bug#42911: 28.0.50; write_region sometimes extremely very slow

[Tino Calancha]

X-Debbugs-Cc: uyennhi.qm@gmail.com

Verified in both emacs-27 and emacs-master

Recipe:
emacs -Q ~/save-buffer-slow.txt
;; insert few characters and run
M-: (benchmark-run nil (save-buffer))

;; Repeat several times: sometimes it takes a reasonable time, but
;; in others it takes huge time
;; (6-10 s to save a buffer with less that 50 characters)

It happens randomly.
I have been observed this maybe 1 week.  I don't recall to see this in
my other laptio runinning Debian 10, so it might be OS related?


In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.21, cairo 
version 1.16.0)
 of 2020-08-18 built on localhost.example.com
Repository revision: 9905001e4b0c9dc0a90cefdd9530a90d07a17b99
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: openSUSE Tumbleweed
uname -a: Linux localhost.example.com 5.7.11-1-default #1 SMP Wed Jul 29 
09:32:21 UTC 2020 (5015994) x86_64 x86_64 x86_64 GNU/Linux


Recent messages:
You can run the command ‘erase-buffer’ with M-x era RET
user-error: Beginning of history; no preceding item
Saving file /home/calancha/save-buffer-slow.txt...
Wrote /home/calancha/save-buffer-slow.txt
(6.688615679 0 0.0)
Saving file /home/calancha/save-buffer-slow.txt...
Wrote /home/calancha/save-buffer-slow.txt
(0.076871944 0 0.0)
Saving file /home/calancha/save-buffer-slow.txt...
Wrote /home/calancha/save-buffer-slow.txt
(10.659738934 0 0.0)
Configured using:
 'configure --with-x-toolkit=gtk3'

Configured features:
XPM JPEG TIFF GIF PNG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY INOTIFY
LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11 XDBE XIM MODULES THREADS PDUMPER

Important settings:
  value of $LC_ALL: 
  value of $LC_COLLATE: en_US.UTF-8
  value of $LC_CTYPE: en_US.UTF-8
  value of $LC_MESSAGES: en_US.UTF-8
  value of $LC_MONETARY: en_US.UTF-8
  value of $LC_NUMERIC: en_US.UTF-8
  value of $LC_TIME: en_US.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Text

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-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

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml mml-sec password-cache epa derived epg epg-config gnus-util
rmail rmail-loaddefs text-property-search mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils cl-extra seq byte-opt
gv bytecomp byte-compile cconv help-mode easymenu novice time-date
subr-x cl-loaddefs cl-lib benchmark tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote threads dbusbind inotify dynamic-setting system-font-setting
font-render-setting cairo move-toolbar gtk x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 16 47888 6325)
 (symbols 48 6098 1)
 (strings 32 16202 1003)
 (string-bytes 1 529670)
 (vectors 16 11018)
 (vector-slots 8 152947 12719)
 (floats 8 33 40)
 (intervals 56 201 0)
 (buffers 992 13))