Re: MAX_BACKENDS size (comment accuracy)

Hi,

On 2025-02-23 10:39:36 +1300, Thomas Munro wrote:
> 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.

Cool. I now pushed them, including static asserts in inval.c and deadlock.

Thanks for the reviews and the complaint leading to these changes.

> > 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...

Heh, that's odd. I left it like that, didn't seem worth changing given that
it's so far from the real limit...

Greetings,

Andres Freund