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, 65 insertions, 17 deletions

diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 3f826532b88..0ab921a169b 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -115,6 +115,7 @@
  * GUC parameters
  */
 bool		autovacuum_start_daemon = false;
+int			autovacuum_worker_slots;
 int			autovacuum_max_workers;
 int			autovacuum_work_mem = -1;
 int			autovacuum_naptime;
@@ -210,7 +211,7 @@ typedef struct autovac_table
 /*-------------
  * This struct holds information about a single worker's whereabouts.  We keep
  * an array of these in shared memory, sized according to
- * autovacuum_max_workers.
+ * autovacuum_worker_slots.
  *
  * wi_links		entry into free list or running list
  * wi_dboid		OID of the database this worker is supposed to work on
@@ -291,7 +292,7 @@ typedef struct
 {
 	sig_atomic_t av_signal[AutoVacNumSignals];
 	pid_t		av_launcherpid;
-	dlist_head	av_freeWorkers;
+	dclist_head av_freeWorkers;
 	dlist_head	av_runningWorkers;
 	WorkerInfo	av_startingWorker;
 	AutoVacuumWorkItem av_workItems[NUM_WORKITEMS];
@@ -349,6 +350,8 @@ static void autovac_report_activity(autovac_table *tab);
 static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
 									const char *nspname, const char *relname);
 static void avl_sigusr2_handler(SIGNAL_ARGS);
+static bool av_worker_available(void);
+static void check_av_worker_gucs(void);
 
 
 
@@ -577,8 +580,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		 * wakening conditions.
 		 */
 
-		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
-								 false, &nap);
+		launcher_determine_sleep(av_worker_available(), false, &nap);
 
 		/*
 		 * Wait until naptime expires or we get some type of signal (all the
@@ -638,7 +640,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 		current_time = GetCurrentTimestamp();
 		LWLockAcquire(AutovacuumLock, LW_SHARED);
 
-		can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);
+		can_launch = av_worker_available();
 
 		if (AutoVacuumShmem->av_startingWorker != NULL)
 		{
@@ -681,8 +683,8 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 					worker->wi_sharedrel = false;
 					worker->wi_proc = NULL;
 					worker->wi_launchtime = 0;
-					dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-									&worker->wi_links);
+					dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+									 &worker->wi_links);
 					AutoVacuumShmem->av_startingWorker = NULL;
 					ereport(WARNING,
 							errmsg("autovacuum worker took too long to start; canceled"));
@@ -747,6 +749,8 @@ HandleAutoVacLauncherInterrupts(void)
 
 	if (ConfigReloadPending)
 	{
+		int			autovacuum_max_workers_prev = autovacuum_max_workers;
+
 		ConfigReloadPending = false;
 		ProcessConfigFile(PGC_SIGHUP);
 
@@ -754,6 +758,14 @@ HandleAutoVacLauncherInterrupts(void)
 		if (!AutoVacuumingActive())
 			AutoVacLauncherShutdown();
 
+		/*
+		 * If autovacuum_max_workers changed, emit a WARNING if
+		 * autovacuum_worker_slots < autovacuum_max_workers.  If it didn't
+		 * change, skip this to avoid too many repeated log messages.
+		 */
+		if (autovacuum_max_workers_prev != autovacuum_max_workers)
+			check_av_worker_gucs();
+
 		/* rebuild the list in case the naptime changed */
 		rebuild_database_list(InvalidOid);
 	}
@@ -1089,7 +1101,7 @@ do_start_worker(void)
 
 	/* return quickly when there are no free workers */
 	LWLockAcquire(AutovacuumLock, LW_SHARED);
-	if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
+	if (!av_worker_available())
 	{
 		LWLockRelease(AutovacuumLock);
 		return InvalidOid;
@@ -1242,7 +1254,7 @@ do_start_worker(void)
 		 * Get a worker entry from the freelist.  We checked above, so there
 		 * really should be a free slot.
 		 */
-		wptr = dlist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
+		wptr = dclist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);
 
 		worker = dlist_container(WorkerInfoData, wi_links, wptr);
 		worker->wi_dboid = avdb->adw_datid;
@@ -1615,8 +1627,8 @@ FreeWorkerInfo(int code, Datum arg)
 		MyWorkerInfo->wi_proc = NULL;
 		MyWorkerInfo->wi_launchtime = 0;
 		pg_atomic_clear_flag(&MyWorkerInfo->wi_dobalance);
-		dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-						&MyWorkerInfo->wi_links);
+		dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+						 &MyWorkerInfo->wi_links);
 		/* not mine anymore */
 		MyWorkerInfo = NULL;
 
@@ -3248,10 +3260,14 @@ AutoVacuumRequestWork(AutoVacuumWorkItemType type, Oid relationId,
 void
 autovac_init(void)
 {
-	if (autovacuum_start_daemon && !pgstat_track_counts)
+	if (!autovacuum_start_daemon)
+		return;
+	else if (!pgstat_track_counts)
 		ereport(WARNING,
 				(errmsg("autovacuum not started because of misconfiguration"),
 				 errhint("Enable the \"track_counts\" option.")));
+	else
+		check_av_worker_gucs();
 }
 
 /*
@@ -3268,7 +3284,7 @@ AutoVacuumShmemSize(void)
 	 */
 	size = sizeof(AutoVacuumShmemStruct);
 	size = MAXALIGN(size);
-	size = add_size(size, mul_size(autovacuum_max_workers,
+	size = add_size(size, mul_size(autovacuum_worker_slots,
 								   sizeof(WorkerInfoData)));
 	return size;
 }
@@ -3295,7 +3311,7 @@ AutoVacuumShmemInit(void)
 		Assert(!found);
 
 		AutoVacuumShmem->av_launcherpid = 0;
-		dlist_init(&AutoVacuumShmem->av_freeWorkers);
+		dclist_init(&AutoVacuumShmem->av_freeWorkers);
 		dlist_init(&AutoVacuumShmem->av_runningWorkers);
 		AutoVacuumShmem->av_startingWorker = NULL;
 		memset(AutoVacuumShmem->av_workItems, 0,
@@ -3305,10 +3321,10 @@ AutoVacuumShmemInit(void)
 							   MAXALIGN(sizeof(AutoVacuumShmemStruct)));
 
 		/* initialize the WorkerInfo free list */
-		for (i = 0; i < autovacuum_max_workers; i++)
+		for (i = 0; i < autovacuum_worker_slots; i++)
 		{
-			dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
-							&worker[i].wi_links);
+			dclist_push_head(&AutoVacuumShmem->av_freeWorkers,
+							 &worker[i].wi_links);
 			pg_atomic_init_flag(&worker[i].wi_dobalance);
 		}
 
@@ -3344,3 +3360,35 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 
 	return true;
 }
+
+/*
+ * Returns whether there is a free autovacuum worker slot available.
+ */
+static bool
+av_worker_available(void)
+{
+	int			free_slots;
+	int			reserved_slots;
+
+	free_slots = dclist_count(&AutoVacuumShmem->av_freeWorkers);
+
+	reserved_slots = autovacuum_worker_slots - autovacuum_max_workers;
+	reserved_slots = Max(0, reserved_slots);
+
+	return free_slots > reserved_slots;
+}
+
+/*
+ * Emits a WARNING if autovacuum_worker_slots < autovacuum_max_workers.
+ */
+static void
+check_av_worker_gucs(void)
+{
+	if (autovacuum_worker_slots < autovacuum_max_workers)
+		ereport(WARNING,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("\"autovacuum_max_workers\" (%d) should be less than or equal to \"autovacuum_worker_slots\" (%d)",
+						autovacuum_max_workers, autovacuum_worker_slots),
+				 errdetail("The server will only start up to \"autovacuum_worker_slots\" (%d) autovacuum workers at a given time.",
+						   autovacuum_worker_slots)));
+}