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

[Dmitry Gutov]

On 19/09/2023 18:37, Eli Zaretskii wrote:
> > Date: Tue, 19 Sep 2023 18:15:50 +0300
> > Cc:acm@muc.de,incal@dataswamp.org,emacs-devel@gnu.org
> > From: Dmitry Gutov<dmitry@gutov.dev>
> >
> > On 19/09/2023 17:14, Eli Zaretskii wrote:
> > > > This is very simple - I suggest to not touch xdisp.c as much as
> > > > possible, just make sure that it is interlocked. And focus on async
> > > > support in Elisp code. Once there is async support in Elisp code, we can
> > > > move further and look into xdisp.
> > > >
> > > > (I suggest this because I feel that xdisp is a rabbit hole we may sink
> > > > in instead of doing something more productive)
> > > I'm actually of the opposite opinion.  I think trying to parallelize
> > > redisplay is a lower-hanging fruit, and if successful, it could bring
> > > non-trivial gains to Emacs even if the rest remains single-threaded.
> > Do we have evidence that the speed of redisplay is a limiting factor in
> > general Emacs usage? I.e. with an optimized build and most of the
> > popular modes (excepting certain display-heavy Org buffers, let's say).
> You already forgot the long-lines problems?

The long-lines problem wasn't solvable with parallel redisplay either, 
it was a case of high algorithm complexity (a combination of them).

> And there are other situations where we'd like faster redisplay, even
> though they are not as bad.  For example, when a session has many
> frames (I believe Stefan Monnier tends to have a lot of them in his
> sessions).  To say nothing of the fact that displays become larger and
> larger, and many people like to have their frames maximized.

It would help to see some info on:

- Whether Stefan's build is optimized,
- How much time the many-frames case spends talking to the windowing 
system, compared to the time our redisplay takes internally. If there 
are many frames displayed, but the current buffer is shown in only one 
of them (or two, etc), then shouldn't the rest of the frames stay mostly 
still anyway?

I also like to have my frames maximized, and I have a 4K display -- even 
so, redisplay seems to be mostly fine (with 1-2 frames anyway). But 
rendering large windows at high resolution, while it can be taxing, 
generally bottlenecks at something else than the layout algorithms. 
Unless you're arguing that the layouts are going to become more complex 
exponentially as well.