Re: cc-mode fontification feels random

[Alan Mackenzie]

Hello, Eli.

On Fri, Jun 11, 2021 at 20:53:10 +0300, Eli Zaretskii wrote:
> > Date: Fri, 11 Jun 2021 16:11:19 +0000
> > Cc: Daniel Colascione <dancol@dancol.org>, rudalics@gmx.at,
> >   monnier@iro.umontreal.ca, rms@gnu.org, emacs-devel@gnu.org
> > From: Alan Mackenzie <acm@muc.de>

> > I have measured CC Mode's scrolling performance using:

> > (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)))

> > together with

> > M-: (time-it (scroll-up-window) (sit-for 0))

> > on regions of text which are not yet fontified.  My window has 65 lines
> > of buffer text.  Starting at the middle of xdisp.c, I see the following
> > timings for the first few scrolls:

> >    0.026s, 0.025s, 0.026s, 0.078s, 0.026s, 0.027s.

> > That is, with the exception of the fourth timing, the scroll operation
> > takes a little over 1/40 second.

> > This is in an Emacs-28 compiled with default optimisation, on a 4
> > year-old first generation Ryzen machine.

> > For me personally, this scrolling speed, in conjunction with
> > fast-but-imprecise-scrolling, is acceptable.  I also accept there are
> > people with slower machines.

> I suggest to compare these times with Emacs 23 to see how we
> regressed.

OK, on emacs-23.3 -Q, otherwise exactly the same circumstances,  I get
these timings:

    0.0093s, 0.0089s, 0.0084s, 0.0144s, 0.0094s, 0.0084s.

So the difference is around a factor of 3, perhaps a little more.  "Half
an order of magnitude" perhaps sums it up best.

-- 
Alan Mackenzie (Nuremberg, Germany).