Re: Some experience with the igc branch

[Eli Zaretskii]

> Date: Fri, 27 Dec 2024 15:01:11 +0000
> From: Pip Cet <pipcet@protonmail.com>
> Cc: Stefan Kangas <stefankangas@gmail.com>, Eli Zaretskii <eliz@gnu.org>, 
> ofv@wanadoo.es, emacs-devel@gnu.org, eller.helmut@gmail.com, acorallo@gnu.org
> 
> > - Concurrent.  The GC runs in its own thread.  There are no explicit
> >  calls to start GC, and Emacs doesn't have to wait for the GC to
> >  complete.
> 
> I don't think that's true right now (it is what I want for Christmas,
> though).  On GNU/Linux, the GC usually runs on the main thread.

Isn't it both, actually?  That is, MPS could be triggered both
synchronously and on a separate thread?  That's what I thought.

At least on Windows, I clearly see new threads starting when MPS
starts GC.

> On
> macOS, the GC can run on the main thread (allocation) or on the SIGSEGV
> handler thread (memory barriers); in both cases, the main thread has to
> wait for it to complete.
> 
> I'm not sure it's ever useful to make the assumption that GC isn't
> concurrent:  it is very hard to do so, but it is possible.
> 
> Maybe Eli knows more; I posted a patch to force concurrent GC for
> debugging a while ago, and Eli told me not to because it would produce
> false positives.  I'm not so sure about the "false" part now.

I just conveyed what a comment in igc.c says (or used to say back in
May).