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

From: Eli Zaretskii
Subject: Re: Emacs design and architecture. How about copy-on-write?
Date: Thu, 21 Sep 2023 17:02:20 +0300

> From: Ihor Radchenko <yantar92@posteo.net>
> Cc: acm@muc.de, incal@dataswamp.org, emacs-devel@gnu.org
> Date: Thu, 21 Sep 2023 10:29:48 +0000
> Eli Zaretskii <eliz@gnu.org> writes:
> >> AFAIU, long-lines performance is largely problematic when running Elisp.
> >
> > No, the slow parts are pure C.
> >
> >> (Also, isn't it solved already?)
> >
> > Not solved, but "bypassed".  The solution does have its disadvantages,
> > although we found them to be a much lesser evil than the original
> > problem.
> May you point to the xdisp.c function that is being slow?

The basic iteration and layout calculations are slow because (a) they
must examine every character between point A and point B to figure out
the relative position of these points on the screen, and (b) because
this examination can only go forward, not back (so to go back, we must
"jump" very far back, and then go forward).

> >> Another one you mentioned is displaying many frames. But isn't it
> >> optimized by the part of the code that delays redisplay of frames that
> >> are not visible? Or does Stefan (or anyone) tend to have so many
> >> _visible_ frames?
> >
> > The latter.
> Ok. But is it possible to redisplay the whole frame without ever calling 
> Elisp?

Probably not "without ever", but why is it important in this context?
Most of the processing is in C, not in Lisp.

> >> > There's more than one reason, that's true.  But it doesn't change the
> >> > fact that all those problems have to be resolved before we have
> >> > reasonably usable threads.
> >> 
> >> I disagree. Yes, _all_ the problems are to be solved eventually. But
> >> solving _some_ of the problems will already be an improvement. IMHO, it
> >> is not all-or-nothing; we can split the problem into several
> >> sub-problems and solve them one by one.
> >
> > You've lost context, and effectively changed the subject, so now this
> > sub-thread is not useful anymore.  To recap, I was trying to explain
> > why the display aspects of any concurrency cannot be disregarded if we
> > want to have a workable solution that can be used in practice.
> I believe that I did not lose the context. Unless I misunderstand
> something.
> What I suggested is to defer the redisplay concurrency and leave it
> synchronous (interlocked) for now. Concurrency of Elisp that does not
> trigger redisplay can also be useful.
> It looks to me that we just disagree about the relative importance of
> redisplay concurrency.

Yes, exactly.  My point is that without having a reasonable solution
for display, any concurrency will be dead in the water from the

> >> I don't forget that. However, we do not have to make each and every
> >> thread _fully_ asynchronous. It is often enough to process the
> >> performance bottlenecks asynchronously. After that processing, we can go
> >> ahead and present the results synchronously to user.
> >
> > This assumes the display is always in the end.  But that is a false
> > assumption in Emacs: there are progress messages, there are prompts
> > and requests for confirmation, there are display updates to show the
> > stuff processed so far, etc.
> Again, not always. I do not think that the _initial_ goal should be
> making every aspect of Elisp working asynchronously without
> interlocking. We certainly want all Elisp to work in threads, possibly
> with global lock. But if we get at least some part of useful Elisp to
> run truly concurrently, it will already be a win.

Not if we want to run the existing Lisp programs in threads with
minimal or no changes.  Because Lisp programs we have now require
display and user interaction all the time, whether by themselves or by
lower-level subroutines they call.

> >> Let me provide a more concrete example.
> >> Consider something as simple as grepping across project files.
> >> The process of grepping involves: (1) opening and searching many files;
> >> (2) presenting results to the user. Stage (1) does not really involve
> >> user interaction or redisplay and can greatly benefit from asynchronous
> >> threads - we can simply search multiple files at the same time. Then,
> >> even if stage (2) has to be synchronous, it does not matter - stage (1)
> >> is what takes most of the time and makes the user wait.
> >
> > You describe an Emacs without async subprocesses -- this is how it
> > works on MSDOS, for example.  On more capable systems we display in
> > parallel with running Grep.
> I think you misunderstood. I did not mean calling GNU grep. I meant
> calling M-x grep - Elisp code searching across many Emacs buffers. It
> would be useful if such code could run concurrently.

It will be much more useful if it could also show the hits as it runs,
instead of requiring the user to wait till it finishes.

> Another example is org-agenda that is running a large number of regexp
> search across many buffers. Some real-world users search across as many
> as 500 buffers, with results accumulated into agenda. It would certainly
> be useful to run that regexp search concurrently, in multiple buffers at
> once. Or maybe even on several regions of a single, large buffer.

And you never want to show the accumulated agenda as it is

> >> > I'm not talking about async redisplay, I'm talking about the ability
> >> > of non-main threads to display something or do something that would
> >> > cause redisplay change the stuff on the glass.
> >> 
> >> Then, how will it be possible without first having async Elisp threads?
> >
> > Async Lisp threads is a much harder problem to solve.  If we solve it,
> > this one will also be solved; but the opposite is not true.
> I agree here. But do note that the interest (and some trial code by Po
> Lu) so far has been focused on Elisp threads.

I'm quite sure people who disregard the display when they are talking
about concurrent Lisp threads are going to repeat the same mistake we
made with the existing Lisp threads.  We must learn from past
experience if we want to succeed.

> The problem with display code, even if it is easier to transform it to
> concurrent, is that understanding display code is by itself is a
> time-consuming task. Making changes in it (even small changes) is no
> less trivial (I am referring to bug#64596).

That is true about any non-trivial part of Emacs internals.  At least
with the display code we have some expertise and experience on board,
something that is not always true elsewhere in Emacs.

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