linux-kernel - Re: Purpose of numa

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Message-ID: <20080131074045.GA13788@linux-sh.org>
Date:	Thu, 31 Jan 2008 16:40:45 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Chris Snook <csnook@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Purpose of numa_node?

On Wed, Jan 30, 2008 at 07:48:13PM -0500, Chris Snook wrote:
> While pondering ways to optimize I/O and swapping on large NUMA machines, I 
> noticed that the numa_node field in struct device isn't actually used 
> anywhere. We just have a couple dozen lines of code to conditionally 
>  create a sysfs file that will always return -1.  Is anyone even working on 
> code to actually use this field?  I think it's a good piece of information 
> to keep track of, so I'm not suggesting we remove it, but I want to make 
> sure I'm not stepping on toes or duplicating effort if I try to make it 
> useful.
> 
It's manipulated with accessors. If you look at the users of
dev_to_node()/set_dev_node() you can see where it's being used. It's
primarily used in allocation paths for node locality, and the existing
set_dev_node() callsites are places where node locality information
already exists (ie, which node a given controller sits on). You can see
this in places like PCI (pcibus_to_node()) and USB, with node allocation
hints used in places like the dmapool and skb alloc paths.

The in-kernel use looks perfectly sane in that regard, though I'm not
sure what the point of exporting this as a RO attribute to userspace is.
Presumably someone has a tool somewhere that cares about this.
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