Re: Existing redisplay profiling tool?

[Eli Zaretskii <eliz@gnu.org>]

> From: Yuan Fu <casouri@gmail.com>
> Date: Sun, 15 Sep 2024 23:16:48 -0700
> Cc: emacs-devel@gnu.org
> 
> > How about using the scrolling benchmark through xdisp.c while
> > profiling?
> > 
> > (defun scroll-up-benchmark ()
> >  (interactive)
> >  (let ((oldgc gcs-done)
> >        (oldtime (float-time)))
> >    (condition-case nil (while t (scroll-up) (redisplay))
> >      (error (message "GCs: %d Elapsed time: %f seconds"
> >                      (- gcs-done oldgc) (- (float-time) oldtime))))))
> > 
> > Evaluate the above, then turn on profiling, then type
> > "M-x scroll-up-benchmark RET" with point at beginning
> > of buffer that visits xdisp.c under c-ts-mode.
> 
> Thanks! I’m hoping to measure the perceived responsiveness when typing text in the buffer, so here’s what I came up with:
> 
> (let ((prev-time (current-time))
>       (measurements nil))
>   (dotimes (_ 100)
>     (insert "s")
>     (redisplay)
>     (push (float-time (time-subtract (current-time) prev-time))
>           measurements)
>     (setq prev-time (current-time)))
>   (message "Average time: %f"
>            (/ (apply #'+ measurements) (length measurements))))
> 
> Do you think this accurately measures the redisplay time between each keystroke? (Obviously this doesn’t take account of post-command-hook, I only want to measure repose & redisplay here.)

I don't understand why you measure only insertion of a single
character.  This kind of change to buffer text is so frequent that it
has special optimizations in the display engine, and you might be
measuring only those special optimizations.

I proposed a scrolling benchmark because it executes the font-lock
code many times, and is more expensive than insertion of a single
character.  You might as well try both, and could learn different
things from each other.

A variant of the above scrolling benchmark is scrolling by many lines
in one go (it causes a larger mismatch between the previous and the
current window, so redisplay is forced to use yet another code path
for that):

(defun scroll-up-by-40-benchmark ()
  (interactive)
  (let ((oldgc gcs-done)
        (oldtime (float-time)))
    (condition-case nil (while t (scroll-up 40) (redisplay))
      (error (message "GCs: %d Elapsed time: %f seconds"
                      (- gcs-done oldgc) (- (float-time) oldtime))))))

> If that’s an accurate measurements, then my optimization doesn’t make any significant difference :-)
> In xdisp.c, inserting 100 “s” takes about 17.8ms per “keystroke” on average. With optimization it’s about 16.7ms, not much difference.

That's because redisplay is careful to redraw only a single screen
line when a single character is added.  I'm not sure this is the only
situation that is interesting for you, but then I don't really know
what you are trying to optimize.

> I guess the takeaway is a) my new optimization doesn’t do anything, and b) tree-sitter font-lock doesn’t add human-perceivable latency when typing.

Try the other benchmarks, and maybe you will arrive at different
conclusions.