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

Zsolt Parragi <zsolt.parragi@percona.com>

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From: Zsolt Parragi <zsolt.parragi@percona.com>
To: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, chaturvedipalak1911@gmail.com
Date: 2026-03-13T23:02:05Z
Lists: pgsql-hackers 
Hello

 dsm_shmem_init(void)
 {
  size_t size = dsm_estimate_size();
- bool found;

  if (size == 0)
  return;

Isn't there an assignment missing from this function now? Size is
calculated but never used. With the current code if
min_dynamic_shared_memory > 0 the server can crash.

+static ShmemHashDesc WaitEventCustomHashByNameDesc =
+{
+ .name = "WaitEventCustom hash by name",
+ .ptr = &WaitEventCustomHashByName,
+
+ .init_size = WAIT_EVENT_CUSTOM_HASH_INIT_SIZE,
+ .max_size = WAIT_EVENT_CUSTOM_HASH_MAX_SIZE,
+ /* key is a NULL-terminated string */
+ .hash_info.keysize = sizeof(char[NAMEDATALEN]),
+ .hash_info.entrysize = sizeof(WaitEventCustomEntryByName),
+ .hash_flags = HASH_ELEM | HASH_BLOBS,
+};

This was HASH_STRINGS originally, and it is used with plain const
char* parameters. Shouldn't it use HASH_SRINGS as before?

  size = add_size(size, ShmemRegisteredSize());

  size = add_size(size, dsm_estimate_size());
- size = add_size(size, DSMRegistryShmemSize());
+
+ size = add_size(size, ShmemRegisteredSize());

ShmemRegisteredSize is now called twice.

+    /* Initialize the lock */
+    tranche_id = LWLockNewTrancheId("my tranche name");
+    LWLockInitialize(&amp;MyShmem->lock);

Second parameter is missing for LWLockInitialize

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.