Re: vc-dir operation is very slow on large git repositories in Emacs 26.1

[Eli Zaretskii <eliz@gnu.org>]

> From: Alex Harsanyi <alexharsanyi@gmail.com>
> Date: Thu, 21 Jun 2018 06:33:57 +0800
> Cc: emacs-devel@gnu.org
> 
> >> The operation that takes a long time is the ls-files
> >> operation, "git --no-pager ls-files -z -c -s --", but running
> >> that command in a terminal completes in less than a second.
> >
> > So you are saying that in Emacs 25.2 this slow Git command was not
> > invoked in your scenario?
> 
> I do not say that.  Emacs 25.2 completes too fast to be able to
> figure what is going on.  I suspect 25.2 uses the same git
> command though, as the command itself completes quickly.

Can you step with Edebug through the vc-git.el function where this
command is invoked, and see if it gets invoked in Emacs 25?

> I suspect this might be a problem with my environment, but I do
> not know what.  It also happens on two machines: my work one and
> my home one.  I would appreciate any hints on how to diagnose
> this.

One possibility is to run "M-x profile-start" before invoking vc-dir,
then run "M-x profile-report" after it finishes, and look at the
profile.  If vc-dir takes 2 minutes, the profiler should be able to
collect enough evidence to point at the guilty parties.  For best
results, load vc-git.el manually, so that Emacs runs the interpreted
Lisp code, not byte-compiled code.

> Both versions were downloaded as compiled binaries from the
> gnu.org website and not modified in any way -- this happens when
> I start Emacs with "-Q -no-site-file".

Are they both 64-bit builds (or both 32-bit)?