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


From: Ergus
Subject: Re: The Emacs master is much slower than the emacs-27 branch.
Date: Thu, 3 Dec 2020 22:09:19 +0100

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


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