Messages in this thread | | | Date | Mon, 21 Nov 2011 21:41:34 +0100 | From | Petr Holasek <> | Subject | Re: NUMA emulation x86_64: numa=fake parameter for custom nodes distance |
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On Sat, 19 Nov 2011, David Rientjes wrote:
> Date: Sat, 19 Nov 2011 18:09:59 -0800 (PST) > From: David Rientjes <rientjes@google.com> > To: Petr Holasek <pholasek@redhat.com> > cc: Andrew Morton <akpm@linux-foundation.org>, Thomas Gleixner > <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" > <hpa@zytor.com>, linux-kernel@vger.kernel.org, x86@kernel.org, Anton > Arapov <anton@redhat.com> > 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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