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

[Nick Piggin]

Martin J. Bligh wrote:

> >    Good stuff, I just gave the patch a spin and things seem a little
> >    calmer. However Im still seeing a lot of balancing going on within a
> >    node.
> >
> > This is a clearly recognizable edge case, so I'll try drawing this up on
> > some paper and see if I can suggest another patch.  There's no good reason
> > to move one lone process from a particular processor to another idle one.
> >
> > But it also approaches a question that's come up before:  if you have 2
> > tasks on processor A and 1 on processor B, do you move one from A to B?
> > One argument is that the two tasks on A will take twice as long as
> > the one on B if you do nothing.  But another says that bouncing a task
> > around can't correct the overall imbalance and so is wasteful.  I know
> > of benchmarks where both behaviors are considered important.  Thoughts?
>
> It's the classic fairness vs throughput thing we've argued about before.
> Most workloads don't have that static a number of processes, but it 
> probably does need to do it if the imbalance is persistent ... but much
> more reluctantly than normal balancing. See the patch I sent out a bit
> earlier to test it - that may be *too* extreme in the other direction,
> but it should confirm what's going on, at least.

Yep. I've argued for fairness here, and that is presently what
we get. Between nodes the threshold should probably be higher
though.

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