Re: last_marked array is now ifdef'ed away

[Eli Zaretskii <eliz@gnu.org>]

> From: Mattias Engdegård <mattias.engdegard@gmail.com>
> Date: Sat, 14 Sep 2024 22:30:11 +0200
> Cc: emacs-devel@gnu.org
> 
> 14 sep. 2024 kl. 19.30 skrev Eli Zaretskii <eliz@gnu.org>:
> 
> > That's the nature of things that are needed rarely.  But when they
> > _are_ needed, there's nothing to replace them, and that's when you
> > appreciate their existence.  Like safety belts, these features are
> > never a waste.
> 
> No, this device doesn't improve safety in any way. At best it's a special-purpose single-channel flight data recorder mounted in such a way that it disrupts the airflow and increases fuel consumption. Let's see if we can make it a bit more streamlined.

I'm okay with FDR analogy.  As anyone who saw Air Crash Investigation
series knows, this is invaluable in investigating the (very rare!)
airplane crashes.

> > 16 is way too few.  GC is deeply recursive, and when I used this, I
> > almost always had to look at much more than 16 objects.
> 
> Note that the GC recursion is now almost entirely handled by mark_stack. 512 entries is an entire x86 page; surely that can be trimmed?

I'd need to see actual evidence for a shorter array, based on actual
GC run of some data structure.

Also, how bad is the penalty for accessing 512-element array vs a
16-element array?  Unless it's close to 10% or exceeds that, I
wouldn't bother.