bug#56682: Fix the long lines font locking related slowdowns

[Dmitry Gutov]

On 06.08.2022 14:17, Eli Zaretskii wrote:
> > Date: Sat, 6 Aug 2022 13:50:41 +0300
> > Cc: 56682@debbugs.gnu.org, gregory@heytings.org, monnier@iro.umontreal.ca
> > From: Dmitry Gutov <dgutov@yandex.ru>
> >
> > > > I'm not seeing any particular sluggishness in these operations when
> > > > visiting dictionary.json.
> > >
> > > Numbers, please.  You have a very fast machine, so what doesn't look
> > > sluggish on your system could very well be so on others.
> >
> > How do you measure these operations including the redisplay lag?
>
> By timing them.  With benchmark-run or similar, if it isn't slow
> enough to be measured "by hand".

How do I time 'M-x'?

Anyway, here are some numbers:

(benchmark 1 '(next-line 1)) =>
Elapsed time: 0.035137s
Elapsed time: 0.061863s
Elapsed time: 0.027261s
(benchmark 1 '(cua-scroll-down 1)) =>
Elapsed time: 0.049362s
Elapsed time: 0.050060s
(benchmark 1 '(cua-scroll-up 1)) =>
Elapsed time: 0.012383s
Elapsed time: 0.039984s
(benchmark 1 '(recenter-top-bottom 1)) =>
Elapsed time: 0.042014s
Elapsed time: 0.013934s

I get very similar numbers on these operations with unmodified master in 
this file.

> > > Linear scaling is less optimal than O(0), which is what the current
> > > solution produces.
> >
> > Correct syntax highlighting requires parsing the buffer from the
> > beginning. Otherwise we get random results, essentially.
> >
> > Just like depicted on my latest screenshot for downloadify.js: the
> > beginning of the narrowed region ended up inside a string, and as a
> > result several screenfuls were entirely mis-fontified, with strings and
> > non-strings inverted.
>
> Yes, and IMNSHO responsiveness is more important than correctness in
> these cases.

That's why I suggested that, possibly after some further optimization 
work on parse-partial-sexp and syntax-propertize, we decide which 
initial length of the file we can fontify correctly with decent speed. 1 
MB, or 10 MB, something like that.

And of course make that customizable.