Re: [PATCH 10/13] remove aggressive idle balancing

[Siddha, Suresh B]

On Tue, Mar 08, 2005 at 07:17:31PM +1100, Nick Piggin wrote:
> Siddha, Suresh B wrote:
> > 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

I agree.

> 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.
> 
> > 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.

With periodic balancer, moved tasks will not be actively running
and by the time it gets a cpu slot, it will most probably be on the
correct cpu (though "most probably" is the key word here ;-)

> 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.

With active_load_balance, we will be moving the currently running
process. So obviously if we can move in one single step, that will be nice.

I agree with the complexity part though. And it will become more complex
with upcoming dual-core requirements.

thanks,
suresh
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