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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: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, chaturvedipalak1911@gmail.com
Date: 2026-04-05T15:50:11Z
Lists: pgsql-hackers 
On 05/04/2026 02:17, Matthias van de Meent wrote:
>> +    pgss->extent = 0;
>> +    pgss->n_writers = 0;
>> +    pgss->gc_count = 0;
>> +    pgss->stats.dealloc = 0;
> 
> Shmem is said to be zero-initialized, should we remove the manual
> zero-initialization?

Yeah, perhaps. We already had initialization like this in many places, 
while others relied on the implicit initialization. Some places even do 
just this:

void
LogicalDecodingCtlShmemInit(void)
{
	bool		found;

	LogicalDecodingCtl = ShmemInitStruct("Logical decoding control",
										 LogicalDecodingCtlShmemSize(),
										 &found);

	if (!found)
		MemSet(LogicalDecodingCtl, 0, LogicalDecodingCtlShmemSize());
}

I think there are two directions we could go here:

1. Document that the memory is zeroed, and you can rely on it. Remove 
silly initializations like that in LogicalDecodingCtlShmemInit(). In 
other places the explicitly zero-initialization might have documentation 
value though.

2. Require the init functions to explicitly zero the memory. Document it 
and add valgrind checks.

I'm inclined to go with 1. But in the name of avoiding scope creep, not 
as part of these patches.

- 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.