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Re: Better shared data structure management and resizable shared data structures

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, chaturvedipalak1911@gmail.com
Date: 2026-04-05T18:35:06Z
Lists: pgsql-hackers 
On 05/04/2026 19:23, Ashutosh Bapat wrote:
>> Both MultiXactShmemInit and MultiXactShmemAttach set
>> OldestMemberMXactId, OldestVisibleMXactId. In future if we add another
>> global variable to point to the shared memory, somebody needs to
>> remember to initialize it in both these functions. Maybe deduplicate
>> it with something like attached? Similarly for PredicateLock related
>> changes.
> 
> Sorry, I attached the wrong patch. Here's the right patch.

Gotcha, yeah I've thought about that too. I even considered making 
ShmemInitRequested() automatically call all the attach callbacks after 
initialization, even in !EXEC_BACKEND builds. That way, you could put 
the backend-private steps only in the attach function, and have 
automatically be called in the postmaster too. I decided against it, 
because only few subsystems need the attach callback at all, and many of 
them need to do the "local" steps earlier in the init callback anyway. 
For example, XLOGShmemInit() uses the WALInsertLocks variable inside the 
function already.

In the end I decided it's OK as it is. I'm not too worried about the 
duplicated code in multixact.c and predicate.c, they fit in the same 
screen in an editor so it's pretty easy to see that they are duplicated 
for a reason. With more complicated logic it would be a different story.

- Heikki

Commits

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

  1. Tidy up #ifdef USE_INJECTION_POINTS guards

  2. Convert all remaining subsystems to use the new shmem allocation API

  3. Convert buffer manager to use the new shmem allocation functions

  4. Add alignment option to ShmemRequestStruct()

  5. Convert AIO to use the new shmem allocation functions

  6. Convert SLRUs to use the new shmem allocation functions

  7. Refactor shmem initialization code in predicate.c

  8. Use the new shmem allocation functions in a few core subsystems

  9. Convert lwlock.c to use the new shmem allocation functions

  10. Introduce a registry of built-in shmem subsystems

  11. Convert pg_stat_statements to use the new shmem allocation functions

  12. Add a test module to test after-startup shmem allocations

  13. Introduce a new mechanism for registering shared memory areas

  14. Move some code from shmem.c and shmem.h

  15. Improve test_lwlock_tranches

  16. Test pg_stat_statements across crash restart

  17. Refactor PredicateLockShmemInit to not reuse var for different things

  18. Refactor ShmemIndex initialization

  19. Add a new shmem_request_hook hook.