Re: Some experience with the igc branch

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  pipcet@protonmail.com,  ofv@wanadoo.es,
>   emacs-devel@gnu.org,  eller.helmut@gmail.com,  acorallo@gnu.org
> Date: Fri, 27 Dec 2024 14:56:06 +0100
> 
> Gerd Möllmann <gerd.moellmann@gmail.com> writes:
> 
> > I've reached the point where I don't know what else to explain. Could
> > always be improved, of course, and so on. Please find attached, with
> > request for feedback.
> 
> Ahem, just remembered that I had already an admin/igc.org in the branch,
> so I've now replaced that with what I've written.

Thanks.  Some questions about that file:

  In contrast, the new igc collector, using MPS, is

  - 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.

Pip says this is not true?  I also thought MPS GC runs concurrently in
its own thread.

  When copying objects, a marker is left in the original object pointing
  to its copy. This marker is also called a "tombstone". A "memory
  barrier" is placed on the original object. Memory barrier means the
  memory is read and/or write protected (e.g. with mprotect). The
  barrier leads to MPS being invoked when an old object is accessed.
  The whole process is called "object forwarding".

This doesn't tell how object forwarding works once triggered by access
to protected memory.  Can you say something about that?  This:

  MPS makes sure that references to old objects are updated to refer to
  their new addresses. Functions defined in the object format are used
  by MPS to perform the lowest-level tasks of object forwarding, so that
  MPS doesn't have to know application-specific details of how objects
  look like. In the end, copying/forwarding is transparent to the
  application.

seems to try to explain that, but AFAIU stops short of telling it.
IOW, how are "functions defined in the object format used by MPS to
perform the lowest-level tasks of object forwarding"?

  AMC implements a "mostly-copying" collector, where "mostly" refers to
  the fact that it supports ambiguous references. Ambiguous references
  are those from ambiguous roots, where we can't tell if a reference is
  real or not. If we would copy such an object, we wouldn't be able to
  update their address in all references because we can't tell if the
  ambiguous reference is real or just some random integer, and changing
  it would have unforeseeable consequences. Ambiguously referenced
  objects are therefore never copied, and their address does not change.

This should be important to understand why some roots are submitted to
MPS as ambiguous -- because want to prevent them from moving, right?

  MPS allows us to specify roots having tailor-made scan functions that
  Emacs implements. Scanning here refers to the process of finding
  references in the memory area of the root, and telling MPS about the
  references.

What is the purpose of "telling MPS about the references"?

  igc provides a number if functions for doing such allocations. For
  example 'igc_xzalloc_ambig', 'igc_xpalloc_exact' and so on. Freeing
  the memory must be done with 'igc_xfree'.

An example of using these to reference Lisp objects in malloc'ed
memory would be great.

Stuff that I'd like added:

 . a few words about the root_create_* functions
 . same for create_*_ap functions
 . why do we need the finalize_* functions?
 . some explanation why pdumper needs special support from igc

Thanks again for writing this.