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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: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, chaturvedipalak1911@gmail.com
Date: 2026-04-05T14:16:51Z
Lists: pgsql-hackers 
On Sun, Apr 5, 2026 at 4:50 PM Ashutosh Bapat
<ashutosh.bapat.oss@gmail.com> wrote:
>

> 3. The test fails one one machine because RssShmem is consistently 8MB
> higher than the allocated_size in all cases. I guess it is because of
> huge page setting. Adding huge_pages = off to the test configuration.
> I think the test can not rely on huge pages anyway since
> allocated_size isn't aligned to huge page size.

Turning huge_pages = off didn't help. The test actually creates a
resizable shared memory structure which is 100s of MBs and adjusts
GUCs so that very minimum shared memory is allocated. This way the
resizable structure dominates the shared memory segment. Any small
variations in RssShmem because of parts of shared memory not accessed
by a backend can be ignored. Then it expects that the RssShmem of a
backend <= sum(allocated_size) from pg_shmem_allocations; usually
sum(allocated_size) - RssShmem ~= 2MB. That's not accurate but it's
the closest I can get to make sure that we do not over allocate memory
for resizable shared structures. There is something on
https://cirrus-ci.com/task/5501660157444096 which is mapping shared
memory worth 10MB, other than the main shared memory segment,
consistently in all the backends. Because of that RssShmem -
sum(allocated_size) is consistently ~= 8MB. I am not able to figure
out where that 10MB is coming from. If we could know that, we could
either disable the test on that machine or disable that allocation. On
all other CFBot VMs, the test is passing, including the platforms
where the feature is not supported.

-- 
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.