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

[dalanicolai <dalanicolai@gmail.com>]

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Thanks again Eli for looking into this quickly. Over here, the memory does
not
even get released after killing the buffer, while the overlays are 'bound'
in a
buffer local list variable. But anyway, I should design/use things like Po
suggested,
in which case there are no issues.

If you are still interested in the GDB c-level backtrace, then of course I
would be
happy to create it (or does your second mail mean you already did this?),
but then I would need to find some time to read about how those
things work first. Anyway, if so, then let me know (I guess dealing with
'very large'
vscroll isn't really a priority, as this probably only occurs in
'designing' image-rolls).

On Tue, 26 Apr 2022 at 15:25, Eli Zaretskii <eliz@gnu.org> wrote:

> > Date: Tue, 26 Apr 2022 15:33:08 +0300
> > From: Eli Zaretskii <eliz@gnu.org>
> > Cc: emacs-devel@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).
>
> What I see here is that the memory footprint indeed goes up quite
> quickly, but then (not sure exactly what triggers that in my case), it
> gets reset back to almost the original small value.
>
> If this doesn't happen for you, then I guess your code somehow
> triggers bad behavior of glibc's malloc, forcing it not to release
> memory back to the OS, due to the particular pattern of allocations
> and deallocations?  Or maybe something else is at work here and I just
> got lucky?
>
> Memory is allocated for dealing with overlays, I think, because
> redisplay needs to have the overlays centered around the position the
> text it is rendering, and moving around many hundreds of overlays
> needs memory for the move.
>
> But that's a guess.  If someone can find out why we allocate large
> amounts of memory in this scenario, that could perhaps help us
> understand better what's going on here.
>

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