bug#11943: 24.1.50; Emacs unusably slow when looking at large files (bidi support at fault)

[Dima Kogan]

I'm observing that when some large text files are loaded, emacs slows to
a crawl. As an example, I have a 14MB file open (with emacs -Q). Every
time I do (next-line) or (previous-line) it takes a few seconds. This is
a > 2GHz Core2 machine, so there's no reason for this to happen. 'M-x
benchmark' says that (previous-line) takes >2s each time. I discovered
that if I do (setq bidi-display-reordering nil) then emacs is snappy
again, with previous-line taking <1ms.

The specific file I'm using to exhibit the bug consists of many repeated
stanzas such as

=========================
{
 {2.222222,2.222222,2.222222,2.2},
 {-2.222222,2.222222,2.222222},
 {-22.222222,22.222222,2.222222}
},
=========================

without the =. Saving a stanza into a file called 'snippet', the 14MB
file can be made with

$ for i in `seq 17`; do cat snippet snippet > xxx; mv xxx snippet; done

Thanks.


In GNU Emacs 24.1.50.1 (x86_64-pc-linux-gnu, GTK+ Version 3.4.2)
 of 2012-06-29 on zelenka, modified by Debian
 (emacs-snapshot package, version 2:20120629-1)
Windowing system distributor `The X.Org Foundation', version 11.0.11201902
Configured using:
 `configure '--build' 'x86_64-linux-gnu' '--host' 'x86_64-linux-gnu'
 '--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib'
 '--localstatedir=/var' '--infodir=/usr/share/info'
 '--mandir=/usr/share/man' '--with-pop=yes'
 
'--enable-locallisppath=/etc/emacs-snapshot:/etc/emacs:/usr/local/share/emacs/24.1.50/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.1.50/site-lisp:/usr/share/emacs/site-lisp'
 '--without-compress-info' '--with-crt-dir=/usr/lib/x86_64-linux-gnu/'
 '--with-x=yes' '--with-x-toolkit=gtk3' '--with-imagemagick=yes'
 'build_alias=x86_64-linux-gnu' 'host_alias=x86_64-linux-gnu'
 'CFLAGS=-DDEBIAN -DSITELOAD_PURESIZE_EXTRA=5000 -g -O2' 'LDFLAGS=-g
 -Wl,--as-needed -znocombreloc' 'CPPFLAGS=-D_FORTIFY_SOURCE=2''

Important settings:
  value of $LC_ALL: C
  value of $LANG: C
  locale-coding-system: nil
  default enable-multibyte-characters: t

Major mode: Fundamental

Minor modes in effect:
  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
  line-number-mode: t
  transient-mark-mode: t

Recent input:
y <down> <down> <down> <down> <down> <down> <up> M-x 
b e n c h m a r k <return> ( p r e v i o u s - l i 
n e ) <return> <help-echo> <help-echo> M-x r e p o 
r t SPC b u g <return>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
File snippet is large (13.1M), really open? (y or n)  y
Elapsed time: 2.232054s

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