Messages in this thread | | | Date | Thu, 12 Jan 2006 09:05:11 +1100 | From | Peter Williams <> | Subject | Re: -mm seems significanty slower than mainline on kernbench |
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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.
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.
Peter PS -- Peter Williams pwil3058@bigpond.net.au
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