linux-kernel - Re: NUMA emulation x86_64: numa=fake parameter for custom nodes distance

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Date:	Mon, 21 Nov 2011 21:41:34 +0100
From:	Petr Holasek <pholasek@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, Anton Arapov <anton@...hat.com>
Subject: Re: 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 <rientjes@...gle.com>
> To: Petr Holasek <pholasek@...hat.com>
> cc: Andrew Morton <akpm@...ux-foundation.org>, Thomas Gleixner
>  <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin"
>  <hpa@...or.com>, linux-kernel@...r.kernel.org, x86@...nel.org, Anton
>  Arapov <anton@...hat.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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