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

[Nick Piggin]

Martin J. Bligh wrote:

> > From a later email ....
> >
> > 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.
>
> I'd side with just removing the addition alltogether ...

By that stage though, it has already passed through the
imbalance_pct filter, and with the higher precision and
averaging of previous loads, it might take a while to
get there.

> > > Moreover, as Rick pointed out, it's particularly futile over idle cpus ;-)
> > I don't follow...
>
> If CPU 7 has 1 task, and cpu 8 has 0 tasks, there's an imbalance of 1.
> There is no point whatsoever in bouncing that task back and forth
> between cpu 7 and 8 - it just makes things slower, and trashes the cache.
> There's *no* fairness issue here.

Right, it should not be moved. I think Anton is seeing a problem
with active balancing, and not so much a problem with the imbalance
calculation though.

> If CPU 8 has 2 tasks, and cpu 1 has 1 task, there's an imbalance of 1.
> *If* that imbalance persists (and it probably won't, given tasks being
> created, destroyed, and blocking for IO), we may want to rotate that 
> to 1 vs 2, and then back to 2 vs 1, etc. in the interests of fairness,
> even though it's slower throughput overall.

Yes, although as long as it's node local and happens a couple of
times a second you should be pretty hard pressed noticing the
difference.

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/