Re: XID formatting and SLRU refactorings (was: Add 64-bit XIDs into PostgreSQL 15)

Hi,

> Here's how I see the development path for this [...]

> So, in my view, the plan should be [...]
> Thoughts?

The plan looks great! I would also explicitly include this:

> As we start to refactor these things, I also think it would be good to
> have more explicit tracking of the valid range of SLRU pages in each
> SLRU. Take pg_subtrans for example: it's not very clear what pages have
> been initialized, especially during different stages of startup. It
> would be good to have clear start and end page numbers, and throw an
> error if you try to look up anything outside those bounds. Same for all
> other SLRUs.

So the plan becomes:

1. Use internal 64 bit page numbers in SLRUs without changing segments naming.
2. Use the larger segment file names in async.c, to lift the current 8
GB limit on the max number of pending notifications.
3. Extend pg_xact to 64-bits.
4. Extend pg_subtrans to 64-bits.
5. Extend pg_multixact so that pg_multixact/members is addressed by
64-bit offsets.
6. Extend pg_twophase.c, to use FullTransactionIds. (a bonus thing)
7. More explicit tracking of the valid range of SLRU pages in each SLRU

Where our main focus for PG16 is going to be 1 and 2, and we may try
to deliver 6 and 7 too if time will permit.

Maxim and I agreed (offlist) that I'm going to submit 1 and 2. The
patch 1 is ready, please see the attachment. I'm currently working on
2 and going to submit it in a bit. It seems to be relatively
straightforward but I don't want to rush things and want to make sure
I didn't miss something.

> I wonder if we should actually add an artificial limit, as a GUC.

Yes, I think we need some sort of limit. Using a GUC would be the most
straightforward approach. Alternatively we could derive the limit from
the existing GUCs, similarly to how we derive the default value of
wal_buffers from shared_buffers [1]. However, off the top of my head
we only have max_wal_size and it doesn't strike me as a good candidate
for deriving something for NOTIFY / LISTEN.

So I'm going to add max_notify_segments GUC with the default value of
65536 as it is now. If the new GUC will receive a push back from the
community we can always use a hard-coded value instead, or no limit at
all.

[1]: https://www.postgresql.org/docs/current/runtime-config-wal.html#GUC-WAL-BUFFERS

--
Best regards,
Aleksander Alekseev