Re: POC: make mxidoff 64 bits

Maxim Orlov <orlovmg@gmail.com>

From: Maxim Orlov <orlovmg@gmail.com>

To: Alexander Korotkov <aekorotkov@gmail.com>

Cc: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, wenhui qiu <qiuwenhuifx@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Postgres hackers <pgsql-hackers@lists.postgresql.org>

Date: 2025-09-15T15:42:00Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Fix partial read handling in pg_upgrade's multixact conversion
- ac94ce8194e5 19 (unreleased) landed
Increase timeout in multixid_conversion upgrade test
- bd43940b02b2 19 (unreleased) landed
Improve sanity checks on multixid members length
- ecb553ae8211 19 (unreleased) landed
Clarify comment on multixid offset wraparound check
- 170361d7b869 14.21 landed
- b0b52b7123ae 15.16 landed
- 7d42e2367c6b 16.12 landed
- cd1a887fe9bf 17.8 landed
- 3fbad030a24d 18.2 landed
- 366dcdaf5779 19 (unreleased) landed
Never store 0 as the nextMXact
- 87a350e1f284 19 (unreleased) landed
Add runtime checks for bogus multixact offsets
- d4b7bde4183b 19 (unreleased) landed
Widen MultiXactOffset to 64 bits
- bd8d9c9bdfa0 19 (unreleased) landed
Move pg_multixact SLRU page format definitions to a separate header
- bb3b1c4f6462 19 (unreleased) landed
Convert confusing macros in multixact.c to static inline functions
- 0099b9408e8c 17.0 landed
Index SLRUs by 64-bit integers rather than by 32-bit integers
- 4ed8f0913bfd 17.0 cited
Cope with possible failure of the oldest MultiXact to exist.
- b6a3444fa635 9.4.4 cited

On Sat, 13 Sept 2025 at 16:34, Alexander Korotkov <aekorotkov@gmail.com>
wrote:

>
> Therefore, we can change from each 8 of 32-bit multixact offsets
> (takes 32-bytes) to one 64-bit offset + 7 of 24-bit offset increments
> (takes 29-bytes).  The actual multixact offsets can be calculated at
> the fly, overhead shouldn't be significant.  What do you think?
>
>
Thank you for your review; I'm pleased to hear from you again.

Yes, because the maximum number of mxoff is limited by the number of
running transactions, we may do it that way.
However, it is a bit wired to have offsets with the 7-byte "base".

I believe we may take advantage of the 64XID patch's notion of putting a
8 byte base followed by 4 byte offsets for particular page.

32kB page may contain then 2^13-2 offsets, each is maxed by 2^18+1.
Therefore, offset from base will never overflow 2^31 and will always
fit uint32.

It appears logical to me.

-- 
Best regards,
Maxim Orlov.