From: PG Doc comments form <noreply@postgresql.org>
To: pgsql-docs@lists.postgresql.org
Cc: sameer.kasi200x@gmail.com
Subject: Misleading description for IPC wait events in PostgreSQL documentation
Date: Thu, 04 May 2023 18:00:22 +0000
Message-ID: <168322322228.395345.6400904841473089995@wrigleys.postgresql.org> (raw)
The following documentation comment has been logged on the website:
Page: https://www.postgresql.org/docs/15/monitoring-stats.html
Description:
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 was looking 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 feature.
**My understanding is**
In order to avoid `XactSLRULock` being passed between backends, backends
waiting for it will add themselves to the queue [3]. The first backend in
the queue (also the leader) will be the only one to acquire `XactSLRULock`
and update the XID status for all those pids which are in the queue. This
wait event `IPC:XactGroupUpdate` is observed in other backened processes in
queue who are waiting for the group leader to update the XID status.
I recommend making a change to PostgreSQL docs to clearly indicate what this
wait event means:
> When concurrent processes are trying to update transaction status in the
same page of commit logs, a group leader is assigned to perform the update
and other processes wanting to perform the update wait on this IPC wait
event.
OR
> When multiple backend processes wait on a group leader to update their
transaction status, so that PostgreSQL can avoid passing around the LwLock
`XactSLRULock` between multiple backend processes.
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").
Thoughts?
[1]
https://www.postgresql.org/docs/current/monitoring-stats.html#WAIT-EVENT-IPC-TABLE
[2]
https://github.com/postgres/postgres/blob/master/src/backend/access/transam/clog.c#L481
[3]
https://github.com/postgres/postgres/blob/master/src/backend/access/transam/clog.c#L399
Thanks,
Sameer
DB Specialist,
Amazon Web Services
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Subject: Re: Misleading description for IPC wait events in PostgreSQL documentation
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