Re: introduce dynamic shared memory registry

On Thu, Jan 11, 2024 at 10:42 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>
> >> 3. IIUC, this feature eventually makes both shmem_request_hook and
> >> shmem_startup_hook pointless, no? Or put another way, what's the
> >> significance of shmem request and startup hooks in lieu of this new
> >> feature? I think it's quite possible to get rid of the shmem request
> >> and startup hooks (of course, not now but at some point in future to
> >> not break the external modules), because all the external modules can
> >> allocate and initialize the same shared memory via
> >> dsm_registry_init_or_attach and its init_callback. All the external
> >> modules will then need to call dsm_registry_init_or_attach in their
> >> _PG_init callbacks and/or in their bg worker's main functions in case
> >> the modules intend to start up bg workers. Am I right?
> >
> > Well, modules might need to do a number of other things (e.g., adding
> > hooks) that can presently only be done when preloaded, in which case I
> > doubt there's much benefit from switching to the DSM registry. I don't
> > really intend for it to replace the existing request/startup hooks, but
> > you're probably right that most, if not all, could use the registry
> > instead. IMHO this is well beyond the scope of this thread, though.
>
> Even if that's not in the scope of this thread, just removing these
> hooks would break a lot of out-of-core things, and they still have a
> lot of value when extensions expect to always be loaded with shared.
> They don't cost in maintenance at this stage.

Adding some notes in the docs on when exactly one needs to use shmem
hooks and the named DSM segments can help greatly.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com