Re: introduce dynamic shared memory registry

On Tue, Dec 19, 2023 at 10:19:11AM -0600, Nathan Bossart wrote:
> On Fri, Dec 08, 2023 at 04:36:52PM +0900, Michael Paquier wrote:
>> 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.
> 
> I'll add a patch for autoprewarm.c.  Even if we don't proceed with that
> change, it'll be a good demonstration.

Cool, thanks.  It could just be a separate change on top of the main
one.

> > +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.
> 
> What is the use-case for only verifying the existence of a segment?

One case I was thinking about is parallel aggregates that can define
combining and serial/deserial functions, where some of the operations
could happen in shared memory, requiring a DSM, and where each process
doing some aggregate combining would expect a DSM to exist before
making use of it.  So a registry wrapper for dshash_find() could be
used as a way to perform sanity checks with what's stored in the
registry.
--
Michael