Re: MAX_BACKENDS size (comment accuracy)

On Sun, Feb 23, 2025 at 4:16 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> We do count the number of lwlock share lockers and the number of buffer
> refcounts within those bits. And obviously 0 lockers/refcounts has to be
> valid. So I think the limit is correct?

Ah, right. That makes perfect sense. The 18 bits need to be able to
hold a count, not just an index, and I confused myself about that from
the moment I thought about the name PROC_NUMBER_BITS, which I retract.

> I didn't yet have enough coffe, but isn't the inval.c limit 2^24-1 rather than
> 2^23-1?

Yeah, it has 24 bits of space, but curiously backend_hi is signed, so
(msg->sm.backend_hi << 16) would be sign-extended, so it wouldn't actually
work if you used all 24 bits... which is obviously not a real
problem...