Re: Adding clarification to description of IPC wait events XactGroupUpdate and ProcArrayGroupUpdate

On Thu, Jul 25, 2024 at 11:13:39AM +0800, SAMEER KUMAR wrote:
> While preparing for my presentation on PostgreSQL Wait Events at
> PgConf India, I was trying to understand *IPC:XactGroupUpdate* in more
> detail. PostgreSQL documentation [1] mentions:
>
>> A process is waiting for the group leader to update the transaction status at the end of a _parallel operation_.
>
> I looked at `TransactionGroupUpdateXidStatus` in PostgreSQL code (`clog.c`)
> Line `481` [2] sets this wait event.
>
> And after reading the code, my understanding is - It does not
> necessarily need to be a "_parallel operation_". Or maybe I am just
> misinterpreting "parallel operation" in this context. But it is
> possible for other users to confuse it with the parallel query (and
> parallel workers) feature.
>
> [...]
>
> We can add more clarity on what this wait event means. A similar
> change should be done for `ProcArrayGroupUpdate` to indicate that the
> wait event is a result of concurrent backend processes trying to clear
> the transaction id (instead of saying "parallel operation").

Both of these wait events had descriptions similar to what you are
proposing when they were first introduced (commits d4116a7 and baaf272),
but they were changed to the current wording by commit 3048898. I skimmed
through the thread for the latter commit [0] but didn't see anything that
explained why it was changed.

[0] https://postgr.es/m/21247.1589296570%40sss.pgh.pa.us

--
nathan