Re: POC: make mxidoff 64 bits

Heikki Linnakangas <hlinnaka@iki.fi>

From: Heikki Linnakangas <hlinnaka@iki.fi>

To: Maxim Orlov <orlovmg@gmail.com>, wenhui qiu <qiuwenhuifx@gmail.com>

Cc: Alexander Korotkov <aekorotkov@gmail.com>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Postgres hackers <pgsql-hackers@lists.postgresql.org>

Date: 2025-10-28T14:17:11Z

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 27/10/2025 17:54, Maxim Orlov wrote:
> Here is a new patch set @ 10b5bb3bffaee8
> 
> As previously stated, the patch set implements the concept of saving the
> "difference" between page offsets in order to save disc space.

Hmm, is that safe? We do the assignment of multixact and offset, in the 
GetNewMultiXactId() function, separately from updating the SLRU pages in 
the RecordNewMultiXact() function. I believe this happen:

To keep the arithmetic simple, let's assume that multixid 100 is the 
first multixid on an offsets SLRU page. So the 'base' on the page is 
initialized when multixid 100 is written.

1. Backend A calls GetNewMultiXactId(), is assigned multixid 100, offset 
1000
2. Backend B calls GetNewMultiXactId(), is assigned multixid 101, offset 
1010
3. Backend B calls RecordNewMultiXact() and sets 'page->offsets[1] = 10'
4. Backend A calls RecordNewMultiXact() and sets 'page->base = 1000' and 
'page->offsets[0] = 0'

If backend C looks up multixid 101 in between steps 3 and 4, it would 
read the offset incorrectly, because 'base' isn't set yet.

- Heikki