bug#44638: [PATCH 1/2] autorevert: don't reuse existing watch descriptors

[Eli Zaretskii <eliz@gnu.org>]

> From: Spencer Baugh <sbaugh@catern.com>
> Date: Sat, 14 Nov 2020 11:54:58 -0500
> Cc: Spencer Baugh <sbaugh@catern.com>
> 
> Previously, when enabling autorevert for a new buffer, we would search
> the buffers already registered with autorevert to see if any of them
> had the same filename.
> 
> This is very slow with a large number of buffers - with 1000, it takes
> 2 seconds on my system. This 2-second overhead is paid for every new
> file opened.
> 
> But this is an unnecesary optimization; registering the same file
> twice with file-notify has minimal or no overhead, depending on the
> implementation.

Emacs actually watches the file's directory, not the file itself.  The
directory is what's registered with inotify and other similar
backends.

> In fact, file-notify has some baked-in overhead to support registering
> the same file twice without problems. For example, inotify on Linux
> returns the same inotify watch descriptor when the same file is
> registered twice; file-notify adds an additional uniquifying id so
> that all watch descriptors are unique in Emacs, even with inotify.

Again, we watch the directory of the file, so what inotify does with
files is not really relevant, IMO.  I wonder what that means for the
changes you propose.