GNU bug report logs - #72723
diff -d can be very slow

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

From: Vincent Lefevre <vincent <at> vinc17.net>
To: bug-diffutils <at> gnu.org
Subject: diff -d can be very slow
Date: Tue, 20 Aug 2024 01:55:22 +0200

[Message part 1 (text/plain, inline)]

When opening a .diff file, GNU Emacs runs "diff -ad" on 2 files
it has built (I suppose that the reason is to get a word diff),
and this can be very slow, even though the original .diff file
is rather simple.

I've attached a slow-diff.tar.xz archive with:
  * diff1L52tn0 and diff2U4TVho (files built be GNU Emacs).
  * file.diff the original .diff file.

When running "/usr/bin/emacs -Q file.diff.xz", I could see what
takes the whole time with ps or top. Here I could see

  diff -ad /tmp/diff1L52tn0 /tmp/diff2U4TVho

As this is slow, I could obtain these files.

On my recent machine, "diff -ad diff1L52tn0 diff2U4TVho" takes
27 seconds.

-- 
Vincent Lefèvre <vincent <at> vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

[slow-diff.tar.xz (application/octet-stream, attachment)]

Message #8 received at 72723 <at> debbugs.gnu.org (full text, mbox):

From: Vincent Lefevre <vincent <at> vinc17.net>
To: 72723 <at> debbugs.gnu.org
Subject: Re: diff -d can be very slow
Date: Tue, 20 Aug 2024 02:02:06 +0200

I forgot to say that this is on a Debian/unstable machine (x86_64),
with the Debian packages, so with diff (GNU diffutils) 3.10 and
GNU Emacs 29.4.

-- 
Vincent Lefèvre <vincent <at> vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

Message #11 received at 72723 <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Vincent Lefevre <vincent <at> vinc17.net>, 72723 <at> debbugs.gnu.org
Subject: Re: [bug-diffutils] bug#72723: diff -d can be very slow
Date: Mon, 19 Aug 2024 17:17:27 -0700

On 2024-08-19 16:55, Vincent Lefevre wrote:
> When opening a .diff file, GNU Emacs runs "diff -ad" on 2 files
> it has built (I suppose that the reason is to get a word diff),
> and this can be very slow

That's inherent to the algorithm, no? I don't know of any faster 
algorithm, if you really want minimal output. If you know of one, please 
let us know.

A simple workaround would be for GNU Emacs to not use the -d (--minimal) 
option.

Message #14 received at 72723 <at> debbugs.gnu.org (full text, mbox):

From: Vincent Lefevre <vincent <at> vinc17.net>
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: 72723 <at> debbugs.gnu.org, Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: Re: [bug-diffutils] bug#72723: diff -d can be very slow
Date: Tue, 20 Aug 2024 03:00:55 +0200

[Cc to Stefan Monnier, who introduced -d in GNU Emacs in 2007]

On 2024-08-19 17:17:27 -0700, Paul Eggert wrote:
> On 2024-08-19 16:55, Vincent Lefevre wrote:
> > When opening a .diff file, GNU Emacs runs "diff -ad" on 2 files
> > it has built (I suppose that the reason is to get a word diff),
> > and this can be very slow
> 
> That's inherent to the algorithm, no? I don't know of any faster algorithm,
> if you really want minimal output. If you know of one, please let us know.
> 
> A simple workaround would be for GNU Emacs to not use the -d (--minimal)
> option.

Perhaps not possible. In the Emacs code (in lisp/vc/smerge-mode.el),
I could find:

  (let ((coding-system-for-read 'utf-8-emacs))
    (call-process diff-command nil t nil
                  (if (and smerge-refine-ignore-whitespace
                           (not smerge-refine-weight-hack))
                      ;; Pass -a so diff treats it as a text file even
                      ;; if it contains \0 and such.
                      ;; Pass -d so as to get the smallest change, but
                      ;; also and more importantly because otherwise it
                      ;; may happen that diff doesn't behave like
                      ;; smerge-refine-weight-hack expects it to.
                      ;; See https://lists.gnu.org/r/emacs-devel/2007-11/msg00401.html
                      "-awd" "-ad")
                  file1 file2))

I suppose that this is the code that is called, as I couldn't
find another occurrence of -ad. So there is a reference to

  https://lists.gnu.org/r/emacs-devel/2007-11/msg00401.html

(that was with diff (GNU diffutils) 2.8.1) and the latest message
(about the use of -d, in particular):

  https://lists.gnu.org/r/emacs-devel/2007-11/msg00522.html

-- 
Vincent Lefèvre <vincent <at> vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

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