * SCHED_LOAD_SCALE and where we need it.
> version of NICE_TO_BIAS_PRIO(0). I'll have another patch later today.

I'm just testing this at the moment.

Peter
--
Peter Williams pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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