linux-kernel - Re: Plumbers: Tweaking scheduler policy micro-conf RFP

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Date:	Sun, 20 May 2012 01:13:01 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	paulmck@...ux.vnet.ibm.com, smuckle@...cinc.com, khilman@...com,
	Robin.Randhawa@....com, suresh.b.siddha@...el.com,
	thebigcorporation@...il.com, venki@...gle.com,
	panto@...oniou-consulting.com, mingo@...e.hu, paul.brett@...el.com,
	pdeschrijver@...dia.com, pjt@...gle.com, efault@....de,
	fweisbec@...il.com, geoff@...radead.org, rostedt@...dmis.org,
	tglx@...utronix.de, amit.kucheria@...aro.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linaro-sched-sig@...ts.linaro.org,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Juri Lelli <juri.lelli@...il.com>
Subject: Re: Plumbers: Tweaking scheduler policy micro-conf RFP

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.

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