GNU logs - #32993, boring messages

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Subject: bug#32993: Pathologically slow operation
Resent-From: Stefan Monnier <monnier@HIDDEN>
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From: Stefan Monnier <monnier@HIDDEN>
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Date: Mon, 08 Oct 2018 17:34:18 -0400
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I recently bumped into a `diff` operation that I killed after several
minutes while diffing two files (on 3.7GHz core i3, which is the fastest
machine I have).

These files were generated as part of Emacs's "refine-hunk" processing
which tries to do word-level diffs (by basically turning every word
into N copies of this word, each one on its own line (where N is the
number of chars in the word, used to indicate to `diff` that long words
are "more costly" than short ones)).

So the files's sizes were:

    % wc tmp/diff-bug-* 
    1038026  851160 4963190 tmp/diff-bug-1
      65041   54877  314788 tmp/diff-bug-2
    1103067  906037 5277978 total
    %

With --speed-large-files, diff still took almost a minute to return an
answer (which is 973026 lines long).

Those file aren't exactly security sensitive, but they contain personal
info that I'd rather not make public (I can make send them in private
upon request, tho).  Is there a chance this performance behavior is the
result of a performance bug, or is the algorithm really that costly?

        Stefan

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From: help-debbugs@HIDDEN (GNU bug Tracking System)
To: Stefan Monnier <monnier@HIDDEN>
Subject: bug#32993: Acknowledgement (Pathologically slow operation)
Message-ID: <handler.32993.B.153903448228030.ack <at> debbugs.gnu.org>
References: <jwv4ldwqh32.fsf-monnier+gmane.linux.debian.user@HIDDEN>
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Date: Mon, 08 Oct 2018 21:35:02 +0000

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Subject: bug#32993: [bug-diffutils] bug#32993: Pathologically slow operation
Resent-From: Paul Eggert <eggert@HIDDEN>
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To: Stefan Monnier <monnier@HIDDEN>, 32993 <at> debbugs.gnu.org
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Date: Mon, 8 Oct 2018 15:49:09 -0700
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On 10/8/18 2:34 PM, Stefan Monnier wrote:
> Is there a chance this performance behavior is the
> result of a performance bug, or is the algorithm really that costly?

It's O(N*D), where N is the input length and D is the number of 
differences. In your case that might be about 10*11, which is getting up 
there.

> tries to do word-level diffs (by basically turning every word
> into N copies of this word, each one on its own line (where N is the
> number of chars in the word, used to indicate to `diff` that long words
> are "more costly" than short ones)

Ouch. That's a really inefficient way of mucking with the cost 
algorithm. Diff should be able to do taht directly, without your having 
to repeat the words. (Just a simple matter of programming....)

Message sent to bug-diffutils@HIDDEN:

Subject: bug#32993: [bug-diffutils] bug#32993: Pathologically slow operation
Resent-From: Stefan Monnier <monnier@HIDDEN>
Original-Sender: "Debbugs-submit" <debbugs-submit-bounces <at> debbugs.gnu.org>
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To: Paul Eggert <eggert@HIDDEN>
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From: Stefan Monnier <monnier@HIDDEN>
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 <0904c3c5-90d3-dd53-680f-7867eb7a60c4@HIDDEN>
Date: Mon, 08 Oct 2018 20:14:27 -0400
In-Reply-To: <0904c3c5-90d3-dd53-680f-7867eb7a60c4@HIDDEN> (Paul Eggert's
 message of "Mon, 8 Oct 2018 15:49:09 -0700")
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>> tries to do word-level diffs (by basically turning every word
>> into N copies of this word, each one on its own line (where N is the
>> number of chars in the word, used to indicate to `diff` that long words
>> are "more costly" than short ones)
> Ouch.  That's a really inefficient way of mucking with the cost
> algorithm.

What other way is there?

> Diff should be able to do taht directly, without your having to
> repeat the words. (Just a simple matter of programming....)

"man diff" doesn't seem to offer much room for "programming".


        Stefan

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Organization: UCLA Computer Science Department
Message-ID: <f71a3817-72fa-dd02-a4ad-52b6910609a1@HIDDEN>
Date: Mon, 8 Oct 2018 17:46:57 -0700
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On 10/8/18 5:14 PM, Stefan Monnier wrote:
> What other way is there?

It'd require hacking the diff source code, alas. Also, understanding the 
algorithm. Although the best guy to do it would probably be the 
algorithm's co-inventor Eugene Myers, he's kind of busy these days 
<https://en.wikipedia.org/wiki/Eugene_Myers>.

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