Allow changing autovacuum_max_workers without restarting.

This commit introduces a new parameter named
autovacuum_worker_slots that controls how many autovacuum worker
slots to reserve during server startup.  Modifying this new
parameter's value does require a server restart, but it should
typically be set to the upper bound of what you might realistically
need to set autovacuum_max_workers.  With that new parameter in
place, autovacuum_max_workers can now be changed with a SIGHUP
(e.g., pg_ctl reload).

If autovacuum_max_workers is set higher than
autovacuum_worker_slots, a WARNING is emitted, and the server will
only start up to autovacuum_worker_slots workers at a given time.
If autovacuum_max_workers is set to a value less than the number of
currently-running autovacuum workers, the existing workers will
continue running, but no new workers will be started until the
number of running autovacuum workers drops below
autovacuum_max_workers.

Reviewed-by: Sami Imseih, Justin Pryzby, Robert Haas, Andres Freund, Yogesh Sharma
Discussion: https://postgr.es/m/20240410212344.GA1824549%40nathanxps13

1 files changed, 3 insertions, 3 deletions

diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index c7a972df7dd..49204f91a20 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -150,7 +150,7 @@ ProcGlobalSemas(void)
  *	  So, now we grab enough semaphores to support the desired max number
  *	  of backends immediately at initialization --- if the sysadmin has set
  *	  MaxConnections, max_worker_processes, max_wal_senders, or
- *	  autovacuum_max_workers higher than his kernel will support, he'll
+ *	  autovacuum_worker_slots higher than his kernel will support, he'll
  *	  find out sooner rather than later.
  *
  *	  Another reason for creating semaphores here is that the semaphore
@@ -284,13 +284,13 @@ InitProcGlobal(void)
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + NUM_SPECIAL_WORKER_PROCS)
+		else if (i < MaxConnections + autovacuum_worker_slots + NUM_SPECIAL_WORKER_PROCS)
 		{
 			/* PGPROC for AV or special worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + NUM_SPECIAL_WORKER_PROCS + max_worker_processes)
+		else if (i < MaxConnections + autovacuum_worker_slots + NUM_SPECIAL_WORKER_PROCS + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);