Re: Plumbers: Tweaking scheduler policy micro-conf RFP

[Peter Zijlstra <peterz@infradead.org>]

On Sat, 2012-05-19 at 10:08 -0700, Linus Torvalds wrote:
> Don't try to build up some perfect NUMA topology and then
> try to see how insanely well you can match a particular machine. Make
> some generic "roughly like this" topology with (say) four three of
> NUMAness, and then have architectures say "this is roughly what my
> machine looks like".

> In particular, don't even try to give random "weights" to how close
> things are to each other. Sure, you can parse (and generate) those
> complex NUMA tables, but nobody is *ever* smart enough to really use
> them. Once you move data between boards/nodes, screw the number of
> hops. You are NOT going to get some scheduling decision right that
> says "node X is closer to node Y than to node Z". Especially since the
> load is invariably going to access non-node memory too *anyway*. 

I suspect this is related to the patch I recently did that creates numa
levels from the node_distance() table.

The fact is, that patch removed arch specific code. And yes initially I
tried to use the weights for more than simply creating the balance
levels but I've already realized that was a mistake and removed that
part.

So currently all it does is create load-balance levels based on how far
apart nodes are said to be and decrease the balance rate roughly
proportional to how many cpus are in each level.

The node_distance() table is mostly already a fabrication of the
arch/firmware; some people do exactly what you suggested, expose simple
groups of board vs rest and not bother with fine details.

I used the node_distance() table simply because this was an existing
arch interface that provides exactly what was needed and is used for
exactly this purpose in the mm/ part of the kernel as well.