linux-kernel - Re: [RFC][PATCH 14/26] sched, numa: Numa balancer

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Date:	Mon, 09 Jul 2012 10:50:50 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Dan Smith <danms@...ibm.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH 14/26] sched, numa: Numa balancer

On 07/09/2012 08:40 AM, Peter Zijlstra wrote:
> On Mon, 2012-07-09 at 14:23 +0200, Peter Zijlstra wrote:
>>> It is not yet clear to me how and why your code converges.
>>
>> I don't think it does.. but since the scheduler interaction is fairly
>> weak it doesn't matter too much from that pov.

Fair enough. It is just that you asked this same question
about Andrea's code, and I was asking myself that question
while reading your code (and failing to figure it out).

> That is,.. it slowly moves along with the cpu usage, only if there's a
> lot of remote memory allocations (memory pressure) things get funny.
>
> It'll try and rotate all tasks around a bit trying, but there's no good
> solution for a memory hole on one node and a cpu hole on another, you're
> going to have to take the remote hits.

Agreed, I suspect both your code and Andrea's code will
end up behaving fairly similarly in that situation.

> Again.. what do we want it to do?

That is a good question.

We can have various situations to deal with:

1) tasks fit nicely inside NUMA nodes
2) some tasks have more memory than what fits
    in a NUMA node
3) some tasks have more threads than what fits
    in a NUMA node
4) a combination of the above

I guess what we want the NUMA code to do to increase
the number of local memory accesses for each thread,
and do so in a relatively light weight way.

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