Hi, we have a Postgres 9.6 setup using replication that has recently
started seeing a lot of processes stuck in "SubtransControlLock" as a
wait_event on the read-replicas. Like this, only usually about 300-800 of

 179706 | LWLockNamed     | SubtransControlLock

 186602 | LWLockNamed     | SubtransControlLock

 186606 | LWLockNamed     | SubtransControlLock

 180947 | LWLockNamed     | SubtransControlLock

 186621 | LWLockNamed     | SubtransControlLock

The server then begins to crawl, with some queries just never finishing
until I finally shut the server down.

Searching for that particular combo of wait_event_type and wait_event only
seems to turn up the page about statistics collection, but no helpful
information on troubleshooting this lock.

Restarting the replica server clears the locks and allows us to start
working again, but it's happened twice now in 12 hours and I'm worried it
will happen again.

Does anyone have any advice on where to start looking?


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