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SubjectRe: [PATCH 1/2] NUMA emulation x86_64: numa=fake parameter for custom nodes distance
On Fri, 18 Nov 2011, Petr Holasek wrote:

> As default, when numa emulation is turned on, node distance table
> uses physical distance, so for 4 nodes emulated on 1 physical table is
>
> node 0 1 2 3
> 0: 10 10 10 10
> 1: 10 10 10 10
> 2: 10 10 10 10
> 3: 10 10 10 10
>

That should only be true if you're booting on a system with one physical
node and an SRAT, otherwise the distance between fake nodes should be
representative of their physical distance. For example, if you boot
with numa=fake=4 on a two symmetrical two-node box, you should get
something like

10 10 20 20
10 10 20 20
20 20 10 10
20 20 10 10

It's done like this intentionally so you can test NUMA without having many
nodes. What you're doing is changing the distance even though there is no
actual difference in latency on the hardware so it's an incorrect
representation.

> This patch adds new [distance] argument to
>
> numa=fake=<number/size of nodes>[,distance]
>
> When distance argument is used, it sets linear distance between nodes
> like that:
>
> __distance__
> ___|___ ____|___ ________ ________
> | | | | | | | |
> | node1 |---| node 2 |---| node 3 |---| node 4 |
> |_______| |________| |________| |________|
> | | |
> | | |
> |____distance * 2________| |
> | |
> |____________distance * 3______________|
>
> This feature might be useful for testing some numa awareness features in
> both user and kernel spaces.
>

I don't see any use case for this other than testing if code can actually
order nodes correctly or not. The distances that you're now adding are,
by definition, incorrect since they aren't the same as exported by the
true SLIT (which is what happens by default now) so nothing other than
functional testing of node ordering is achieved with this patch.

So nack on this approach.

What you could do, however, and would be generally useful even outside of
NUMA emulation, is to add fake SLIT functionality so that you can define
it yourself on the command line. You could use that either with or
without NUMA emulation if you know the physical SLIT is incorrect in some
way. Then, you get the same functionality as your patch here by using it
in combination with numa=fake and the added bonus is that you don't need
any of the "distance * 2" or "distance * 3" limitations.


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