Hi, On 2025-02-22 18:54:08 +1300, Thomas Munro wrote: > On Sat, Feb 22, 2025 at 7:27 AM Andres Freund <and...@anarazel.de> wrote: > +#define MAX_BACKENDS_BITS 18 > +#define MAX_BACKENDS ((1 << MAX_BACKENDS_BITS)-1) > > +1 for working forwards from the bits. But why not call it > PROC_NUMBER_BITS?
Partially just because it was named MAX_BACKENDS historically. But also because it seems like it could be misleading - ProcNumber has more bits than PROC_NUMBER_BITS would indicate. > Hmm. Why shouldn't the highest usable ProcNumber (that you might call > PROC_NUMBER_MAX, like INT_MAX et all) FWIW, I don't think we should name it _MAX, precisely because of INT_MAX etc. INT_MAX indicate the actual range of the type, which isn't what we're dealing with here. > be (1 << PROC_NUMBER_BITS) - 1, and wouldn't that imply that MAX_BACKENDS > should actually be 1 << PROC_NUMBER_BITS and that any valid ProcNumber (a > 0-based index) should be *less than* MAX_BACKENDS (a count)? I don't *think* so, but I'm good at off-by-one-ing: > In other words, doesn't the current coding arbitrarily prevent the use of > one more backend, the one that has the highest ProcNumber representable in > 18 bits? If I'm right about that I think it is perhaps related to the use > of 0 as an invalid/unset BackendId before the ProcNumber-only redesign. 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? > + * currently realistic configurations. Even if that limitation were removed, > + * we still could not a) exceed 2^23-1 because inval.c stores the ProcNumber > + * as a 3-byte signed integer, b) INT_MAX/4 because some places compute > + * 4*MaxBackends without any overflow check. This is rechecked in the > > Should those two constraints have their own assertions? Probably wouldn't hurt, even though I think it's unlikely to matter anytime soon. I didn't yet have enough coffe, but isn't the inval.c limit 2^24-1 rather than 2^23-1? Greetings, Andres Freund
