bug#64391: 30.0.50; buffer narrowing slowdown regression in emacs 29

[Eli Zaretskii <eliz@gnu.org>]

> Cc: Gregory Heytings <gregory@heytings.org>
> From: Andrew Cohen <acohen@ust.hk>
> Date: Sat, 01 Jul 2023 08:15:25 +0800
> 
> I have noticed a huge slowdown in parsing email headers in gnus. After
> some debugging with help from Mattias Engdegård, the problem has been
> traced to the narrowing code introduced in
> 
> commit ba9315b1641b483f2bf843c38dcdba0cd1643a55 (HEAD)
> Merge: aef803d6c3d a3b654e069e
> Author: Gregory Heytings <gregory@heytings.org>
> Date:   Thu Nov 24 14:21:30 2022 +0100
> 
>     Merge master into feature/improved-locked-narrowing.
> 
> Gnus populates a buffer with headers from a set of email messages and
> then parses them by: narrowing the buffer to the headers for an
> individual message; parsing the headers; widening; and then repeating
> for the next message.  As part of the parsing the headers are "unfolded"
> so that each header doesn't include line breaks. I noticed that for a
> long list of messages (10,000) this takes between one and two orders of
> magnitude more time in Emacs 30 than in Emacs 28. Unfolding all the
> headers in the full buffer before the parsing process removes most of
> the slowdown. The slowdown seems to grow quadratically with the size of
> the buffer.
> 
> The problem seems fairly general and Mattias has produced a simple test
> case to demonstrate the issue (code at the end of this message):
> 
> 1. Create a buffer with 100,000 lines each containing two characters  "ab".
> 2. Loop through the buffer narrowing to each line, and immediately widening
>     back to the full buffer (so no change is made to the buffer
>     contents).
> 3.  Loop through the buffer removing the first character of each line.
> 
> This takes a very long time compared with reversing the order of 2. and 3.

Does this problem go away if you set long-line-threshold to nil?

Can you show a profile of the processing that takes a long time?