Date: Thu, 3 Nov 2005 09:20:14 +0100 (CET)
From: Simon Derr <Simon.Derr@bull.net>
To: Paul Jackson <pj@sgi.com>
Cc: Daniel J Blueman <daniel.blueman@gmail.com>, rdunlap@xenotime.net,
       linux-kernel@vger.kernel.org, Simon.Derr@bull.net,
       Sylvain.Jeaugey@bull.net
Subject: Re: cpuset - question
In-Reply-To: <20051102102834.0a038576.pj@sgi.com>
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On Wed, 2 Nov 2005, Paul Jackson wrote:

> Randy asked:
> > Just for info, why is this in /dev at all, instead of, say,
> > /sys ??
> 
> Daniel added:
> > I'm not sure of the true answer; it is likely that CPUSETS was
> > designed in the 2.4 timeframe and compatibility was preferred over the
> > clean sysfs interface.
> 
> No .. cpusets was a fresh design for Linux 2.6.  The two primary
> authors were Simon Derr of Bull and myself of SGI.  So far as I
> know, Bull did not have Linux 2.4 precedents.  SGI had both Linux
> 2.4 precedents and Irix precedents.  I chose not to propose either
> of these SGI precedent API's for the Linux mainline kernel.
> 
> Simon proposed the primary interface for the /dev/cpuset, and I gladly
> joined him as his design was superior.  Simon had this file system
> mounted under /proc, and Christoph Hellwig (our primary reviewer -
> thanks!) objected, recommending /dev/cpuset as the mount point instead.
> 
> In Christoph's own words on May 13, 2004:
> 
>  - don't mount the filesystem in procfs.  the whole point of a new
>    fs is to move away from the procfs mess!  /dev/cpuset/ sounds like
>    a saner mtpnt.
> 
> In any case, there are two aspects to this question.  Should the
> cpuset hierarchy be a separate virtual file system of its own, or part
> of the sysfs file system?  Then, if it is separate, where should it
> be mounted.
> 

There were also a few technical reasons.

The first was the desire to create cpusets with 'mkdir my_cpuset'.
But this was not a sufficient reason to have a new filesystem, so after my 
first version of the cpuset patch I reworked it to use sysfs.

However then I ran into a wall: sysfs does not support files larger than a 
page. And this was a showstopper as the size of the `tasks' file can be 
large.

So I had to drop sysfs.

	Simon.

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