Re: Emacs design and architecture. How about copy-on-write?

[Ihor Radchenko <yantar92@posteo.net>]

Dmitry Gutov <dmitry@gutov.dev> writes:

>> I think that there is at least one way to address long lines using
>> asynchronous redisplay - put a placeholder on the problematic line and
>> continue calculating the actual rendering in the background instead.
>> That will not force us to compromise between rendering time and
>> accuracy, as we do now, with long-line-threshold.
>
> Until you're laid out the long line, you don't know which screen line it 
> will finish at, or at which height specifically (it might have images, 
> or taller text due to faces, etc). So you can't render the remainder of 
> the buffer either.

What I had in mind is the processing to happen in two stages: (1)
calculate "quick-and-dirty" layout; (2) start calculating "accurate"
layout. If calculating "accurate" layout takes long time, go ahead and
draw the "quick-and-dirty" version on the glass, while "accurate"
version continues being computed in the background.

Of course, this threshold should be configurable.

>> Similar approach might be used to render mode lines - render a
>> placeholder until it is fully calculated, keeping Emacs responsive.
>
> I hope by "placeholder" you mean the previous rendered image of it.

More precisely, what I had in mind is the old glyph matrix + some
indication that the rendering is not up-to-date (like classic sand clock
icon)

> ... But then the mode-line won't be refreshed at 60fps still, which
> some said is a good goal? (I tentatively agree).

I also agree that it is a good goal.

The scenario I had in mind is poorly written third-party mode line
packages that, for example, query git on every redisplay. There is
nothing we can do about poor Elisp code in third-party packages, but the
proposed approach will at least prevent Emacs being laggy or hanging in
the eyes of the user.

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
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