Date: Tue, 11 Nov 2025 10:36:32 -0600
From: Nathan Bossart <nathandbossart@gmail.com>
To: David Rowley <dgrowleyml@gmail.com>
Cc: Sami Imseih <samimseih@gmail.com>, Robert Haas <robertmhaas@gmail.com>,
	Jeremy Schneider <schneider@ardentperf.com>,
	pgsql-hackers@postgresql.org
Subject: Re: another autovacuum scheduling thread
Message-ID: <aRNmEPQ18qZlLowV@nathan>
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Still catching up on the latest discussion, but here's a v8 patch that
amends the DEBUG3 in relation_needs_vacanalyze() to also log the score.  I
might attempt to add some sort of brief documentation about autovacuum
prioritization next.

From skimming the latest discussion, I gather we might want to consider
re-sorting the list periodically.  Is the idea that we'll re-sort the
remaining tables in the list, or that we'll basically restart
do_autovacuum()?  If it's the latter, then we'll need to come up with some
way to decide when to stop for the current database.  Right now, we just go
through pg_class and call it a day.

-- 
nathan

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	filename=v8-0001-autovacuum-scheduling-improvements.patch

From fdb05afb9445dd00f4cad9071ef51af4f22854b0 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Fri, 10 Oct 2025 12:28:37 -0500
Subject: [PATCH v8 1/1] autovacuum scheduling improvements

---
 src/backend/postmaster/autovacuum.c | 211 +++++++++++++++++++++-------
 src/tools/pgindent/typedefs.list    |   1 +
 2 files changed, 165 insertions(+), 47 deletions(-)

diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index ed19c74bb19..326055f962f 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -310,6 +310,12 @@ static AutoVacuumShmemStruct *AutoVacuumShmem;
 static dlist_head DatabaseList = DLIST_STATIC_INIT(DatabaseList);
 static MemoryContext DatabaseListCxt = NULL;
 
+typedef struct
+{
+	Oid			oid;
+	double		score;
+} TableToProcess;
+
 /*
  * Dummy pointer to persuade Valgrind that we've not leaked the array of
  * avl_dbase structs.  Make it global to ensure the compiler doesn't
@@ -351,7 +357,8 @@ static void relation_needs_vacanalyze(Oid relid, AutoVacOpts *relopts,
 									  Form_pg_class classForm,
 									  PgStat_StatTabEntry *tabentry,
 									  int effective_multixact_freeze_max_age,
-									  bool *dovacuum, bool *doanalyze, bool *wraparound);
+									  bool *dovacuum, bool *doanalyze, bool *wraparound,
+									  double *score);
 
 static void autovacuum_do_vac_analyze(autovac_table *tab,
 									  BufferAccessStrategy bstrategy);
@@ -1889,6 +1896,15 @@ get_database_list(void)
 	return dblist;
 }
 
+static int
+TableToProcessComparator(const ListCell *a, const ListCell *b)
+{
+	TableToProcess *t1 = (TableToProcess *) lfirst(a);
+	TableToProcess *t2 = (TableToProcess *) lfirst(b);
+
+	return (t2->score < t1->score) ? -1 : (t2->score > t1->score) ? 1 : 0;
+}
+
 /*
  * Process a database table-by-table
  *
@@ -1902,7 +1918,7 @@ do_autovacuum(void)
 	HeapTuple	tuple;
 	TableScanDesc relScan;
 	Form_pg_database dbForm;
-	List	   *table_oids = NIL;
+	List	   *tables_to_process = NIL;
 	List	   *orphan_oids = NIL;
 	HASHCTL		ctl;
 	HTAB	   *table_toast_map;
@@ -2014,6 +2030,7 @@ do_autovacuum(void)
 		bool		dovacuum;
 		bool		doanalyze;
 		bool		wraparound;
+		double		score = 0.0;
 
 		if (classForm->relkind != RELKIND_RELATION &&
 			classForm->relkind != RELKIND_MATVIEW)
@@ -2054,11 +2071,19 @@ do_autovacuum(void)
 		/* Check if it needs vacuum or analyze */
 		relation_needs_vacanalyze(relid, relopts, classForm, tabentry,
 								  effective_multixact_freeze_max_age,
-								  &dovacuum, &doanalyze, &wraparound);
+								  &dovacuum, &doanalyze, &wraparound,
+								  &score);
 
-		/* Relations that need work are added to table_oids */
+		/* Relations that need work are added to tables_to_process */
 		if (dovacuum || doanalyze)
-			table_oids = lappend_oid(table_oids, relid);
+		{
+			TableToProcess *table = palloc(sizeof(TableToProcess));
+
+			table->oid = relid;
+			table->score = score;
+
+			tables_to_process = lappend(tables_to_process, table);
+		}
 
 		/*
 		 * Remember TOAST associations for the second pass.  Note: we must do
@@ -2114,6 +2139,7 @@ do_autovacuum(void)
 		bool		dovacuum;
 		bool		doanalyze;
 		bool		wraparound;
+		double		score = 0.0;
 
 		/*
 		 * We cannot safely process other backends' temp tables, so skip 'em.
@@ -2146,11 +2172,19 @@ do_autovacuum(void)
 
 		relation_needs_vacanalyze(relid, relopts, classForm, tabentry,
 								  effective_multixact_freeze_max_age,
-								  &dovacuum, &doanalyze, &wraparound);
+								  &dovacuum, &doanalyze, &wraparound,
+								  &score);
 
 		/* ignore analyze for toast tables */
 		if (dovacuum)
-			table_oids = lappend_oid(table_oids, relid);
+		{
+			TableToProcess *table = palloc(sizeof(TableToProcess));
+
+			table->oid = relid;
+			table->score = score;
+
+			tables_to_process = lappend(tables_to_process, table);
+		}
 
 		/* Release stuff to avoid leakage */
 		if (free_relopts)
@@ -2274,6 +2308,8 @@ do_autovacuum(void)
 		MemoryContextSwitchTo(AutovacMemCxt);
 	}
 
+	list_sort(tables_to_process, TableToProcessComparator);
+
 	/*
 	 * Optionally, create a buffer access strategy object for VACUUM to use.
 	 * We use the same BufferAccessStrategy object for all tables VACUUMed by
@@ -2302,9 +2338,9 @@ do_autovacuum(void)
 	/*
 	 * Perform operations on collected tables.
 	 */
-	foreach(cell, table_oids)
+	foreach_ptr(TableToProcess, table, tables_to_process)
 	{
-		Oid			relid = lfirst_oid(cell);
+		Oid			relid = table->oid;
 		HeapTuple	classTup;
 		autovac_table *tab;
 		bool		isshared;
@@ -2535,7 +2571,7 @@ deleted:
 		pg_atomic_test_set_flag(&MyWorkerInfo->wi_dobalance);
 	}
 
-	list_free(table_oids);
+	list_free_deep(tables_to_process);
 
 	/*
 	 * Perform additional work items, as requested by backends.
@@ -2937,6 +2973,7 @@ recheck_relation_needs_vacanalyze(Oid relid,
 								  bool *wraparound)
 {
 	PgStat_StatTabEntry *tabentry;
+	double		score;
 
 	/* fetch the pgstat table entry */
 	tabentry = pgstat_fetch_stat_tabentry_ext(classForm->relisshared,
@@ -2944,15 +2981,12 @@ recheck_relation_needs_vacanalyze(Oid relid,
 
 	relation_needs_vacanalyze(relid, avopts, classForm, tabentry,
 							  effective_multixact_freeze_max_age,
-							  dovacuum, doanalyze, wraparound);
+							  dovacuum, doanalyze, wraparound,
+							  &score);
 
 	/* Release tabentry to avoid leakage */
 	if (tabentry)
 		pfree(tabentry);
-
-	/* ignore ANALYZE for toast tables */
-	if (classForm->relkind == RELKIND_TOASTVALUE)
-		*doanalyze = false;
 }
 
 /*
@@ -2993,6 +3027,32 @@ recheck_relation_needs_vacanalyze(Oid relid,
  * autovacuum_vacuum_threshold GUC variable.  Similarly, a vac_scale_factor
  * value < 0 is substituted with the value of
  * autovacuum_vacuum_scale_factor GUC variable.  Ditto for analyze.
+ *
+ * This function also returns a score that can be used to sort the list of
+ * tables to process.  The idea is to have autovacuum prioritize tables that
+ * are furthest beyond their thresholds (e.g., a table nearing transaction ID
+ * wraparound should be vacuumed first).  This prioritization scheme is
+ * certainly far from perfect; there are simply too many possibilities for any
+ * scoring technique to work across all workloads, and the situation might
+ * change significantly between the time we calculate the score and the time
+ * that autovacuum gets to processing it.  However, we have attempted to
+ * develop something that is expected to work for a large portion of workloads
+ * with reasonable parameter settings.
+ *
+ * The score is calculated as the maximum of the ratios of each of the table's
+ * relevant values to its threshold.  For example, if the number of inserted
+ * tuples is 100, and the insert threshold for the table is 80, the insert
+ * score is 1.25.  If all other scores are below that value, the returned score
+ * will be 1.25.  The other criteria considered for the score are the table
+ * ages (both relfrozenxid and relminmxid) compared to the corresponding
+ * freeze-max-age setting, the number of updated/deleted tuples compared to the
+ * vacuum threshold, and the number of inserted/updated/deleted tuples compared
+ * to the analyze threshold.
+ *
+ * One exception to the previous paragraph is for tables nearing wraparound,
+ * i.e., those that have surpassed the effective failsafe ages.  In that case,
+ * the relfrozen/relminmxid-based score is scaled aggressively so that the
+ * table has a decent chance of sorting to the top of the list.
  */
 static void
 relation_needs_vacanalyze(Oid relid,
@@ -3003,7 +3063,8 @@ relation_needs_vacanalyze(Oid relid,
  /* output params below */
 						  bool *dovacuum,
 						  bool *doanalyze,
-						  bool *wraparound)
+						  bool *wraparound,
+						  double *score)
 {
 	bool		force_vacuum;
 	bool		av_enabled;
@@ -3032,11 +3093,16 @@ relation_needs_vacanalyze(Oid relid,
 	int			multixact_freeze_max_age;
 	TransactionId xidForceLimit;
 	TransactionId relfrozenxid;
+	TransactionId relminmxid;
 	MultiXactId multiForceLimit;
 
 	Assert(classForm != NULL);
 	Assert(OidIsValid(relid));
 
+	/* initialize variables that aren't guaranteed to be set below */
+	*score = 0.0;
+	*doanalyze = false;
+
 	/*
 	 * Determine vacuum/analyze equation parameters.  We have two possible
 	 * sources: the passed reloptions (which could be a main table or a toast
@@ -3084,32 +3150,73 @@ relation_needs_vacanalyze(Oid relid,
 
 	av_enabled = (relopts ? relopts->enabled : true);
 
+	relfrozenxid = classForm->relfrozenxid;
+	relminmxid = classForm->relminmxid;
+
 	/* Force vacuum if table is at risk of wraparound */
 	xidForceLimit = recentXid - freeze_max_age;
 	if (xidForceLimit < FirstNormalTransactionId)
 		xidForceLimit -= FirstNormalTransactionId;
-	relfrozenxid = classForm->relfrozenxid;
 	force_vacuum = (TransactionIdIsNormal(relfrozenxid) &&
 					TransactionIdPrecedes(relfrozenxid, xidForceLimit));
 	if (!force_vacuum)
 	{
-		MultiXactId relminmxid = classForm->relminmxid;
-
 		multiForceLimit = recentMulti - multixact_freeze_max_age;
 		if (multiForceLimit < FirstMultiXactId)
 			multiForceLimit -= FirstMultiXactId;
 		force_vacuum = MultiXactIdIsValid(relminmxid) &&
 			MultiXactIdPrecedes(relminmxid, multiForceLimit);
 	}
-	*wraparound = force_vacuum;
+	*wraparound = *dovacuum = force_vacuum;
+
+	/* Update the score. */
+	if (force_vacuum)
+	{
+		Oid			xid_age;
+		Oid			mxid_age;
+		double		xid_score;
+		double		mxid_score;
+		int			effective_xid_failsafe_age;
+		int			effective_mxid_failsafe_age;
+
+		/*
+		 * To calculate the (M)XID age portion of the score, divide the age by
+		 * its respective *_freeze_max_age parameter.
+		 */
+		xid_age = TransactionIdIsNormal(relfrozenxid) ? recentXid - relfrozenxid : 0;
+		mxid_age = MultiXactIdIsValid(relminmxid) ? recentMulti - relminmxid : 0;
+
+		xid_score = (double) xid_age / freeze_max_age;
+		mxid_score = (double) mxid_age / multixact_freeze_max_age;
+
+		/*
+		 * To ensure tables are given increased priority once they begin
+		 * approaching wraparound, we scale the score aggressively if the ages
+		 * surpass vacuum_failsafe_age or vacuum_multixact_failsafe_age.
+		 *
+		 * As in vacuum_xid_failsafe_check(), the effective failsafe age is no
+		 * less than 105% the value of the respective *_freeze_max_age
+		 * parameter.  Note that per-table settings could result in a low
+		 * score even if the table surpasses the failsafe settings.  However,
+		 * this is a strange enough corner case that we don't bother trying to
+		 * handle it.
+		 */
+		effective_xid_failsafe_age = Max(vacuum_failsafe_age,
+										 autovacuum_freeze_max_age * 1.05);
+		effective_mxid_failsafe_age = Max(vacuum_multixact_failsafe_age,
+										  autovacuum_multixact_freeze_max_age * 1.05);
+
+		if (xid_age >= effective_xid_failsafe_age)
+			xid_score = pow(xid_score, Max(1.0, (double) xid_age / 100000000));
+		if (mxid_age >= effective_mxid_failsafe_age)
+			mxid_score = pow(mxid_score, Max(1.0, (double) mxid_age / 100000000));
+
+		*score = Max(xid_score, mxid_score);
+	}
 
 	/* User disabled it in pg_class.reloptions?  (But ignore if at risk) */
 	if (!av_enabled && !force_vacuum)
-	{
-		*doanalyze = false;
-		*dovacuum = false;
 		return;
-	}
 
 	/*
 	 * If we found stats for the table, and autovacuum is currently enabled,
@@ -3163,34 +3270,44 @@ relation_needs_vacanalyze(Oid relid,
 		 * reset, because if that happens, the last vacuum and analyze counts
 		 * will be reset too.
 		 */
+
+		/*
+		 * Determine if this table needs vacuum, and update the score if it
+		 * does.
+		 */
+		if (vactuples > vacthresh)
+		{
+			*dovacuum = true;
+			*score = Max(*score, (double) vactuples / Max(vacthresh, 1));
+		}
+
+		if (vac_ins_base_thresh >= 0 && instuples > vacinsthresh)
+		{
+			*dovacuum = true;
+			*score = Max(*score, (double) instuples / Max(vacinsthresh, 1));
+		}
+
+		/*
+		 * Determine if this table needs analyze, and update the score if it
+		 * does.  Note that we don't analyze TOAST tables and pg_statistic.
+		 */
+		if (anltuples > anlthresh &&
+			relid != StatisticRelationId &&
+			classForm->relkind != RELKIND_TOASTVALUE)
+		{
+			*doanalyze = true;
+			*score = Max(*score, (double) anltuples / Max(anlthresh, 1));
+		}
+
 		if (vac_ins_base_thresh >= 0)
-			elog(DEBUG3, "%s: vac: %.0f (threshold %.0f), ins: %.0f (threshold %.0f), anl: %.0f (threshold %.0f)",
+			elog(DEBUG3, "%s: vac: %.0f (threshold %.0f), ins: %.0f (threshold %.0f), anl: %.0f (threshold %.0f), score: %.3f",
 				 NameStr(classForm->relname),
-				 vactuples, vacthresh, instuples, vacinsthresh, anltuples, anlthresh);
+				 vactuples, vacthresh, instuples, vacinsthresh, anltuples, anlthresh, *score);
 		else
-			elog(DEBUG3, "%s: vac: %.0f (threshold %.0f), ins: (disabled), anl: %.0f (threshold %.0f)",
+			elog(DEBUG3, "%s: vac: %.0f (threshold %.0f), ins: (disabled), anl: %.0f (threshold %.0f), score %.3f",
 				 NameStr(classForm->relname),
-				 vactuples, vacthresh, anltuples, anlthresh);
-
-		/* Determine if this table needs vacuum or analyze. */
-		*dovacuum = force_vacuum || (vactuples > vacthresh) ||
-			(vac_ins_base_thresh >= 0 && instuples > vacinsthresh);
-		*doanalyze = (anltuples > anlthresh);
-	}
-	else
-	{
-		/*
-		 * Skip a table not found in stat hash, unless we have to force vacuum
-		 * for anti-wrap purposes.  If it's not acted upon, there's no need to
-		 * vacuum it.
-		 */
-		*dovacuum = force_vacuum;
-		*doanalyze = false;
+				 vactuples, vacthresh, anltuples, anlthresh, *score);
 	}
-
-	/* ANALYZE refuses to work with pg_statistic */
-	if (relid == StatisticRelationId)
-		*doanalyze = false;
 }
 
 /*
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 432509277c9..3d2b5b6925a 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -3010,6 +3010,7 @@ TableScanDesc
 TableScanDescData
 TableSpaceCacheEntry
 TableSpaceOpts
+TableToProcess
 TablespaceList
 TablespaceListCell
 TapeBlockTrailer
-- 
2.39.5 (Apple Git-154)


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