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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