Fix leakage of cost_limit when multiple autovacuum workers are active.

When using default autovacuum_vac_cost_limit, autovac_balance_cost relied
on VacuumCostLimit to contain the correct global value ... but after the
first time through in a particular worker process, it didn't, because we'd
trashed it in previous iterations.  Depending on the state of other autovac
workers, this could result in a steady reduction of the effective
cost_limit setting as a particular worker processed more and more tables,
causing it to go slower and slower.  Spotted by Simon Poole (bug #5759).
Fix by saving and restoring the GUC variables in the loop in do_autovacuum.

In passing, improve a few comments.

Back-patch to 8.3 ... the cost rebalancing code has been buggy since it was
put in.

src/backend/postmaster/autovacuum.c

@@ -190,7 +190,7 @@ typedef struct autovac_table
  *
  * wi_links		entry into free list or running list
  * wi_dboid		OID of the database this worker is supposed to work on
- * wi_tableoid	OID of the table currently being vacuumed
+ * wi_tableoid	OID of the table currently being vacuumed, if any
  * wi_proc		pointer to PGPROC of the running worker, NULL if not started
  * wi_launchtime Time at which this worker was launched
  * wi_cost_*	Vacuum cost-based delay parameters current in this worker
@@ -1629,7 +1629,7 @@ FreeWorkerInfo(int code, Datum arg)
 		 * limit setting of the remaining workers.
 		 *
 		 * We somewhat ignore the risk that the launcher changes its PID
-		 * between we reading it and the actual kill; we expect ProcKill to be
+		 * between us reading it and the actual kill; we expect ProcKill to be
 		 * called shortly after us, and we assume that PIDs are not reused too
 		 * quickly after a process exits.
 		 */
@@ -1673,16 +1673,18 @@ AutoVacuumUpdateDelay(void)
 
 /*
  * autovac_balance_cost
- *		Recalculate the cost limit setting for each active workers.
+ *		Recalculate the cost limit setting for each active worker.
  *
  * Caller must hold the AutovacuumLock in exclusive mode.
  */
 static void
 autovac_balance_cost(void)
 {
-	WorkerInfo	worker;
-
 	/*
+	 * The idea here is that we ration out I/O equally.  The amount of I/O
+	 * that a worker can consume is determined by cost_limit/cost_delay, so
+	 * we try to equalize those ratios rather than the raw limit settings.
+	 *
 	 * note: in cost_limit, zero also means use value from elsewhere, because
 	 * zero is not a valid value.
 	 */
@@ -1692,6 +1694,7 @@ autovac_balance_cost(void)
 								autovacuum_vac_cost_delay : VacuumCostDelay);
 	double		cost_total;
 	double		cost_avail;
+	WorkerInfo	worker;
 
 	/* not set? nothing to do */
 	if (vac_cost_limit <= 0 || vac_cost_delay <= 0)
@@ -1718,7 +1721,7 @@ autovac_balance_cost(void)
 		return;
 
 	/*
-	 * Adjust each cost limit of active workers to balance the total of cost
+	 * Adjust cost limit of each active worker to balance the total of cost
 	 * limit to autovacuum_vacuum_cost_limit.
 	 */
 	cost_avail = (double) vac_cost_limit / vac_cost_delay;
@@ -1734,14 +1737,19 @@ autovac_balance_cost(void)
 			(cost_avail * worker->wi_cost_limit_base / cost_total);
 
 			/*
-			 * We put a lower bound of 1 to the cost_limit, to avoid division-
-			 * by-zero in the vacuum code.
+			 * We put a lower bound of 1 on the cost_limit, to avoid division-
+			 * by-zero in the vacuum code.  Also, in case of roundoff trouble
+			 * in these calculations, let's be sure we don't ever set
+			 * cost_limit to more than the base value.
 			 */
-			worker->wi_cost_limit = Max(Min(limit, worker->wi_cost_limit_base), 1);
+			worker->wi_cost_limit = Max(Min(limit,
+											worker->wi_cost_limit_base),
+										1);
 
-			elog(DEBUG2, "autovac_balance_cost(pid=%u db=%u, rel=%u, cost_limit=%d, cost_delay=%d)",
-				 worker->wi_proc->pid, worker->wi_dboid,
-				 worker->wi_tableoid, worker->wi_cost_limit, worker->wi_cost_delay);
+			elog(DEBUG2, "autovac_balance_cost(pid=%u db=%u, rel=%u, cost_limit=%d, cost_limit_base=%d, cost_delay=%d)",
+				 worker->wi_proc->pid, worker->wi_dboid, worker->wi_tableoid,
+				 worker->wi_cost_limit, worker->wi_cost_limit_base,
+				 worker->wi_cost_delay);
 		}
 
 		worker = (WorkerInfo) SHMQueueNext(&AutoVacuumShmem->av_runningWorkers,
@@ -2125,6 +2133,8 @@ do_autovacuum(void)
 		autovac_table *tab;
 		WorkerInfo	worker;
 		bool		skipit;
+		int			stdVacuumCostDelay;
+		int			stdVacuumCostLimit;
 
 		CHECK_FOR_INTERRUPTS();
 
@@ -2198,11 +2208,15 @@ do_autovacuum(void)
 		MyWorkerInfo->wi_tableoid = relid;
 		LWLockRelease(AutovacuumScheduleLock);
 
-		/* Set the initial vacuum cost parameters for this table */
-		VacuumCostDelay = tab->at_vacuum_cost_delay;
-		VacuumCostLimit = tab->at_vacuum_cost_limit;
+		/*
+		 * Remember the prevailing values of the vacuum cost GUCs.  We have
+		 * to restore these at the bottom of the loop, else we'll compute
+		 * wrong values in the next iteration of autovac_balance_cost().
+		 */
+		stdVacuumCostDelay = VacuumCostDelay;
+		stdVacuumCostLimit = VacuumCostLimit;
 
-		/* Last fixups before actually starting to work */
+		/* Must hold AutovacuumLock while mucking with cost balance info */
 		LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);
 
 		/* advertise my cost delay parameters for the balancing algorithm */
@@ -2213,6 +2227,9 @@ do_autovacuum(void)
 		/* do a balance */
 		autovac_balance_cost();
 
+		/* set the active cost parameters from the result of that */
+		AutoVacuumUpdateDelay();
+
 		/* done */
 		LWLockRelease(AutovacuumLock);
 
@@ -2290,10 +2307,20 @@ deleted:
 			pfree(tab->at_relname);
 		pfree(tab);
 
-		/* remove my info from shared memory */
+		/*
+		 * Remove my info from shared memory.  We could, but intentionally
+		 * don't, clear wi_cost_limit and friends --- this is on the
+		 * assumption that we probably have more to do with similar cost
+		 * settings, so we don't want to give up our share of I/O for a very
+		 * short interval and thereby thrash the global balance.
+		 */
 		LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);
 		MyWorkerInfo->wi_tableoid = InvalidOid;
 		LWLockRelease(AutovacuumLock);
+
+		/* restore vacuum cost GUCs for the next iteration */
+		VacuumCostDelay = stdVacuumCostDelay;
+		VacuumCostLimit = stdVacuumCostLimit;
 	}
 
 	/*