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: Fri, 22 Jun 2018 09:12:29 +0800
> Cc: emacs-devel@gnu.org
> 
> I set `vc-command-messages`, which prints out the subprocesses run by
> `vc-do-command` and also updated `vc-do-command` to show the time it took for
> each process.  It turns out that Emacs 26.1 does run one extra command:
> 
> In Emacs 25.2, a "vc-dir" command runs the following:
> 
> Running git --no-pager update-index --refresh . in background... done
> 0.48 seconds
> Running git --no-pager diff-index --relative -z -M HEAD -- . in
> background... done 0.42 seconds
> Running git --no-pager ls-files -z -o --directory --no-empty-directory
> --exclude-standard -- . in background... done 0.10 seconds
> 
> In Emacs 26.2, a "vc-dir" command runs the following:
> 
> Running git --no-pager update-index --refresh . in background... done
> 0.46 seconds
> Running git --no-pager diff-index --relative -z -M HEAD -- . in
> background... done 0.46 seconds
> Running git --no-pager ls-files -z -c -s -- . in background... done 4.49 seconds
> Running git --no-pager ls-files -z -o --directory --no-empty-directory
> --exclude-standard -- . in background... done 0.09 seconds
> 
> The extra command is "git --no-pager ls-files -z -c -s -- ." and it takes 4.5
> seconds to run (I ran each test multiple times and the times are consistent).
> However, when I run that command in a terminal, it finishes in less than a
> second.

But the times you cited originally were much longer: 2 min.  What
caused such a stark difference, from 2 min to just 4.5 sec?

> An Emacs-Lisp program to run this script, but ignore all output, it takes 16
> seconds, in both 25.2 and 26.1:

Once again, 16 sec is much shorter than 2 min you originally cited.

> It seems reading output from a process in Emacs is somehow limited to 64 Kb /
> second -- I verified this with procmon.  It looks like the behavior is the
> same in 25.2 and 26.1, it is just that in vc-dir 25.2 does not run a git
> command that produces a lot of output.

Does it help to set w32-pipe-buffer-size to some large value, like
16384, and/or decrease w32-pipe-read-delay?

Thanks.