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
