Re: Is Emacs buffer scroll/redisplay slow?

[Fabrice Popineau <fabrice.popineau@gmail.com>]

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Hi Alan,

Le dim. 19 janv. 2020 à 12:51, Alan Mackenzie <acm@muc.de> a écrit :

> Hello, Fabrice.
>
> On Sun, Jan 19, 2020 at 10:34:07 +0100, Fabrice Popineau wrote:
> > Hi,
>
> > Out of curiosity, I timed 'emacs -Q' ....
>
> Which version of Emacs?
>
> Current master.
Emacs is compiled with gcc 9.2.0 (MSYS2) and 7.4.0 (Ubuntu)
and CFLAGS = -g3 -O2 -gdwarf-2


> > .... scrolling through an elisp buffer: I started the timer by hand and
> > scrolled down for 1000 lines ....
>
> How, exactly, did you do the scrolling?  How did you stop the scrolling
> at 1000 lines?  Do you get this slowness on any elisp buffer, or is it a
> particular one?
>
> I used my phone timer. It was my init.el file but I get the same result
with dired.el

> .... and it took 45s on Windows/WSL and 35s on Windows/native.
>
> That is slow indeed.
>

Definitely.


>
> Have you tried profiling this scrolling, yet?  If not, I suggest you do,
> with M-x profiler-start <CR> <CR>, running the scrolling, then M-x
> profiler-report.  In the report buffer, hit <CR> to get successively
> deeper results.
>
> This may well give you an idea where the slowness is happening.
>

You are right. I should have tried it right away.

[image: image.png]

I am not sure there is anything unexpected in there.
The lisp machinery interpreting commands accounts for most of the time,
especially `completing-read-default' and I wonder why? Is it because of the
`make-composed-keymap' call
which occurs in `completing-read-default'?

Best regards,

Fabrice

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