Remove multixact offset freeze threshold computation.

Alexander Korotkov · Alexander Korotkov · commit 2b0a919d5ce3 · 2016-11-27T20:16:07.000+03:00
diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
@@ -2314,68 +2314,6 @@ ReadMultiXactCounts(uint64 *multixacts, MultiXactOffset *members)
 	return true;
 }
 
-/*
- * Multixact members can be removed once the multixacts that refer to them
- * are older than every datminxmid.  autovacuum_multixact_freeze_max_age and
- * vacuum_multixact_freeze_table_age work together to make sure we never have
- * too many multixacts; we hope that, at least under normal circumstances,
- * this will also be sufficient to keep us from using too many offsets.
- * However, if the average multixact has many members, we might exhaust the
- * members space while still using few enough members that these limits fail
- * to trigger full table scans for relminmxid advancement.  At that point,
- * we'd have no choice but to start failing multixact-creating operations
- * with an error.
- *
- * To prevent that, if more than a threshold portion of the members space is
- * used, we effectively reduce autovacuum_multixact_freeze_max_age and
- * to a value just less than the number of multixacts in use.  We hope that
- * this will quickly trigger autovacuuming on the table or tables with the
- * oldest relminmxid, thus allowing datminmxid values to advance and removing
- * some members.
- *
- * As the fraction of the member space currently in use grows, we become
- * more aggressive in clamping this value.  That not only causes autovacuum
- * to ramp up, but also makes any manual vacuums the user issues more
- * aggressive.  This happens because vacuum_set_xid_limits() clamps the
- * freeze table and the minimum freeze age based on the effective
- * autovacuum_multixact_freeze_max_age this function returns.  In the worst
- * case, we'll claim the freeze_max_age to zero, and every vacuum of any
- * table will try to freeze every multixact.
- *
- * It's possible that these thresholds should be user-tunable, but for now
- * we keep it simple.
- */
-int64
-MultiXactMemberFreezeThreshold(void)
-{
-	MultiXactOffset members;
-	uint64		multixacts;
-	uint64		victim_multixacts;
-	double		fraction;
-
-	/* If we can't determine member space utilization, assume the worst. */
-	if (!ReadMultiXactCounts(&multixacts, &members))
-		return 0;
-
-	/* If member space utilization is low, no special action is required. */
-	if (members <= MULTIXACT_MEMBER_SAFE_THRESHOLD)
-		return autovacuum_multixact_freeze_max_age;
-
-	/*
-	 * Compute a target for relminmxid advancement.  The number of multixacts
-	 * we try to eliminate from the system is based on how far we are past
-	 * MULTIXACT_MEMBER_SAFE_THRESHOLD.
-	 */
-	fraction = (double) (members - MULTIXACT_MEMBER_SAFE_THRESHOLD) /
-		(MULTIXACT_MEMBER_DANGER_THRESHOLD - MULTIXACT_MEMBER_SAFE_THRESHOLD);
-	victim_multixacts = multixacts * fraction;
-
-	/* fraction could be > 1.0, but lowest possible freeze age is zero */
-	if (victim_multixacts > multixacts)
-		return 0;
-	return multixacts - victim_multixacts;
-}
-
 typedef struct mxtruncinfo
 {
 	int			earliestExistingPage;
diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
@@ -551,7 +551,7 @@ vacuum_set_xid_limits(Relation rel,
 	 * normally autovacuum_multixact_freeze_max_age, but may be less if we are
 	 * short of multixact member space.
 	 */
-	effective_multixact_freeze_max_age = MultiXactMemberFreezeThreshold();
+	effective_multixact_freeze_max_age = autovacuum_multixact_freeze_max_age;
 
 	/*
 	 * Determine the minimum multixact freeze age to use: as specified by
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
@@ -1130,7 +1130,7 @@ do_start_worker(void)
 
 	/* Also determine the oldest datminmxid we will consider. */
 	recentMulti = ReadNextMultiXactId();
-	multiMembersThreshold = MultiXactMemberFreezeThreshold();
+	multiMembersThreshold = autovacuum_multixact_freeze_max_age;
 	if (recentMulti > FirstMultiXactId + multiMembersThreshold)
 		multiForceLimit = recentMulti - multiMembersThreshold;
 	else
@@ -1935,7 +1935,7 @@ do_autovacuum(void)
 	 * normally autovacuum_multixact_freeze_max_age, but may be less if we are
 	 * short of multixact member space.
 	 */
-	effective_multixact_freeze_max_age = MultiXactMemberFreezeThreshold();
+	effective_multixact_freeze_max_age = autovacuum_multixact_freeze_max_age;
 
 	/*
 	 * Find the pg_database entry and select the default freeze ages. We use
diff --git a/src/include/access/multixact.h b/src/include/access/multixact.h
@@ -138,7 +138,6 @@ extern void MultiXactSetNextMXact(MultiXactId nextMulti,
 extern void MultiXactAdvanceNextMXact(MultiXactId minMulti,
 						  MultiXactOffset minMultiOffset);
 extern void MultiXactAdvanceOldest(MultiXactId oldestMulti, Oid oldestMultiDB);
-extern int64 MultiXactMemberFreezeThreshold(void);
 
 extern void multixact_twophase_recover(TransactionId xid, uint16 info,
 						   void *recdata, uint32 len);
