Re: The Emacs master is much slower than the emacs-27 branch.

[Ergus <spacibba@aol.com>]

On Thu, Dec 03, 2020 at 06:15:30PM +0000, Alan Mackenzie wrote:
>Hello, Emacs.
>
>I've got pretty much up to date copies of both the master and the
>emacs-27 branches.
>
>When I run my customary benchmark (see below) on master's src/xdisp.c in
>master, it's taking around 34s.  On emacs-27, it's taking 22s.
>
>That's a massive slowdown.  Does anybody know why this is happening?
>Should I raise a bug report?
>
>The benchmark I run is this:
>
>(defmacro time-it (&rest forms)
>  "Time the running of a sequence of forms using `float-time'.
>Call like this: \"M-: (time-it (foo ...) (bar ...) ...)\"."
>  `(let ((start (float-time)))
>    ,@forms
>    (- (float-time) start)))
>
>(defun time-scroll (&optional arg)
>  (interactive "P")
>  (message "%s"
>           (time-it
>            (condition-case nil
>                (while t
>                  (if arg (scroll-down) (scroll-up))
>                  (sit-for 0))
>              (error nil)))))
>
>On a freshly loaded buffer, do M-: (time-scroll).
>
>-- 
>Alan Mackenzie (Nuremberg, Germany).
>
Hi Alan:

I have observed the slowdown, but haven't have time to profile it
properly. Maybe adding to your time-scroll function (profile-start 'cpu)
just before and (profiler-stop) at the end to see the profile-report
could provide useful information.

The other thing I would check may be to compare the values of gcs-done
in both cases.

Best,
Ergus