GNU bug report logs - #69233
30.0.50; project-files + project-find-file is slow in large repositories

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Spencer Baugh <sbaugh <at> janestreet.com>
To: bug-gnu-emacs <at> gnu.org
Cc: Dmitry Gutov <dmitry <at> gutov.dev>
Subject: 30.0.50; project-files + project-find-file is slow in large
 repositories
Date: Thu, 15 Feb 2024 17:55:46 -0500

(project-files (project-current)) takes around 1 second in Linux (80k
files) and 7 seconds in my larger (500k file) repository.

With this patch:
diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index c7c07c3d34c..037beaa835a 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -667,12 +667,15 @@
                                               (setq i (concat i "**"))))
                                         i)))
                                    extra-ignores)))))
-       (setq files
-             (mapcar
-              (lambda (file) (concat default-directory file))
-              (split-string
-               (apply #'vc-git--run-command-string nil "ls-files" args)
-               "\0" t)))
+       (with-temp-buffer
+         (let ((ok (apply #'vc-git--out-ok "ls-files" args))
+               (pt (point-min)))
+           (unless ok
+             (error "File listing failed: %s" (buffer-string)))
+           (goto-char pt)
+           (while (search-forward "\0" nil t)
+             (push (concat default-directory (buffer-substring-no-properties pt (1- (point)))) files)
+             (setq pt (point)))))
        (when (project--vc-merge-submodules-p default-directory)
          ;; Unfortunately, 'ls-files --recurse-submodules' conflicts with '-o'.
          (let* ((submodules (project--git-submodules))

project-files in Linux takes around .75 seconds.

If I further remove the (concat default-directory ...) around each file,
it speeds up to .5 seconds.

(Note that git ls-files itself takes only around 20 milliseconds)

My large repository (which uses Mercurial) has a custom project-files
which is basically:

(with-temp-buffer
  (unless (zerop (apply #'call-process "rhg" nil t nil "files"))
    (error "File listing failed: %s" (buffer-string)))
  (goto-char (point-min))
  (let ((pt (point))
        res)
    (while (search-forward "\n" nil t)
      (push (file-name-concat default-directory (buffer-substring-no-properties pt (1- (point)))) res)
      (setq pt (point)))
    res))

Likewise, removing the (concat default-directory ...) speeds my
project-files up from 7 seconds to 4.5 seconds.

This is especially silly because project-find-file then just removes
this default-directory again from all the files, which has yet more
overhead.

My proposal: Could we find a way to make the default-directory not
necessary for the files returned from project-files?

Perhaps project-files could be allowed to return relative file paths
which are relative to the project root.  Then in the common case where
all the files are within the project root, project-find-file would be
way faster.  Happy to implement this, if it makes sense.

Another optimization I've considered: We could run the process
asynchronously so project-files parsing can be parallel with the
process; but the process is usually very fast anyway, that's not most of
the overhead, so that won't be a big win.

However, that would make it easy for project-files as a whole to be
asynchronous.  Then that would allow project-find-file to start the
listing in the background, and then we'd write a completion table which
completes only over whatever files we've already read into Emacs.  I
think this would be a lot nicer for most use-cases, and I'd again be
happy to implement this.

Also happy to implement any other optimizations you think might make
sense.


In GNU Emacs 30.0.50 (build 37, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.15.12, Xaw scroll bars) of 2024-02-13 built on
 igm-qws-u22796a
Repository revision: a24a2b1ceb12f11c9d345190fbf554f27c4ec186
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Rocky Linux 8.9 (Green Obsidian)

Configured using:
 'configure -C --with-x-toolkit=lucid 'CFLAGS=-O0 -g3'
 --without-native-compilation --without-gif'

Configured features:
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XINPUT2 XPM LUCID ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
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Message #8 received at 69233 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Spencer Baugh <sbaugh <at> janestreet.com>
Cc: dmitry <at> gutov.dev, 69233 <at> debbugs.gnu.org
Subject: Re: bug#69233: 30.0.50;
 project-files + project-find-file is slow in large repositories
Date: Sun, 18 Feb 2024 20:56:43 +0200

merge 69233 69188
thanks

> Cc: Dmitry Gutov <dmitry <at> gutov.dev>
> From: Spencer Baugh <sbaugh <at> janestreet.com>
> Date: Thu, 15 Feb 2024 17:55:46 -0500
> 
> 
> (project-files (project-current)) takes around 1 second in Linux (80k
> files) and 7 seconds in my larger (500k file) repository.

This is a duplicate of another bug report you submitted not long ago.

Message #13 received at 69233 <at> debbugs.gnu.org (full text, mbox):

From: Dmitry Gutov <dmitry <at> gutov.dev>
To: Eli Zaretskii <eliz <at> gnu.org>, Spencer Baugh <sbaugh <at> janestreet.com>
Cc: 69233 <at> debbugs.gnu.org
Subject: Re: bug#69233: 30.0.50; project-files + project-find-file is slow in
 large repositories
Date: Sun, 18 Feb 2024 21:42:37 +0200

On 18/02/2024 20:56, Eli Zaretskii wrote:
> This is a duplicate of another bug report you submitted not long ago.

Any reason I didn't receive the first one to my inbox?

Message #16 received at 69233 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Dmitry Gutov <dmitry <at> gutov.dev>
Cc: sbaugh <at> janestreet.com, 69233 <at> debbugs.gnu.org
Subject: Re: bug#69233: 30.0.50; project-files + project-find-file is slow in
 large repositories
Date: Sun, 18 Feb 2024 21:45:22 +0200

> Date: Sun, 18 Feb 2024 21:42:37 +0200
> Cc: 69233 <at> debbugs.gnu.org
> From: Dmitry Gutov <dmitry <at> gutov.dev>
> 
> On 18/02/2024 20:56, Eli Zaretskii wrote:
> > This is a duplicate of another bug report you submitted not long ago.
> 
> Any reason I didn't receive the first one to my inbox?

I don't have the foggiest, sorry.

Message #19 received at 69233 <at> debbugs.gnu.org (full text, mbox):

From: Dmitry Gutov <dmitry <at> gutov.dev>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: sbaugh <at> janestreet.com, 69233 <at> debbugs.gnu.org
Subject: Re: bug#69233: 30.0.50; project-files + project-find-file is slow in
 large repositories
Date: Sun, 18 Feb 2024 22:11:43 +0200

On 18/02/2024 21:45, Eli Zaretskii wrote:
>> Date: Sun, 18 Feb 2024 21:42:37 +0200
>> Cc:69233 <at> debbugs.gnu.org
>> From: Dmitry Gutov<dmitry <at> gutov.dev>
>>
>> On 18/02/2024 20:56, Eli Zaretskii wrote:
>>> This is a duplicate of another bug report you submitted not long ago.
>> Any reason I didn't receive the first one to my inbox?
> I don't have the foggiest, sorry.

It seems Spencer didn't get the confirmation email either, or he 
wouldn't resubmit.

Message #22 received at 69233 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Dmitry Gutov <dmitry <at> gutov.dev>
Cc: sbaugh <at> janestreet.com, 69233 <at> debbugs.gnu.org
Subject: Re: bug#69233: 30.0.50; project-files + project-find-file is slow in
 large repositories
Date: Sun, 18 Feb 2024 22:18:06 +0200

> Date: Sun, 18 Feb 2024 22:11:43 +0200
> Cc: sbaugh <at> janestreet.com, 69233 <at> debbugs.gnu.org
> From: Dmitry Gutov <dmitry <at> gutov.dev>
> 
> On 18/02/2024 21:45, Eli Zaretskii wrote:
> >> Date: Sun, 18 Feb 2024 21:42:37 +0200
> >> Cc:69233 <at> debbugs.gnu.org
> >> From: Dmitry Gutov<dmitry <at> gutov.dev>
> >>
> >> On 18/02/2024 20:56, Eli Zaretskii wrote:
> >>> This is a duplicate of another bug report you submitted not long ago.
> >> Any reason I didn't receive the first one to my inbox?
> > I don't have the foggiest, sorry.
> 
> It seems Spencer didn't get the confirmation email either, or he 
> wouldn't resubmit.

One can know if debbugs received a report via the Web interface.

Message #25 received at 69233 <at> debbugs.gnu.org (full text, mbox):

From: Spencer Baugh <sbaugh <at> janestreet.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: Dmitry Gutov <dmitry <at> gutov.dev>, 69233 <at> debbugs.gnu.org,
 69188 <at> debbugs.gnu.org
Subject: Re: bug#69188: 30.0.50; project-files + project-find-file is slow
 in large repositories
Date: Fri, 23 Feb 2024 16:34:38 -0500

Eli Zaretskii <eliz <at> gnu.org> writes:
>> Date: Sun, 18 Feb 2024 22:11:43 +0200
>> Cc: sbaugh <at> janestreet.com, 69233 <at> debbugs.gnu.org
>> From: Dmitry Gutov <dmitry <at> gutov.dev>
>> 
>> On 18/02/2024 21:45, Eli Zaretskii wrote:
>> >> Date: Sun, 18 Feb 2024 21:42:37 +0200
>> >> Cc:69233 <at> debbugs.gnu.org
>> >> From: Dmitry Gutov<dmitry <at> gutov.dev>
>> >>
>> >> On 18/02/2024 20:56, Eli Zaretskii wrote:
>> >>> This is a duplicate of another bug report you submitted not long ago.
>> >> Any reason I didn't receive the first one to my inbox?
>> > I don't have the foggiest, sorry.
>> 
>> It seems Spencer didn't get the confirmation email either, or he 
>> wouldn't resubmit.
>
> One can know if debbugs received a report via the Web interface.

Yes, it seems that all my email was backed up for a day or so, for
whatever reason.  Sorry for the noise.

(Or maybe I just think this is such an important bug that I submitted it
twice :) )

Message #28 received at 69233 <at> debbugs.gnu.org (full text, mbox):

From: Spencer Baugh <sbaugh <at> janestreet.com>
To: Dmitry Gutov <dmitry <at> gutov.dev>
Cc: 69233 <at> debbugs.gnu.org, 69188 <at> debbugs.gnu.org
Subject: Re: bug#69233: 30.0.50; project-files + project-find-file is slow
 in large repositories
Date: Mon, 29 Apr 2024 16:27:01 -0400

Dmitry Gutov <dmitry <at> gutov.dev> writes:
> On 13/04/2024 05:34, Dmitry Gutov wrote:
>> Both options are relatively clunky, and the second one might also
>> fail to work when DIRS is non-nil (or would have to fall back to
>> absolute names anyway), so I'm leaning toward the first one. It
>> might also allow certain code to be written supporting both relative
>> and absolute names (e.g. a process call both binds default-directory
>> to root and keeps the file names as-is -- the relative ones would be
>> interpreted as such, the rest just as they are interpreted now).
>
> Here's how that change can look.
>
> The patch should demonstrate both the performance improvements for
> project-find-file and project-find-regexp, and some awkwardness in the
> implementation, chiefly due to backward compatibility.
>
> Guess more tests will be required, at the very least.

I see almost a 50% performance improvement with this patch in my large
private repository, once adding support for project-files-relative-names
in my internal project backend.  Seems great so far.

My benchmarking:

(let ((proj (project-current)))
  (list (benchmark-run 10 (let ((project-files-relative-names t)) (length (project-files proj))))
        (benchmark-run 10 (let ((project-files-relative-names nil)) (length (project-files proj))))))

((17.605295389 28 7.647366087000023)
 (29.918302167 57 19.246283027999993))

Message #31 received at 69233 <at> debbugs.gnu.org (full text, mbox):

From: Spencer Baugh <sbaugh <at> janestreet.com>
To: Dmitry Gutov <dmitry <at> gutov.dev>
Cc: 69233 <at> debbugs.gnu.org, 69188 <at> debbugs.gnu.org
Subject: Re: bug#69233: 30.0.50; project-files + project-find-file is slow
 in large repositories
Date: Mon, 29 Apr 2024 17:04:24 -0400

Dmitry Gutov <dmitry <at> gutov.dev> writes:

> Hi Spencer,
>
> Sorry about the wait.
>
> On 16/02/2024 00:55, Spencer Baugh wrote:
>> (project-files (project-current)) takes around 1 second in Linux
>> (80k
>> files) and 7 seconds in my larger (500k file) repository.
>> With this patch:
>> diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
>> index c7c07c3d34c..037beaa835a 100644
>> --- a/lisp/progmodes/project.el
>> +++ b/lisp/progmodes/project.el
>> @@ -667,12 +667,15 @@
>>                                                 (setq i (concat i "**"))))
>>                                           i)))
>>                                      extra-ignores)))))
>> -       (setq files
>> -             (mapcar
>> -              (lambda (file) (concat default-directory file))
>> -              (split-string
>> -               (apply #'vc-git--run-command-string nil "ls-files" args)
>> -               "\0" t)))
>> +       (with-temp-buffer
>> +         (let ((ok (apply #'vc-git--out-ok "ls-files" args))
>> +               (pt (point-min)))
>> +           (unless ok
>> +             (error "File listing failed: %s" (buffer-string)))
>> +           (goto-char pt)
>> +           (while (search-forward "\0" nil t)
>> +             (push (concat default-directory (buffer-substring-no-properties pt (1- (point)))) files)
>> +             (setq pt (point)))))
>>          (when (project--vc-merge-submodules-p default-directory)
>>            ;; Unfortunately, 'ls-files --recurse-submodules' conflicts with '-o'.
>>            (let* ((submodules (project--git-submodules))
>> project-files in Linux takes around .75 seconds.
>
> The patch makes sense (and the approach works okay in
> project--files-in-directory), so this is something I've made a few
> attempts to use in the past.
>
> However, the measurements on my machine show a much smaller
> improvement -- just 3-4%. I.e. if I just evaluate the functions
> interpreted or run them just once, the variations between the runs far
> exceed the difference in runtimes (around ~450ms with a Linux
> repository checkout from 2021, 70k files).
>
> A stricter comparison works out like this:
>
> 1. Apply the patch (or not),
> 2. M-x byte-compile-file
> 3. (load "project.elc")
> 4. (benchmark-run 10 (project-files (project-current)))
>
> When run these in my working session one after another, the 10
> iteration benchmark works out to 4.09s vs 3.93s (master vs your
> patch).
>
> (4.093848777 44 1.6119981489999944)
>
> vs
>
> (3.9392906549999998 41 1.499010061)
>
> With 'emacs -Q', however, it's vice versa:
>
> (3.777694389 130 1.2422826310000001)
>
> vs
>
> (3.889905663 165 1.46846598)
>
> It seems like, maybe, the longer running session is more sensitive to
> the allocation of the initial long string than the fresh session.
>
> In any case, I don't mind switching to the other approach. Just
> wondering where the difference between our machines might come from.
>
> Last but not least, when/if we apply this, we should keep the fix for
> bug#66806 in there. Good news is it doesn't seem to affect
> performance.

Oh, interesting, I see roughly the same result.

Benchmarking with:
(benchmark-run 10 (project-files (project-current)))

Running in my long-lived existing Emacs 29 session:
Old:
(4.434228319 14 2.850654906999921)
New:
(4.983809167 16 3.2989908669999295)

In Emacs 29 emacs -Q:
Old:
(3.5112438729999997 130 1.9230644630000002)
New:
(3.819248509 171 2.309731412)

But, in Emacs 30 emacs -Q:
Old:
(7.949549188 65 3.3445626799999992)
New:
(7.270785783999999 87 4.0610532379999995)

So... the performance improvement seems highly unreliable.  Probably not
worth changing this area, then - the other patch to allow relative files
will probably be more worth it.

>> My proposal: Could we find a way to make the default-directory not
>> necessary for the files returned from project-files?
>> Perhaps project-files could be allowed to return relative file paths
>> which are relative to the project root.  Then in the common case where
>> all the files are within the project root, project-find-file would be
>> way faster.  Happy to implement this, if it makes sense.
>
> Yep, that should make sense. Originally the idea was to keep it more
> universal so that lists of files coming from the "external roots"
> could be handled the same way (used in the two *-or-external-*
> commands).
>
> But indeed it's the relatively rare case, so it'd be better to avoid
> paying the performance penalty, especially when the subsequent
> handling could do without the added prefix. And even the "rare case"
> could be split into separate calls instead of having all files
> returned at once.
>
> My main concern is backward compatibility, so that 3rd party callers
> don't break after the update.
>
> I think there are basically two approaches:
> - A new devar like project-use-relative-names,
> - Or a new argument for 'project-files', e.g. called RELATIVE.
>
> Both options are relatively clunky, and the second one might also fail
> to work when DIRS is non-nil (or would have to fall back to absolute
> names anyway), so I'm leaning toward the first one. It might also
> allow certain code to be written supporting both relative and absolute
> names (e.g. a process call both binds default-directory to root and
> keeps the file names as-is -- the relative ones would be interpreted
> as such, the rest just as they are interpreted now).
>
> Both project-find-file and project-find-regexp should be able to
> benefit. Although the former might require a bigger update, given that
> the current project-read-file-name-function options don't expect
> relative names. Ideally we'd have a smoother migration for custom
> p-r-f-n-f functions, but I don't have any good ideas there.

I think the defvar approach seems reasonable.

The existing project-read-file-name-function certainly don't expect
relative names, but they do actually work OK.  e.g.

(project--read-file-cpd-relative "" '("foo/bar" "foo1/bar") nil 'minibuffer-history)
(project--read-file-absolute "" '("foo/bar" "foo1/bar") nil 'minibuffer-history)

Both complete fine and return a filename fine.  read-file-cpd-relative
returns an absolute filename, read-file-absolute reutrns a relative
filename.

Maybe the same is true for any custom project-read-file-name-functions
that exist?  Maybe they will just work?

>> Another optimization I've considered: We could run the process
>> asynchronously so project-files parsing can be parallel with the
>> process; but the process is usually very fast anyway, that's not most of
>> the overhead, so that won't be a big win.
>
> Right. This came up in bug#64735, and together with patch in bug#66020
> the asynchronous file listing can run a bit faster than the
> synchronous implementation.
>
> I'm guessing the difference won't be huge in your case, since either
> way most time remains spent in Lisp code and GC. But if we take
> advantage of this by improving the UIs at the same time, this can be a
> real win.

Right.

> This should go into a separate discussion, I think, but to quickly sum
> up my thinking on the subject:
>
> - Ideally project-files implementations for sync and async UIs should
>   always look the same. Hopefully the "async" implementation looks the
>   same or almost the same as the "sync" one. Threads might help.
> - project-find-regexp could benefit from this a lot, first by running
>   the search in parallel to the file listing, and second by showing
>   the results right away (the current advantage of 'M-x grep'). The
>   difficult part is have the "async" Xref interface as well (can we do
>   this without extending the current one? probably not). The UI also
>   needs to have some "running ..." indicator, as well as a way to
>   abort the search, killing both processes - that adds requirements to
>  "async Xref" as well.

All seems reasonable.

>> However, that would make it easy for project-files as a whole to be
>> asynchronous.  Then that would allow project-find-file to start the
>> listing in the background, and then we'd write a completion table which
>> completes only over whatever files we've already read into Emacs.  I
>> think this would be a lot nicer for most use-cases, and I'd again be
>> happy to implement this.
>
> Could this be that simple?
>
> Whatever the source of the file listing, as soon as the UI (or
> completion styles) calls try-completion or all-completions, the search
> has to finish first, shouldn't it? That seems like the semantics of
> this API. Or if perhaps we allow it to operate on incomplete results,
> how would we indicate to the user at the end that the scan has
> finished, and they can press TAB once more to refresh the results? Or
> perhaps to be able to find a file they hadn't managed to find in the
> incomplete set.
>
> This seems like it might require both a new UI and an extension of
> completion table API. E.g. in certain cases we could say that we only
> need N matches, so if the current incomplete set can provide as many,
> we don't have to wait until the end. But 'try-completion' would become
> unreliable either way.

Yes, that's all true, and this is definitely not the intended semantics
of the API, but I vaguely suspect it might be fine in practice?  That
vague suspicion can wait until later, though, because I think the more
conservative approach you suggest is also a good improvement on its own.

> Even if keeping to the most conservative approach, though, it should
> be possible to at least render the prompt before the file listing is
> finished. That could make the UI look a bit more responsive.

True, that would be pretty nice.  And further I suppose in the case of
the default completion UI (which doesn't automatically display
completions), the user can even type some input before hitting TAB and
waiting.

Also, I suppose that even non-default completion UIs would allow the
user to type input, if the non-default completion UI uses
while-no-input.  So it would be a pretty responsive experience for such
UIs (assuming we are careful in our implementation and don't have bugs
when being interrupted).

That sounds pretty great, actually.  We avoid the blocking part of the
UI without needing to think about how to surface "incomplete completion"
in all the different completion UIs.

Message #34 received at 69233 <at> debbugs.gnu.org (full text, mbox):

From: Dmitry Gutov <dmitry <at> gutov.dev>
To: Spencer Baugh <sbaugh <at> janestreet.com>
Cc: 69233 <at> debbugs.gnu.org, 69188 <at> debbugs.gnu.org
Subject: Re: bug#69233: 30.0.50; project-files + project-find-file is slow in
 large repositories
Date: Sun, 5 May 2024 03:29:12 +0300

On 29/04/2024 23:27, Spencer Baugh wrote:
> Dmitry Gutov<dmitry <at> gutov.dev>  writes:
>> On 13/04/2024 05:34, Dmitry Gutov wrote:
>>> Both options are relatively clunky, and the second one might also
>>> fail to work when DIRS is non-nil (or would have to fall back to
>>> absolute names anyway), so I'm leaning toward the first one. It
>>> might also allow certain code to be written supporting both relative
>>> and absolute names (e.g. a process call both binds default-directory
>>> to root and keeps the file names as-is -- the relative ones would be
>>> interpreted as such, the rest just as they are interpreted now).
>> Here's how that change can look.
>>
>> The patch should demonstrate both the performance improvements for
>> project-find-file and project-find-regexp, and some awkwardness in the
>> implementation, chiefly due to backward compatibility.
>>
>> Guess more tests will be required, at the very least.
> I see almost a 50% performance improvement with this patch in my large
> private repository, once adding support for project-files-relative-names
> in my internal project backend.  Seems great so far.
> 
> My benchmarking:
> 
> (let ((proj (project-current)))
>    (list (benchmark-run 10 (let ((project-files-relative-names t)) (length (project-files proj))))
>          (benchmark-run 10 (let ((project-files-relative-names nil)) (length (project-files proj))))))
> 
> ((17.605295389 28 7.647366087000023)
>   (29.918302167 57 19.246283027999993))

Nice!

Too bad it's still takes ~1.7s to list all the files in the project. 
Well above the comfortable wait time (ideally <100ms or at least <500ms, 
I guess).

Message #37 received at 69233 <at> debbugs.gnu.org (full text, mbox):

From: Dmitry Gutov <dmitry <at> gutov.dev>
To: Spencer Baugh <sbaugh <at> janestreet.com>
Cc: 69233 <at> debbugs.gnu.org, 69188 <at> debbugs.gnu.org
Subject: Re: bug#69233: 30.0.50; project-files + project-find-file is slow in
 large repositories
Date: Sun, 5 May 2024 06:32:12 +0300

On 30/04/2024 00:04, Spencer Baugh wrote:

> Oh, interesting, I see roughly the same result.
> 
> Benchmarking with:
> (benchmark-run 10 (project-files (project-current)))
> 
> Running in my long-lived existing Emacs 29 session:
> Old:
> (4.434228319 14 2.850654906999921)
> New:
> (4.983809167 16 3.2989908669999295)
> 
> In Emacs 29 emacs -Q:
> Old:
> (3.5112438729999997 130 1.9230644630000002)
> New:
> (3.819248509 171 2.309731412)
> 
> But, in Emacs 30 emacs -Q:
> Old:
> (7.949549188 65 3.3445626799999992)
> New:
> (7.270785783999999 87 4.0610532379999995)
> 
> So... the performance improvement seems highly unreliable.  Probably not
> worth changing this area, then - the other patch to allow relative files
> will probably be more worth it.

All right then, let's hold off on this potential change for now, and 
maybe revisit it later. Maybe the new GC engine will swing the needle in 
one or the other direction.

> I think the defvar approach seems reasonable.
> 
> The existing project-read-file-name-function certainly don't expect
> relative names, but they do actually work OK.  e.g.
> 
> (project--read-file-cpd-relative "" '("foo/bar" "foo1/bar") nil 'minibuffer-history)

Evaluating this one with the version in master results in

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  expand-file-name(nil)

hence the associated change in the patch.

> (project--read-file-absolute "" '("foo/bar" "foo1/bar") nil 'minibuffer-history)

No errors here, but two problems are that a) it doesn't show the 
default-directory [meaning no indication in which project the read is 
happening], and b) returning the relative name will mess up the 
file-name-history entry.

Good thing you noted the latter, it needs explicit handling. The former 
can be be shown in the prompt, at least.

> Both complete fine and return a filename fine.  read-file-cpd-relative
> returns an absolute filename, read-file-absolute reutrns a relative
> filename.
> 
> Maybe the same is true for any custom project-read-file-name-functions
> that exist?  Maybe they will just work?

So, apparently not.

Anyway, I've pushed the patch in commit 370b216f086. Here's hoping the 
breakage will be minimal.

>>> However, that would make it easy for project-files as a whole to be
>>> asynchronous.  Then that would allow project-find-file to start the
>>> listing in the background, and then we'd write a completion table which
>>> completes only over whatever files we've already read into Emacs.  I
>>> think this would be a lot nicer for most use-cases, and I'd again be
>>> happy to implement this.
>>
>> Could this be that simple?
>>
>> Whatever the source of the file listing, as soon as the UI (or
>> completion styles) calls try-completion or all-completions, the search
>> has to finish first, shouldn't it? That seems like the semantics of
>> this API. Or if perhaps we allow it to operate on incomplete results,
>> how would we indicate to the user at the end that the scan has
>> finished, and they can press TAB once more to refresh the results? Or
>> perhaps to be able to find a file they hadn't managed to find in the
>> incomplete set.
>>
>> This seems like it might require both a new UI and an extension of
>> completion table API. E.g. in certain cases we could say that we only
>> need N matches, so if the current incomplete set can provide as many,
>> we don't have to wait until the end. But 'try-completion' would become
>> unreliable either way.
> 
> Yes, that's all true, and this is definitely not the intended semantics
> of the API, but I vaguely suspect it might be fine in practice?  That
> vague suspicion can wait until later, though, because I think the more
> conservative approach you suggest is also a good improvement on its own.

Some async stuff could make a big improvement on top of it, but it seems 
to require a fair bit more complexity.

>> Even if keeping to the most conservative approach, though, it should
>> be possible to at least render the prompt before the file listing is
>> finished. That could make the UI look a bit more responsive.
> 
> True, that would be pretty nice.  And further I suppose in the case of
> the default completion UI (which doesn't automatically display
> completions), the user can even type some input before hitting TAB and
> waiting.

It could be advantageous if the search process starts right when (or 
before) the prompt is shown, then by the type the first input is entered 
the search could either be finished or have found some matches at least.

> Also, I suppose that even non-default completion UIs would allow the
> user to type input, if the non-default completion UI uses
> while-no-input.  So it would be a pretty responsive experience for such
> UIs (assuming we are careful in our implementation and don't have bugs
> when being interrupted).

Not sure about this one:

1) If you only do the search while the user is not typing, it will 
finish later compared to the scheme in the previous paragraph.
2) Suppose you type a char, pause, then another one. Will the search 
start, abort, and then start again? That seems wasteful.

I'd ultimately prefer a scheme where work isn't thrown away - but that 
would require a more complex API. Including a way to abort the 
background computation (since typing won't do that anymore).

For some UIs and commands that makes sense (e.g. incremental interfaces 
like counsel-rg) because they perform the search with different inputs 
each time you type a new character. That kinds of works for 
small-to-medium projects, and you can enjoy the responsiveness of the 
process. I'm not sure about this approach for big projects.

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