Ok, in that case I wonder if this is actually a bug.

On Tue, Oct 30, 2018 at 1:33 PM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Justin Van Winkle <justin.vanwinkle@gmail.com>
> > Date: Tue, 30 Oct 2018 12:18:34 -0400
> > Cc: michael.albinus@gmx.de, 33194@debbugs.gnu.org
> >
> > To clarify, I was not writing to a file that was open in emacs.  In
> fact, emacs would use 100% cpu even with no
> > files opened, so long as global-auto-revert-mode was activated.
>
> That was clear.  Like I said: auto-revert mode makes Emacs watch
> changes to _any_ file in that file's directory.
>