Re: cross-cpu balancing with the new scheduler

[Ingo Molnar]

On Mon, 14 Jan 2002, Anton Blanchard wrote:

> Rusty and I were talking about this recently. Would it make sense for
> the load balancer to use a weighted queue length (sum up all
> priorities in the queue?) instead of just balancing the queue length?

something like this would work, but it's not an easy task to *truly*
balance priorities (or timeslice lengths instead) between CPUs.

Eg. in the following situation:

	CPU#0			CPU#1

	prio 1			prio 1
	prio 1			prio 1
	prio 20			prio 1

if the load-balancer only looks at the tail of the runqueue then it finds
that it cannot balance things any better - by moving the prio 20 task over
to CPU#1 it will not create a better-balanced situation. If it would look
at other runqueue entries then it could create the following,
better-balanced situation:

	CPU#0			CPU#1

	prio 20			prio 1
				prio 1
				prio 1
				prio 1
				prio 1

the solution would be to search the whole runqueue and migrate the task
with the shortest timeslice - but that is a pretty slow and
cache-intensive thing to do.

	Ingo