Separate multixact freezing parameters from xid's

Previously we were piggybacking on transaction ID parameters to freeze
multixacts; but since there isn't necessarily any relationship between
rates of Xid and multixact consumption, this turns out not to be a good
idea.

Therefore, we now have multixact-specific freezing parameters:

vacuum_multixact_freeze_min_age: when to remove multis as we come across
them in vacuum (default to 5 million, i.e. early in comparison to Xid's
default of 50 million)

vacuum_multixact_freeze_table_age: when to force whole-table scans
instead of scanning only the pages marked as not all visible in
visibility map (default to 150 million, same as for Xids).  Whichever of
both which reaches the 150 million mark earlier will cause a whole-table
scan.

autovacuum_multixact_freeze_max_age: when for cause emergency,
uninterruptible whole-table scans (default to 400 million, double as
that for Xids).  This means there shouldn't be more frequent emergency
vacuuming than previously, unless multixacts are being used very
rapidly.

Backpatch to 9.3 where multixacts were made to persist enough to require
freezing.  To avoid an ABI break in 9.3, VacuumStmt has a couple of
fields in an unnatural place, and StdRdOptions is split in two so that
the newly added fields can go at the end.

Patch by me, reviewed by Robert Haas, with additional input from Andres
Freund and Tom Lane.

src/backend/commands/cluster.c

@@ -850,7 +850,7 @@ copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex, bool verbose,
 	 * Since we're going to rewrite the whole table anyway, there's no reason
 	 * not to be aggressive about this.
 	 */
-	vacuum_set_xid_limits(0, 0, OldHeap->rd_rel->relisshared,
+	vacuum_set_xid_limits(0, 0, 0, 0, OldHeap->rd_rel->relisshared,
 						  &OldestXmin, &FreezeXid, NULL, &MultiXactCutoff,
 						  NULL);
 

src/backend/commands/vacuum.c

@@ -55,6 +55,8 @@
  */
 int			vacuum_freeze_min_age;
 int			vacuum_freeze_table_age;
+int			vacuum_multixact_freeze_min_age;
+int			vacuum_multixact_freeze_table_age;
 
 
 /* A few variables that don't seem worth passing around as parameters */
@@ -398,6 +400,8 @@ get_rel_oids(Oid relid, const RangeVar *vacrel)
 void
 vacuum_set_xid_limits(int freeze_min_age,
 					  int freeze_table_age,
+					  int multixact_freeze_min_age,
+					  int multixact_freeze_table_age,
 					  bool sharedRel,
 					  TransactionId *oldestXmin,
 					  TransactionId *freezeLimit,
@@ -406,9 +410,11 @@ vacuum_set_xid_limits(int freeze_min_age,
 					  MultiXactId *mxactFullScanLimit)
 {
 	int			freezemin;
+	int			mxid_freezemin;
 	TransactionId limit;
 	TransactionId safeLimit;
-	MultiXactId	mxactLimit;
+	MultiXactId mxactLimit;
+	MultiXactId safeMxactLimit;
 
 	/*
 	 * We can always ignore processes running lazy vacuum.	This is because we
@@ -462,13 +468,36 @@ vacuum_set_xid_limits(int freeze_min_age,
 	*freezeLimit = limit;
 
 	/*
-	 * simplistic MultiXactId removal limit: use the same policy as for
-	 * freezing Xids (except we use the oldest known mxact instead of the
-	 * current next value).
+	 * Determine the minimum multixact freeze age to use: as specified by
+	 * caller, or vacuum_multixact_freeze_min_age, but in any case not more
+	 * than half autovacuum_multixact_freeze_max_age, so that autovacuums to
+	 * prevent MultiXact wraparound won't occur too frequently.
 	 */
-	mxactLimit = GetOldestMultiXactId() - freezemin;
+	mxid_freezemin = multixact_freeze_min_age;
+	if (mxid_freezemin < 0)
+		mxid_freezemin = vacuum_multixact_freeze_min_age;
+	mxid_freezemin = Min(mxid_freezemin,
+						 autovacuum_multixact_freeze_max_age / 2);
+	Assert(mxid_freezemin >= 0);
+
+	/* compute the cutoff multi, being careful to generate a valid value */
+	mxactLimit = GetOldestMultiXactId() - mxid_freezemin;
 	if (mxactLimit < FirstMultiXactId)
 		mxactLimit = FirstMultiXactId;
+
+	safeMxactLimit =
+		ReadNextMultiXactId() - autovacuum_multixact_freeze_max_age;
+	if (safeMxactLimit < FirstMultiXactId)
+		safeMxactLimit = FirstMultiXactId;
+
+	if (MultiXactIdPrecedes(mxactLimit, safeMxactLimit))
+	{
+		ereport(WARNING,
+				(errmsg("oldest multixact is far in the past"),
+				 errhint("Close open transactions with multixacts soon to avoid wraparound problems.")));
+		mxactLimit = safeMxactLimit;
+	}
+
 	*multiXactCutoff = mxactLimit;
 
 	if (xidFullScanLimit != NULL)
@@ -501,9 +530,23 @@ vacuum_set_xid_limits(int freeze_min_age,
 		*xidFullScanLimit = limit;
 
 		/*
-		 * Compute MultiXactId limit to cause a full-table vacuum, being
-		 * careful not to generate an invalid multi. We just copy the logic
-		 * (and limits) from plain XIDs here.
+		 * Similar to the above, determine the table freeze age to use for
+		 * multixacts: as specified by the caller, or
+		 * vacuum_multixact_freeze_table_age, but in any case not more than
+		 * autovacuum_multixact_freeze_table_age * 0.95, so that if you have
+		 * e.g. nightly VACUUM schedule, the nightly VACUUM gets a chance to
+		 * freeze multixacts before anti-wraparound autovacuum is launched.
+		 */
+		freezetable = multixact_freeze_table_age;
+		if (freezetable < 0)
+			freezetable = vacuum_multixact_freeze_table_age;
+		freezetable = Min(freezetable,
+						  autovacuum_multixact_freeze_max_age * 0.95);
+		Assert(freezetable >= 0);
+
+		/*
+		 * Compute MultiXact limit causing a full-table vacuum, being careful
+		 * to generate a valid MultiXact value.
 		 */
 		mxactLimit = ReadNextMultiXactId() - freezetable;
 		if (mxactLimit < FirstMultiXactId)
@@ -511,6 +554,10 @@ vacuum_set_xid_limits(int freeze_min_age,
 
 		*mxactFullScanLimit = mxactLimit;
 	}
+	else
+	{
+		Assert(mxactFullScanLimit == NULL);
+	}
 }
 
 /*

src/backend/commands/vacuumlazy.c

@@ -205,6 +205,8 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
 	vac_strategy = bstrategy;
 
 	vacuum_set_xid_limits(vacstmt->freeze_min_age, vacstmt->freeze_table_age,
+						  vacstmt->multixact_freeze_min_age,
+						  vacstmt->multixact_freeze_table_age,
 						  onerel->rd_rel->relisshared,
 						  &OldestXmin, &FreezeLimit, &xidFullScanLimit,
 						  &MultiXactCutoff, &mxactFullScanLimit);
@@ -212,8 +214,8 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
 	/*
 	 * We request a full scan if either the table's frozen Xid is now older
 	 * than or equal to the requested Xid full-table scan limit; or if the
-	 * table's minimum MultiXactId is older than or equal to the requested mxid
-	 * full-table scan limit.
+	 * table's minimum MultiXactId is older than or equal to the requested
+	 * mxid full-table scan limit.
 	 */
 	scan_all = TransactionIdPrecedesOrEquals(onerel->rd_rel->relfrozenxid,
 											 xidFullScanLimit);