Re: last_marked array is now ifdef'ed away

[Andrea Corallo]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Stefan Kangas <stefankangas@gmail.com>
>> Date: Sat, 14 Sep 2024 15:50:03 -0700
>> Cc: emacs-devel@gnu.org
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> > I'm quite confident that I want this be turned on again.
>> 
>> Since this affects performance by quite a bit (4.5-6.5 %, reportedly),
>> how about the suggestion to "hitch it to another existing config option
>> like ENABLE_CHECKING, or even make this an independent config option"?
>
> This feature is useless if it is optional, because its main (perhaps
> the only) use is when Emacs crashes during GC on a user machine,
> especially without enough debug info.  Crashes during GC are rarely if
> at all reproducible, so the only hope to investigate them is to look
> at the objects recorded in the last_marked array, and asking users to
> rebuild with some non-default option will not help.
>
> If we are going to make it a compilation option, we might as well
> remove it entirely.
>
> 5% of slowdown during GC (if that's a well-established number) is a
> small price to pay for the possibility of investigating rare crashes.
> A typical GC cycle takes tens or hundreds of milliseconds, so these
> few percents are just a couple of msec.

5% slowdown during GC is at worst 2.5% slowdown overall for GC intensive
workloads.

I think as well we should not loose this debug tool for a slowdown that
no-one would notice.  So I'm for re-enabling the code without it being
under a default-off flag.

  Andrea