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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: Robert Haas <robertmhaas@gmail.com>
Cc: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, chaturvedipalak1911@gmail.com
Date: 2026-03-13T11:41:33Z
Lists: pgsql-hackers

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On 12/03/2026 22:05, Robert Haas wrote:
> On Thu, Mar 12, 2026 at 3:21 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>>>> I'm currently leaning towards _PG_init(), except for allocations that
>>>> depend on MaxBackends. For those, you can install a shmem_request_hook
>>>> that sets the size in the descriptor. In other words, you can leave the
>>>> 'size' as empty in _PG_init(), but set it later in the shmem_request_hook.
>>>
>>> Why can't you just do the whole thing later?
>>
>> shmem_request_hook won't work in EXEC_BACKEND mode, because in
>> EXEC_BACKEND mode, ShmemRegisterStruct() also needs to be called at
>> backend startup.
>>
>> One of my design goals is to avoid EXEC_BACKEND specific steps so that
>> if you write your extension oblivious to EXEC_BACKEND mode, it will
>> still usually work with EXEC_BACKEND. For example, if it was necessary
>> to call a separate AttachShmem() function for every shmem struct in
>> EXEC_BACKEND mode, but which was not needed on Unix, that would be bad.
> 
> That's *definitely* a good goal. A less important but still valuable
> goal is to maximize the notational simplicity of the mechanism. Your
> callback idea is elegant in theory but in practice it seems like it
> might make it harder for people to get started quickly on a new
> module, and having to create the object in one place and then fill in
> the size in another sort of has the same problem. I don't really know
> what to do about that, but it's something to think about. The
> complexity of getting the details right is annoyingly high in this
> area.

Yeah. IMHO the existing shmem_request/startup_hook mechanism is pretty 
awkward too, and in most cases, the new mechanism is more convenient. It 
might be slightly less convenient for some things, but mostly it's 
better. Would you agree with that, or do you actually like the old hooks 
and ShmemInitStruct() better?

One such wrinkle with ShmemRegisterStruct() in the patch now is that 
it's harder to do initialization that touches multiple structs or hash 
tables. Currently the callbacks are called in the same order that the 
structs are registered, so you can do all the initialization in the last 
struct's callback. The single pair of shmem_request/startup_hooks per 
module was more clear in that aspect. Fortunately, that kind of 
cross-struct dependencies are pretty rare. So I think it's fine. (The 
order that the callbacks are called needs be documented explicitly though).

If we want to improve on that, one idea would be to introduce a 
ShmemRegisterCallbacks() function to register callbacks that are not 
tied to any particular struct and are called after all the per-struct 
callbacks.

>>> Yeah, I think RequestNamedLWLockTranche() might be fine if you just
>>> need LWLocks, but if you need a bunch of resources, putting them all
>>> into the same chunk of memory seems cleaner.
>>
>> Agreed. Then again, how often do you need just a LWLock (or multiple
>> LWLocks)? Surely you have a struct you want to protect with the lock. I
>> guess having shmem hash table but no struct would be pretty common, though.
> 
> Yeah, we've developed an annoying number of different ways to do this
> stuff. I don't entirely know how to fix that.

Here's a new version that doubles down on the 
LWLockNewTrancheId+LWLockInitialize method, by changing the example in 
the docs, and contrib/pg_stat_statements, to use that method. 
RequestNamedLWLockTranche() still works, there are no changes to it, 
it's just not as convenient to use with ShmemRegisterStruct(). This has 
the advantage that we don't introduce yet another way of allocating LWLocks.

P.S. Thanks for chiming in on this. It's pretty subjective how natural a 
new API like this feels like, so opinions are very welcome.

- 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.