Re: Support to define custom wait events for extensions

On Tue, Aug 08, 2023 at 09:39:02AM +0900, Masahiro Ikeda wrote:
> I am thinking a bit that we also need another hash where the key
> is a custom string. For extensions that have no dependencies
> with shared_preload_libraries, I think we need to avoid that
> WaitEventExtensionNew() is called repeatedly and a new eventId
> is issued each time.
> 
> So, is it better to have another hash where the key is
> a custom string and uniqueness is identified by it to determine
> if a new eventId should be issued?

Yeah, I was also considering if something like that is really
necessary, but I cannot stop worrying about adding more contention to
the hash table lookup each time an extention needs to retrieve an
event ID to use for WaitLatch() or such.  The results of the hash
table lookups could be cached in each backend, still it creates an
extra cost when combined with queries running in parallel on
pg_stat_activity that do the opposite lookup event_id -> event_name.
My suggestion adds more load to AddinShmemInitLock instead.

Hence, I was just thinking about relying on AddinShmemInitLock to
insert new entries in the hash table, only if its shmem state is not
found when calling ShmemInitStruct().  Or perhaps it is just OK to not
care about the impact event_name -> event_id lookup for fresh
connections, and just bite the bullet with two lookup keys instead of
relying on AddinShmemInitLock for the addition of new entries in the
hash table?  Hmm, perhaps you're right with your approach here, at the
end.
--
Michael