Re: why is my cd command on eshell on Windows so slow?

[Wayne Harris]

Eli Zaretskii <eliz@gnu.org> writes:

>> Date: Fri, 16 Apr 2021 15:40:29 -0300
>> From: Wayne Harris via Users list for the GNU Emacs text editor
>> <help-gnu-emacs@gnu.org>
>> 
>> So when I cd into a directory, I get:
>> 
>> %cd dir/
>> 0.001 secs
>> %
>> 
>> However, it actually took about 3 seconds for me to get my prompt back.
>> There is something before cd actually operates (or afterwards) that
>> makes me wait --- *sometimes*.  If I immediately ``cd ..'' and then cd
>> into dir/ once again, it's pretty fast.
>> 
>> %which cd
>> eshell/cd is a compiled Lisp function in `em-dirs.el'
>> %
>> 
>> Can you point me in the right direction?  This is a Windows system.
>
> Profile it, then look at the profile to see what takes most of the
> time.

Thank you.  Didn't know there was a profiler.  I'm not sure I know how
to use it.  The output is below, but before I will summarize what I
found with the profiler.

(*) Summary of findings

It appears that the delay has nothing to do with Windows or with the
cd-command.  It seems to be the procedure

  completion-in-region--postch

from minibuffer.el.  To certify, I commented out all the code in this
procedure and tried various commands, then uncommented it again and saw
more delays, so it seems reproducible.

In the next sections I describe what I did in chronological order that
has led me to this observation.

(*) What I did

I prepared a ``cd some-dir/'' on eshell --- thinking this directory
would cause a slow down.  I was often wrong, so I tried different
directories until I saw a delay.  Before issuing the cd-command, I said

  M-x profiler-start

then I pressed enter to issue the cd-command.  Then I said

  M-x profiler-report
  M-x profiler-stop.

Looking at the output, it looks like most of the time is spent not in
executing the command.  I read it as 72% of the time is spent outside of
command-execute, that is, in completion-in-region--postch.

--8<---------------cut here---------------start------------->8---
- completion-in-region--postch                                    119  72%
 - #<compiled 0x34cb2ad>                                          119  72%
  - pcomplete-completions-at-point                                119  72%
   - pcomplete-completions                                        119  72%
    - #<compiled 0x254143d>                                       119  72%
     - pcomplete--here                                            119  72%
      - #<compiled 0x2541429>                                     119  72%
       - eshell-complete-commands-list                            104  63%
        - eshell-winnow-list                                        1   0%
         - eshell-find-alias-function                               1   0%
            file-name-base                                          1   0%
- command-execute                                                  40  24%
 - call-interactively                                              40  24%
  - funcall-interactively                                          26  15%
   - execute-extended-command                                      26  15%
    - execute-extended-command--shorter                            25  15%
     - completion-try-completion                                   24  14%
      - completion--nth-completion                                 24  14%
       - completion--some                                          24  14%
        - #<compiled 0x34c8a4d>                                    24  14%
         - completion-pcm-try-completion                           16   9%
          - completion-pcm--find-all-completions                   15   9%
             completion-pcm--all-completions                       15   9%
            completion-pcm--merge-try                               1   0%
           completion-basic-try-completion                          8   4%
    - sit-for                                                       1   0%
       redisplay                                                    1   0%
  - byte-code                                                      14   8%
   - read-extended-command                                         14   8%
    - completing-read                                              14   8%
     - completing-read-default                                     14   8%
      - read-from-minibuffer                                       10   6%
       - command-execute                                            6   3%
        - call-interactively                                        6   3%
         - funcall-interactively                                    6   3%
          - minibuffer-complete                                     6   3%
           - completion-in-region                                   6   3%
            - completion--in-region                                 6   3%
             - #<compiled 0x2646831>                                6   3%
              - apply                                               6   3%
               - #<compiled 0x1013d81>                              6   3%
                - completion--in-region-1                           6   3%
                 - completion--do-completion                        6   3%
                  - completion-try-completion                       4   2%
                   - completion--nth-completion                     4   2%
                    - completion--some                              4   2%
                     - #<compiled 0x264f8fd>                        4   2%
                      - completion-basic-try-completion             2   1%
                       - try-completion                             2   1%
                        - #<compiled 0x1046b41>                     2   1%
                           complete-with-action                     2   1%
                      - completion-pcm-try-completion               1   0%
                       - completion-pcm--find-all-completions       1   0%
                        - completion-pcm--all-completions           1   0%
                         - all-completions                          1   0%
                          - #<compiled 0x1046b41>                   1   0%
                             complete-with-action                   1   0%
                      - completion-emacs22-try-completion           1   0%
                       - try-completion                             1   0%
                        - #<compiled 0x1046b41>                     1   0%
                           complete-with-action                     1   0%
                  - completion--message                             2   1%
                   - minibuffer-message                             2   1%
                    - sit-for                                       2   1%
                       redisplay                                    1   0%
- ...                                                               6   3%
   Automatic GC                                                     6   3%
--8<---------------cut here---------------end--------------->8---

The procedure

  completion-in-region--postch

belongs to minibuffer.el.  Here is its source code.

--8<---------------cut here---------------start------------->8---
;; It is difficult to know when to exit completion-in-region-mode (i.e. hide
;; the *Completions*).  Here's how previous packages did it:
;; - lisp-mode: never.
;; - comint: only do it if you hit SPC at the right time.
;; - pcomplete: pop it down on SPC or after some time-delay.
;; - semantic: use a post-command-hook check similar to this one.
(defun completion-in-region--postch ()
  (or unread-command-events ;Don't pop down the completions in the middle of
                            ;mouse-drag-region/mouse-set-point.
      (and completion-in-region--data
           (and (eq (marker-buffer (nth 0 completion-in-region--data))
                    (current-buffer))
                (>= (point) (nth 0 completion-in-region--data))
                (<= (point)
                    (save-excursion
                      (goto-char (nth 1 completion-in-region--data))
                      (line-end-position)))
                (funcall completion-in-region-mode--predicate)))
      (completion-in-region-mode -1)))
--8<---------------cut here---------------end--------------->8---

(*) An attempt at confirming what appears to be

I commented out all the code of this procedure, so calling it should not
delay at all.  Indeed, after that I don't see any delay at all and it's
much more pleasant to use eshell.  Uncommenting the code does bring back
the delay sometimes.  Either I'm terribly unlucky or the performance
penalty here is confirmed.

(*) What I see in GNU Emacs 24.3

It seems a lot simpler.

--8<---------------cut here---------------start------------->8---
- call-interactively                                                4  66%
  - list                                                            4  66%
    - read-extended-command                                         4  66%
      - completing-read                                             4  66%
        - completing-read-default                                   4  66%
          - read-from-minibuffer                                    4  66%
              redisplay_internal (C function)                       1  16%
  redisplay_internal (C function)                                   2  33%
  Automatic GC                                                      0   0%
--8<---------------cut here---------------end--------------->8---