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bug#13675: 24.2.93; Extremely slow redisplay when lines are very long

From: Eli Zaretskii
Subject: bug#13675: 24.2.93; Extremely slow redisplay when lines are very long
Date: Sun, 10 Feb 2013 18:26:14 +0200

This is a very long-standing deficiency of the Emacs display engine:
it is awfully slow in buffers with very long (thousands of characters)
lines.  Specifically, many simple movement commands, scrolling, or
even typing "M-x" can take several seconds(!) to complete.

A simple Awk script attached below can be used to generate such files.

For the latest discussions of this and some data, see this thread:


The solution for this bug should produce algorithmic changes in the
display engine and possibly also supporting changes in data structures
that would prevent such a terrible slow-down with long lines.
Ideally, redisplay of such buffers should not be much slower than
buffers with "normal" line length.

Here's a script that can be used to produce test files for this bug:

    # 500 lines
    for (i = 1; i <= 500; i++)
        # Line length between 10K and 20K characters + newline
        line_len = 10000 * rand() + 10000;
        for (j = 1; j <= line_len; j++)
            # 15% of punctuation and digit charcaters, the rest letters
            if (rand() < 0.15)
                # Start at SPACE
                lbase = 32;
                llen = 33;
                # Start at 'a'
                lbase = 97;
                llen = 26;
            printf "%c", llen * rand() + lbase;
        printf "\n";


In GNU Emacs (i386-mingw-nt5.1.2600)
 of 2013-02-07 on HOME-C4E4A596F7
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (3.4) --cflags -Id:/usr/include/libxml2'

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1255
  default enable-multibyte-characters: t

Major mode: Mail

Minor modes in effect:
  shell-dirtrack-mode: t
  diff-auto-refine-mode: t
  flyspell-mode: t
  desktop-save-mode: t
  show-paren-mode: t
  display-time-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  temp-buffer-resize-mode: t
  line-number-mode: t
  auto-fill-function: mail-mode-auto-fill
  abbrev-mode: t

Recent input:
e t a i l s . <return> <up> <up> <C-right> <C-right> 
<C-right> C-x C-e <help-echo> <help-echo> <down> <down> 
<down> <return> T h e SPC r e a s o n SPC f o r SPC 
t h e SPC d e f a u l t SPC v a l u e SPC i s SPC t 
o SPC a v o i d SPC t h e SPC a n n o y i n g SPC j 
u m p s SPC o f SPC t h e SPC m o d e SPC l i n e <up> 
<M-right> <C-left> <C-left> <C-left> <left> SPC a <backspace> 
u p SPC a n d SPC d o w n M-q <down> SPC w h e n SPC 
t h e SPC e c h o SPC a r e a SPC d i s p l a y s SPC 
m e s s a g e s SPC o f SPC d i f f e r e n t SPC l 
e n g t h . <return> <C-home> C-c C-s <help-echo> <switch-frame> 
d d d d d d d d d SPC d d o P O <tab> <return> d d 
d d d d d d d d n d SPC d d d SPC <prior> <next> <next> 
d SPC d d d d d C-z C-z C-z C-z C-z C-z C-z C-z d d 
d d d d SPC SPC <prior> <next> <next> <next> <next> 
<next> <next> <next> <next> <next> <next> <next> <next> 
<next> <next> <next> <next> <next> <next> <next> <next> 
<next> d <C-home> C-x C-s <switch-frame> <switch-frame> 
<help-echo> <help-echo> <switch-frame> <switch-frame> 
<help-echo> <switch-frame> <help-echo> M-x r e p o 
r t - e m a c s - b u <tab> <return>

Recent messages:
Added to d:/usr/eli/rmail/SENT.MAIL
Sending email 
Sending email done
Added to d:/usr/eli/rmail/PORTS.rmail
No following nondeleted message
Mark set
Saving file d:/usr/eli/rmail/INBOX...
Wrote d:/usr/eli/rmail/INBOX [2 times]

Load-path shadows:
None found.

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make-network-process w32 multi-tty emacs)

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