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

Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>

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From: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
To: Heikki Linnakangas <hlinnaka@iki.fi>
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: 2026-04-05T14:08:09Z
Lists: pgsql-hackers

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On Sun, Apr 5, 2026 at 11:18 AM Ashutosh Bapat
<ashutosh.bapat.oss@gmail.com> wrote:
>
>
> I will post my resizable shmem structures patch in a separate email in
> this thread but continue to review your patches.

I reviewed the SLRU patch. This is the first time I am looking at SLRU
code, so my review may not be sufficient. As far as I understand, the
patch faithfully copies the functionality from the old system to the
new system. I didn't find any issues there.

I think calls to SimpleLruRequest() reads much better than SimpleLruInit().

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.

shmem_slru_init and shmem_slru_attach() also have the following
duplicate lines, which can be deduplicated in a similar fashion.
desc->shared = shared;
desc->nbanks = nbanks;
memcpy(&desc->options, options, sizeof(SlruOpts));

Including "access/slru.h" in shmem.h is circular inclusion. I am
wondering whether we need to create shmem_slru.h like shmem_hash.h to
handle shared memory APIs related to SLRU. Given that SLRU also has a
disk component, the bifurcation may not be straightforward. I haven't
looked into this aspect in detail.


--
Best Wishes,
Ashutosh Bapat

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.