Messages in this thread Patch in this message | | | Date | Thu, 12 Jan 2006 11:54:30 +1100 | From | Peter Williams <> | Subject | Re: -mm seems significanty slower than mainline on kernbench |
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Peter Williams wrote: > Con Kolivas wrote: > >> On Wednesday 11 January 2006 23:24, Peter Williams wrote: >> >>> Martin J. Bligh wrote: >>> >>>> That seems broken to me ? >>> >>> >>> But, yes, given that the problem goes away when the patch is removed >>> (which we're still waiting to see) it's broken. I think the problem is >>> probably due to the changed metric (i.e. biased load instead of simple >>> load) causing idle_balance() to fail more often (i.e. it decides to not >>> bother moving any tasks more often than it otherwise would) which would >>> explain the increased idle time being seen. This means that the fix >>> would be to review the criteria for deciding whether to move tasks in >>> idle_balance(). >> >> >> >> Look back on my implementation. The problem as I saw it was that one >> task alone with a biased load would suddenly make a runqueue look much >> busier than it was supposed to so I special cased the runqueue that >> had precisely one task. > > > OK. I'll look at that.
Addressed in a separate e-mail.
> > 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>
Peter -- Peter Williams pwil3058@bigpond.net.au
"Learning, n. The kind of ignorance distinguishing the studious." -- Ambrose Bierce Index: MM-2.6.X/kernel/sched.c =================================================================== --- MM-2.6.X.orig/kernel/sched.c 2006-01-12 09:23:38.000000000 +1100 +++ MM-2.6.X/kernel/sched.c 2006-01-12 10:44:50.000000000 +1100 @@ -2116,11 +2116,11 @@ find_busiest_group(struct sched_domain * (avg_load - this_load) * this->cpu_power) / SCHED_LOAD_SCALE; - if (*imbalance < SCHED_LOAD_SCALE) { + if (*imbalance < NICE_TO_BIAS_PRIO(0) * SCHED_LOAD_SCALE) { unsigned long pwr_now = 0, pwr_move = 0; unsigned long tmp; - if (max_load - this_load >= SCHED_LOAD_SCALE*2) { + if (max_load - this_load >= NICE_TO_BIAS_PRIO(0) * SCHED_LOAD_SCALE*2) { *imbalance = NICE_TO_BIAS_PRIO(0); return busiest; } | |