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Date: Tue, 08 Aug 2023 09:39:02 +0900
From: Masahiro Ikeda <ikedamsh@oss.nttdata.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: Andres Freund <andres@anarazel.de>, Bharath Rupireddy
 <bharath.rupireddyforpostgres@gmail.com>, Tristan Partin
 <tristan@neon.tech>, pgsql-hackers@lists.postgresql.org
Subject: Re: Support to define custom wait events for extensions
In-Reply-To: <ZNGEImKhwtgL2zlq@paquier.xyz>
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On 2023-08-08 08:54, Michael Paquier wrote:
> On Wed, Aug 02, 2023 at 06:34:15PM +0900, Masahiro Ikeda wrote:
>> On 2023-08-01 12:23, Andres Freund wrote:
>>> This is why the scheme as implemented doesn't really make sense to 
>>> me.
>>> It'd be
>>> much easier to use if we had a shared hashtable with the identifiers
>>> than
>>> what's been merged now.
>>> 
>>> In plenty of cases it's not realistic for an extension library to run 
>>> in
>>> each
>>> backend, while still needing to wait for things.
>> 
>> OK, I'll try to make a PoC patch.
> 
> Hmm.  There are a few things to take into account here:
> - WaitEventExtensionShmemInit() should gain a dshash_create(), to make
> sure that the shared table is around, and we are going to have a
> reference to it in WaitEventExtensionCounterData, saved from
> dshash_get_hash_table_handle().
> - The hash table entries could just use nextId as key to look at the
> entries, with entries added during WaitEventExtensionNew(), and use as
> contents the name of the wait event.  We are going to need a fixed
> size for these custom strings, but perhaps a hard limit of 256
> characters for each entry of the hash table is more than enough for
> most users?
> - WaitEventExtensionRegisterName() could be removed, I guess, replaced
> by a single WaitEventExtensionNew(), as of:
> uint32 WaitEventExtensionNew(const char *event_name);
> - GetWaitEventExtensionIdentifier() needs to switch to a lookup of the
> shared hash table, based on the eventId.
> 
> All that would save from the extra WaitEventExtensionRegisterName()
> needed in the backends to keep a track of the names, indeed.

Thank you for pointing the direction of implementation.

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?

Regards,
-- 
Masahiro Ikeda
NTT DATA CORPORATION