Messages in this thread | | | Date | Tue, 08 Mar 2005 19:17:31 +1100 | From | Nick Piggin <> | Subject | Re: [PATCH 10/13] remove aggressive idle balancing |
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Siddha, Suresh B wrote: > Nick, > > On Mon, Mar 07, 2005 at 07:28:23PM +1100, Nick Piggin wrote: > >>Siddha, Suresh B wrote: >> >>>We are resetting the nr_balance_failed to cache_nice_tries after kicking >>>active balancing. But can_migrate_task will succeed only if >>>nr_balance_failed > cache_nice_tries. >>> >> >>It is indeed, thanks for catching that. We should probably make it >>reset the count to the point where it will start moving cache hot >>tasks (ie. cache_nice_tries+1). > > > That still might not be enough. We probably need to pass push_cpu's > sd to move_tasks call in active_load_balance, instead of current busiest_cpu's > sd. Just like push_cpu, we need to add one more field to the runqueue which > will specify the domain level of the push_cpu at which we have an imbalance. >
It should be the lowest domain level that spans both this_cpu and push_cpu, and has the SD_BALANCE flag set. We could possibly be a bit more general here, but so long as nobody is coming up with weird and wonderful sched_domains schemes, push_cpu should give you all the info needed.
>>Ah yep, right you are there, too. I obviously hadn't looked closely >>enough at the recent active_load_balance patches that had gone in :( >>What should probably do is heed the "push_cpu" prescription (push_cpu >>is now unused). > > > push_cpu might not be the ideal destination in all cases. Take a NUMA domain > above SMT+SMP domains in my above example. Assume P0, P1 is in node-0 and > P2, P3 in node-1. Assume Loads of P0,P1,P2 are same as the above example,with P3 > containing one process load. Now any idle thread in P2 or P3 can trigger > active load balance on P0. We should be selecting thread in P2 ideally > (currently this is what we get with idle package check). But with push_cpu, > we might move to the idle thread in P3 and then finally move to P2(it will be a > two step process) >
Hmm yeah. It is a bit tricky. We don't currently do exceptionally well at this sort of "balancing over multiple domains" very well in the periodic balancer either.
But at this stage I prefer to not get overly complex, and allow some imperfect task movement, because it should rarely be a problem, and is much better than it was before. The main place where it can go wrong is multi-level NUMA balancing, where moving a task twice (between different nodes) can cause more problems.
Nick
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