Feedback-ID: i0fe9450f:Fastmail
Date: Fri, 8 Dec 2023 16:36:52 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: pgsql-hackers@postgresql.org
Subject: Re: introduce dynamic shared memory registry
Message-ID: <ZXLHlNuTiFaVkTNt@paquier.xyz>
References: <20231205034647.GA2705267@nathanxps13>
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On Mon, Dec 04, 2023 at 09:46:47PM -0600, Nathan Bossart wrote:
> The attached 0001 introduces a "DSM registry" to solve this problem.  The
> API provides an easy way to allocate/initialize a segment or to attach to
> an existing one.  The registry itself is just a dshash table that stores
> the handles keyed by a module-specified string.  0002 adds a test for the
> registry that demonstrates basic usage.
>=20
> I don't presently have any concrete plans to use this for anything, but I
> thought it might be useful for extensions for caching, etc. and wanted to
> see whether there was any interest in the feature.

Yes, tracking that in a more central way can have many usages, so your
patch sounds like a good idea.  Note that we have one case in core
that be improved and make use of what you have here: autoprewarm.c.

The module supports the launch of dynamic workers but the library may
not be loaded with shared_preload_libraries, meaning that it can
allocate a chunk of shared memory worth a size of
AutoPrewarmSharedState without having requested it in a
shmem_request_hook.  AutoPrewarmSharedState could be moved to a DSM
and tracked with the shared hash table introduced by the patch instead
of acquiring AddinShmemInitLock while eating the plate of other
facilities that asked for a chunk of shmem, leaving any conflict
handling to dsm_registry_table.

+dsm_registry_init_or_attach(const char *key, void **ptr, size_t size,
+                           void (*init_callback) (void *ptr))

This is shaped around dshash_find_or_insert(), but it looks like you'd
want an equivalent for dshash_find(), as well.
--
Michael

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