Обсуждение: Misleading description for IPC wait events in PostgreSQL documentation
Hi,
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
> 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
Sameer
DB Specialist,
Amazon Web Services