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Subject: Re: [ANNOUNCEMENT] emacs 27.1-1
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From: Ken Brown via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Ken Brown <kbrown AT cornell DOT edu>
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On 8/15/2020 11:07 AM, Achim Gratz wrote:
> Ken Brown via Cygwin-announce writes:
>> The following packages have been uploaded to the Cygwin distribution:
>>
>> * emacs-27.1-1
>> * emacs-common-27.1-1
>> * emacs-X11-27.1-1
>> * emacs-w32-27.1-1
>> * emacs-lucid-27.1-1
> 
> This release seems to have a serious performance regression when using
> diff-mode on large diffs with long lines (e.g. created by comparing the
> logs of two compilations), especially when combined with
> toggle-truncate-lines.  Emacs becomes unresponsive with 100% CPU load
> when scrolling fast, sometimes for several seconds.  The CPU time seems
> to be spent in redisplay().

emacs has long had problems with slow redisplay in files with long lines.  See, 
for example,

   https://debbugs.gnu.org/cgi/bugreport.cgi?bug=13675

and all the bugs that have been merged with it.  One suggestion I've seen 
recently is to try so-long-mode.  Does that help?

Do you have reason to think this is Cygwin-specific?  If so, it might be related 
to the new pty code.  Does it help to set CYGWIN=disable_pcon before starting 
any Cygwin processes? (*)

Finally, does it matter which version of the emacs binary you use (emacs-w32, 
emacs-X11,...)?

Ken

(*) Alternatively, if you don't mind building your own cygwin1.dll, you could 
try Takashi's new version of the pty code:

   https://cygwin.com/pipermail/cygwin-developers/2020-August/011941.html

I've seen dramatic speedups with this patch installed, though not specifically 
with emacs.
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