bug#56682: locked narrowing

[Dmitry Gutov]

On 29/11/2022 21:51, Eli Zaretskii wrote:
> > Date: Tue, 29 Nov 2022 21:29:59 +0200
> > Cc: 56682@debbugs.gnu.org, gregory@heytings.org, monnier@iro.umontreal.ca
> > From: Dmitry Gutov <dgutov@yandex.ru>
> >
> > > > That's the regression.
> > > The faster ones use a different major-mode, btw.
> >
> > The old (faster) revision uses js-mode, the new one -- the optimized
> > js-json-mode. I'm sure I could have made a mistake doing that
> > optimization, but the behavior doesn't indicate that: the delays are
> > higher near BOB and seem absent near EOB (in master/emacs-29), and they
> > don't show up in the profiler in separate leaf nodes (the previous ones
> > -- which I worked on optimizing -- did).
>
> I didn't say the new mode was the culprit (you could have assumed I tried
> the old mode as soon as I discovered this change since July).  I just
> mentioned this for completeness, so we are aware what we are comparing.

For completeness we could also mention that js-mode also had one 
significant change in font-lock rules (also an optimization) which is 
not as easily transplantable.

> Anyway, do you see any effect if you set long-line-threshold to nil?  Here
> it makes the redisplay after insertion instantaneous (and this is an
> unoptimized build, where without setting that variable to nil each insertion
> in dictionary-pp.json takes about half a second).

Yep, that helps.

So the delay is due to it looking for long lines in the buffer, I guess?

Curious why it doesn't happen when near EOB. The benchmark reports a 
delay once, but if you run it multiple times, the timing go down to 
"instant".