Re: [PATCH] Load balancing problem in 2.6.2-mm1

[Nick Piggin]

Rick Lindsley wrote:

>    OK, but do you agree that the rate we rebalance things like 2 vs 1 should
>    be slower than the rate we rebalance 3 vs 1 ? Fairness is only relevant
>    over a long term imbalance anyway, so there should be a big damper on
>    "fairness only" rebalances.
>
> I think, given the precision we're granted via SCHED_LOAD_SCALE, in
> combination with the new "load average" (cpu_load) code, that we can
> achieve what we want.
>
> If cpu0 has 2 runnable tasks and cpu1 has 1 runnable task, won't we see
> the "load average" of cpu0 slowly approach 2, but not jump there?

Yep

> Right now, we round up on all fractions and Martin has proposed a patch
> which takes it the other way and rounds down.  What if in marginal
> cases like this where this is a small but persistent difference, we
> could bump the task to another cpu when it reaches (say) 1.8 or 1.9?
> That would keep it there longer for shorter-lived tasks, but for those
> long-runners, they'd eventually spread the pain around a little.
>
> And yes, a cpu_load of even 1.0 should *never* get migrated to a cpu
> with a load 0.0.  Instead of

This isn't the load though, but imbalance. It has already passed
through our imbalance_pct filter (or we are idle), so we can pretty
safely assume that we want to try to move at least one task.

>    *imbalance = (*imbalance + SCHED_LOAD_SCALE - 1) >> SCHED_LOAD_SHIFT;
>
> how about, for instance,
>
>    if (max_load <= SCHED_LOAD_SCALE)
> 	*imbalance = 0;
>    else
> 	*imbalance = (*imbalance + (SCHED_LOAD_SCALE / 6) - 1)
> 	    >> SCHED_LOAD_SHIFT;
>
> The intent is to never move anything if max_load is 1 or less (what
> advantage is there?) and to create a slight tendency to round up at
> loads greater than that, which would still tend to leave things where
> they were until they'd been there a while.  In fact the "bonus"
> (SCHED_LOAD_SCALE / 6 - 1) could be another configurable in the scheduling
> domain so that at some level you're not interested in fairness and
> they just don't bounce at all.

Hopefully just tending to round down more would damp it better.
*imbalance = (*imbalance + SCHED_LOAD_SCALE/2) >> SCHED_LOAD_SHIFT;
Or even remove the addition all together.

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