diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 4daa5ae9b0..377d0842bd 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -977,10 +977,7 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
 	 * Note these are pretty much the same protections in GetNewTransactionId.
 	 *----------
 	 */
-	if (!MultiXactIdPrecedes(result, MultiXactState->multiVacLimit) ||
-		!MultiXactState->oldestOffsetKnown ||
-		(MultiXactState->nextOffset - MultiXactState->oldestOffset
-		 > MULTIXACT_MEMBER_SAFE_THRESHOLD))
+	if (!MultiXactIdPrecedes(result, MultiXactState->multiVacLimit))
 	{
 		/*
 		 * For safety's sake, we release MultiXactGenLock while sending
@@ -996,19 +993,17 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
 
 		LWLockRelease(MultiXactGenLock);
 
-		/*
-		 * To avoid swamping the postmaster with signals, we issue the autovac
-		 * request only once per 64K multis generated.  This still gives
-		 * plenty of chances before we get into real trouble.
-		 */
-		if (IsUnderPostmaster && (result % 65536) == 0)
-			SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
-
 		if (IsUnderPostmaster &&
 			!MultiXactIdPrecedes(result, multiStopLimit))
 		{
 			char	   *oldest_datname = get_database_name(oldest_datoid);
 
+			/*
+			 * Immediately kick autovacuum into action as we're already
+			 * in ERROR territory.
+			 */
+			SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
+
 			/* complain even if that DB has disappeared */
 			if (oldest_datname)
 				ereport(ERROR,
@@ -1025,7 +1020,16 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
 				 errhint("Execute a database-wide VACUUM in that database.\n"
 						 "You might also need to commit or roll back old prepared transactions.")));
 		}
-		else if (!MultiXactIdPrecedes(result, multiWarnLimit))
+
+		/*
+		 * To avoid swamping the postmaster with signals, we issue the autovac
+		 * request only once per 64K multis generated.  This still gives
+		 * plenty of chances before we get into real trouble.
+		 */
+		if (IsUnderPostmaster && (result % 65536) == 0)
+			SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
+
+		if (!MultiXactIdPrecedes(result, multiWarnLimit))
 		{
 			char	   *oldest_datname = get_database_name(oldest_datoid);
 
@@ -1096,6 +1100,10 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
 	if (MultiXactState->offsetStopLimitKnown &&
 		MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit, nextOffset,
 								 nmembers))
+	{
+		/* see comment in the corresponding offsets wraparound case */
+		SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
+
 		ereport(ERROR,
 				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
 				 errmsg("multixact \"members\" limit exceeded"),
@@ -1106,10 +1114,33 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
 						   MultiXactState->offsetStopLimit - nextOffset - 1),
 				 errhint("Execute a database-wide VACUUM in database with OID %u with reduced vacuum_multixact_freeze_min_age and vacuum_multixact_freeze_table_age settings.",
 						 MultiXactState->oldestMultiXactDB)));
-	else if (MultiXactState->offsetStopLimitKnown &&
-			 MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,
-									  nextOffset,
-									  nmembers + MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT * OFFSET_WARN_SEGMENTS))
+	}
+
+	/*
+	 * Check whether we should kick autovacuum into action, to prevent members
+	 * wraparound. NB we use a much larger window to trigger autovacuum than
+	 * just the warning limit. The warning is just a measure of last resort -
+	 * this is in line with GetNewTransactionId's behaviour.
+	 */
+	if (!MultiXactState->oldestOffsetKnown ||
+		(MultiXactState->nextOffset - MultiXactState->oldestOffset
+		 > MULTIXACT_MEMBER_SAFE_THRESHOLD))
+	{
+		/*
+		 * To avoid swamping the postmaster with signals, we issue the autovac
+		 * request only when crossing a segment boundary. With default
+		 * compilation settings that's rougly after 50k members.  This still
+		 * gives plenty of chances before we get into real trouble.
+		 */
+		if ((MXOffsetToMemberPage(nextOffset) / SLRU_PAGES_PER_SEGMENT) !=
+			(MXOffsetToMemberPage(nextOffset + nmembers) / SLRU_PAGES_PER_SEGMENT))
+			SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
+	}
+
+	if (MultiXactState->offsetStopLimitKnown &&
+		MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,
+								 nextOffset,
+								 nmembers + MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT * OFFSET_WARN_SEGMENTS))
 		ereport(WARNING,
 				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
 				 errmsg("database with OID %u must be vacuumed before %d more multixact members are used",