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    SubjectRe: NUMA emulation x86_64: numa=fake parameter for custom nodes distance
    On Sat, 19 Nov 2011, David Rientjes wrote:

    > Date: Sat, 19 Nov 2011 18:09:59 -0800 (PST)
    > From: David Rientjes <>
    > To: Petr Holasek <>
    > cc: Andrew Morton <>, Thomas Gleixner
    > <>, Ingo Molnar <>, "H. Peter Anvin"
    > <>,,, Anton
    > Arapov <>
    > Subject: Re: NUMA emulation x86_64: numa=fake parameter for custom nodes
    > distance
    > On Sat, 19 Nov 2011, Petr Holasek wrote:
    > > A lot of developers still have no access to large NUMA machines and
    > > possibility of NUMA emulation could involve more of them to thinking
    > > about NUMA awareness of their apps/kernel code.
    > >
    > That's a bogus argument, numa=fake already allows you to construct as
    > large of a NUMA box as you want in a faked environment. The distances
    > have nothing to do with that.
    > The distances you're adding here are, by definition, incorrect because it
    > doesn't respect the actual distance between physical nodes that numa=fake
    > uses already. If you're using numa=fake on an UMA machine, then the
    > performance of the kernel will be just that, you won't actual see any
    > introduced latency between fake nodes just by changing the distance. So
    > you're completely invalidating what internode distances actually mean.
    > I'd much rather see an option to fake the SLIT that could do all of this
    > without limitation and would be possible to debug issues in the future.

    This patch was designed as nothing more than helper for debugging/testing
    purposes, e.g. when it is useful to have more values in exports than only
    LOCAL_DISTANCEs. So that's the reason why it disregards former distances
    between physical nodes.

    Faking the SLIT table is a really good point, if this patch would be
    eventually rejected, I will rework the patch in that manner.

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