Re: Gap buffer problem?

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: Óscar Fuentes <ofv@wanadoo.es>,  Pip Cet
>  <pipcet@protonmail.com>
> Date: Wed, 11 Dec 2024 06:27:43 +0100
> 
> Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
> writes:
> 
> > To be fair, part of that may be the gap buffer problem rather than GC.
> 
> Could you please tell more about the gap buffer problem?
> 
> I've read a little about the tradeoffs between gap buffers, piece
> tables, ropes, but I'm wondering if there is something concrete already
> known for sure that is a performance problem in Emacs. Maybe a bug that
> has been analyzed or something.
> 
> (I'm asking because I just recently encountered a performance problem
> when adding something to xdisp.c:27339 (with cc-mode, Eglot, Corfu), and
> editing there was so slow that it was absolutely no fun, and that on a
> an M1 pro. Haven't investigated the reason.)

Unless you have a huge (and I mean a HUGE) buffer, and some Lisp that
moves point, then inserts a small number of characters, then moves
point far away and again inserts a small number of characters, etc.,
I'd be very surprised if the gap buffer caused significant performance
problems on a modern CPU.

Can you profile that case and post the expanded profile?  I'm always
happy to be wrong about performance bottlenecks, and profiles are good
at proving me wrong.