linux-kernel - Re: [RFC PATCH 0/3] Fix SLQB on memoryless configurations V2

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Date:	Tue, 22 Sep 2009 02:30:34 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...e.de>,
	Pekka Enberg <penberg@...helsinki.fi>,
	heiko.carstens@...ibm.com, sachinp@...ibm.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Tejun Heo <tj@...nel.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>
Subject: Re: [RFC PATCH 0/3] Fix SLQB on memoryless configurations V2

On Mon, 21 Sep 2009, David Rientjes wrote:

> I disagree that we need kernel support for memoryless nodes on x86 and
> probably on all architectures period.  "NUMA nodes" will always contain
> memory by definition and I think hijacking the node abstraction away from
> representing anything but memory affinity is wrong in the interest of a
> long-term maintainable kernel and will continue to cause issues such as
> this in other subsystems.

Amen. Sadly my past opinions on this did not seem convincing enough.

> I do understand the asymmetries of these machines, including the ppc that
> is triggering this particular hang with slqb.  But I believe the support
> can be implemented in a different way: I would offer an alternative
> representation based entirely on node distances.  This would isolate each
> region of memory that has varying affinity to cpus, pci busses, etc., into
> nodes and then report a distance, whether local or remote, to other nodes
> much in the way the ACPI specification does with proximity domains.

Good idea.

> Using node distances instead of memoryless nodes would still be able to
> represent all asymmetric machines that currently benefit from the support
> by binding devices to memory regions to which they have the closest
> affinity and then reporting relative distances to other nodes via
> node_distance().

How would you deal with a memoryless node that has lets say 4 processors
and some I/O devices? Now the memory policy is round robin and there are 4
nodes at the same distance with 4G memory each. Does one of the nodes now
become priviledged under your plan? How do you equally use memory from all
these nodes?


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