Re: Question about (excessive?) ram usage when many overlays (with large vscroll)

[Eli Zaretskii <eliz@gnu.org>]

> From: dalanicolai <dalanicolai@gmail.com>
> Date: Tue, 26 Apr 2022 14:21:07 +0200
> 
> I have a question about ram usage when using overlays.
> 
> So I have created `image-roll.el` for displaying documents/books (see here
> <https://lists.gnu.org/archive/html/emacs-devel/2022-04/msg00975.html>).
> However, I have just noticed that it uses a large amount of RAM when
> viewing (or
> trying to) pages in the back of 'large' books. But even if RAM usage still
> looks
> perfectly fine, Emacs crashes when trying to scroll to higher page numbers.
> 
> I have looked a little into it, and have found that it is a consequence of
> using
> large overlays. There is no problem when creating a buffer containing many
> overlays, however, when trying to scroll to some overlay at the end of the
> buffer,
> Emacs will use huge amounts of RAM.
> 
> So I am wondering if this is 'necessary' behavior; and then why is it
> necessary?
> The issue occurs even when displaying the same 'empty' svg-image on each
> overlay, but it occurs also when simply displaying some 'specified space' on
> each overlay.

I didn't yet try to reproduce this, but could you perhaps run this
test under GDB, and when Emacs crashes, show the C-level backtrace?
This could give good hints about the possible reason(s).

Thanks.