bug#44007: 26.3; Strange shell mode performance

["Herman, Geza" <geza.herman@gmail.com>]

Sure!

1. execute emacs by "emacs -Q"
2. M-x shell RET
3. in the shell, execute "seq 100000" (or cat a large file)
4. immediately after you hit enter on "seq 100000" press enter again

It doesn't reproduce 100% unfortunately. It seems that if it doesn't 
reproduce, you need to start from step 1., so it has a higher chance to 
happen.

Another strange thing is that enter can have the opposite effect as 
well: if I press it during "seq 100000" several times (not immediately 
after "seq 100000", but after ~1 sec), it can make seq slower. Instead 
of the usual 3-4 sec, it slows down to >10 sec.
(I measured by "time seq 100000"). It seems that the more enter I press, 
the slower it gets. But maybe it's a different issue.

On 2020-10-15 16:10, Eli Zaretskii wrote:
>> From: Herman@debbugs.gnu.org, Géza <geza.herman@gmail.com>
>> Date: Thu, 15 Oct 2020 11:53:16 +0200
>>
>> I'm experiencing strange shell (comint) mode performance. If I cat a 10
>> MB file, it takes 1:20 to cat it. Emacs uses only 30% cpu. But, if I
>> press enter after I send the cat command, it speeds up, and it only
>> takes 8 sec (emacs cpu usage goes up to 100%).
>>
>> Same thing: if I execute "seq 100000" in a small emacs window, it takes
>> ~3 seconds. But, if I press enter after executing seq, it takes only 0.4
>> sec (this is not 100% reproducible, sometimes pressing enter doesn't
>> make a difference).
>> (I executed comint-clear-buffer before measurements)
> Can you please describe a detailed reproduction recipe, starting from
> "emacs -Q"?  It is not exactly clear from the above whether you do
> this in a shell-mode buffer or in some other mode, where exactly do
> you press Enter, etc.  A detailed recipe will resolve all those
> unclear aspects.
>
> Thanks.