Messages in this thread | | | Date | Fri, 13 Jan 2006 09:20:38 +1100 | From | Peter Williams <> | Subject | Re: -mm seems significanty slower than mainline on kernbench |
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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
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