Re: last_marked array is now ifdef'ed away

["Mattias Engdegård" <mattias.engdegard@gmail.com>]

13 sep. 2024 kl. 17.21 skrev Eli Zaretskii <eliz@gnu.org>:

> But if a user reports a crash inside GC, that's basically the only
> trace we have to try and understand what caused the crash, especially
> if the debug info was stripped.  That's why this code was always on by
> default.

Yes, I understand that -- nevertheless it is a bit of a waste to spend time on something that is only rarely used (apparently it took over 2 years before anyone noticed, and I know you aren't one for keeping quiet about this sort of thing).

It surprised me how expensive this silly thing is: enabling GC_REMEMBER_LAST_MARKED made GC 4.5-6.5 % slower in some simple benchmarks, and the difference showed up in some actual useful work type of code that I measured.

And that is with LAST_MARKED_SIZE downgraded from the default 512 to just 16 which seemed more reasonable. (512 entries is 4 KiB of L1D cache which seems extravagant, but I presume the size wasn't given much thought at the time.)

Perhaps it's not a big surprise after all: the GC marking loop has data-dependent branches and data-dependent loads, and we are likely to be bound by instructions in flight rather than execution resources; maybe any added instructions and memory traffic will just make it worse.

> My main concern is not bitrot, it's actually being able to debug
> crashes inside GC.

I'm quite sympathetic to that goal but it would be good if we could do it more cheaply. You can inspect the mark_stack structure; I've found this to help quite a bit.