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

On Fri, Apr 3, 2026 at 3:40 AM Matthias van de Meent
<boekewurm+postgres(at)gmail(dot)com> wrote:
>
> On Wed, 1 Apr 2026 at 20:17, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> >
>
> While I do think it's an improvement over the current APIs, the
> improvement seems to be mostly concentrated in the RequestStruct/Hash
> department, with only marginal improvements in RegisterShmemCallbacks.
> I feel like it's missing the important part: I'd like
> direct-from-_PG_init() ShmemRequestStruct/Hash calls. If
> ShmemRequestStruct/Hash had a size callback as alternative to the size
> field (which would then be called after preload_libraries finishes)
> then that would be sufficient for most shmem allocations, and it'd
> simplify shmem management for most subsystems.
> We'd still need the shmem lifecycle hooks/RegisterShmemCallbacks to
> allow conditionally allocated shmem areas (e.g. those used in aio),
> but I think that, in general, we shouldn't need a separate callback
> function just to get started registering shmem structures.
>
> I also noticed that ShmemCallbacks.%_arg are generally undocumented,
> and I couldn't find any users in core (at the end of the patchset)
> that actually use the argument. Could it be I missed something?
>
> I don't understand the use of ShmemStructDesc. They generally/always
> are private to request_fn(), and their fields are used exclusively
> inside the shmem mechanisms, with no reads of its fields that can't
> already be deduced from context. Why do we need that struct
> everywhere?

My resizable shared memory structure patches use it as a handle to the
structure to be resized.

--
Best Wishes,
Ashutosh Bapat