Horton

Re: POC: make mxidoff 64 bits

Maxim Orlov <orlovmg@gmail.com>

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From: Maxim Orlov <orlovmg@gmail.com>
To: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: wenhui qiu <qiuwenhuifx@gmail.com>, Alexander Korotkov <aekorotkov@gmail.com>, Postgres hackers <pgsql-hackers@lists.postgresql.org>
Date: 2025-11-14T15:40:50Z
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 →

  1. Fix partial read handling in pg_upgrade's multixact conversion

  2. Increase timeout in multixid_conversion upgrade test

  3. Improve sanity checks on multixid members length

  4. Clarify comment on multixid offset wraparound check

  5. Never store 0 as the nextMXact

  6. Add runtime checks for bogus multixact offsets

  7. Widen MultiXactOffset to 64 bits

  8. Move pg_multixact SLRU page format definitions to a separate header

  9. Convert confusing macros in multixact.c to static inline functions

  10. Index SLRUs by 64-bit integers rather than by 32-bit integers

  11. Cope with possible failure of the oldest MultiXact to exist.

On Wed, 12 Nov 2025 at 16:00, Heikki Linnakangas <hlinnaka@iki.fi> wrote:

>
> I added an
> inlined fast path to SlruReadSwitchPage and SlruWriteSwitchPage to
> eliminate the function call overhead of those in the common case that no
> page switch is needed. With that, the 100 million mxid test case I used
> went from 1.2 s to 0.9 s. We could optimize this further but I think
> this is good enough.
>
I agree with you.


- I added an SlruFileName() helper function to slru_io.c, and support
> for reading SLRUs with long_segment_names==true. It's not needed
> currently, but it seemed like a weird omission. AllocSlruRead() actually
> left 'long_segment_names' uninitialized which is error-prone. We
> could've just documented it, but it seems just as easy to support it.
>
Yeah, I didn't particularly like that place either. But then I decided it
was
overkill to do it for the sake of symmetry and would raise questions.
It turned out much better this way.



> I kept all the new test cases for now. We need to decide which ones are
> worth keeping, and polish and speed up the ones we decide to keep.
>
I think of two cases here.
A) Upgrade from "new cluster":
    * created cluster with pre 32-bit overflow mxoff
    * consume around of 2k of mxacts (1k before 32-bit overflow
      and 1k after)
    * run pg_upgrade
    * check upgraded cluster is working
    * check data invariant
B)  Same as A), but for an "old cluster" with oldinstall env.


On Thu, 13 Nov 2025 at 19:04, Heikki Linnakangas <hlinnaka@iki.fi> wrote:

>
> Here's a new patch version that addresses the above issue. I resurrected
> MultiXactMemberFreezeThreshold(), using the same logic as before, just
> using pretty arbitrary thresholds of 1 and 2 billion offsets instead of
> the safe/danger thresholds derived from MaxMultiOffset. That gives
> roughly the same behavior wrt. calculating effective freeze age as before.
>
Yes, I think it's okay for now. This reflects the existing logic well.
I wonder what the alternative solution might be? Can we make a
"vacuum freeze" also do pg_multixact segments truncation?
In any case, this can be discussed later.

-- 
Best regards,
Maxim Orlov.