Change the way latency is calculated with pgbench --rate option.

The reported latency values now include the "schedule lag" time, that is,
the time between the transaction's scheduled start time and the time it
actually started. This relates better to a model where requests arrive at a
certain rate, and we are interested in the response time to the end user or
application, rather than the response time of the database itself.

Also, when --rate is used, include the schedule lag time in the log output.

The --rate option is new in 9.4, so backpatch to 9.4. It seems better to
make this change in 9.4, while we're still in the beta period, than ship a
9.4 version that calculates the values differently than 9.5.

Author: hlinnaka

contrib/pgbench/pgbench.c

@@ -210,10 +210,10 @@ typedef struct
 								 * sent */
 	int			sleeping;		/* 1 indicates that the client is napping */
 	bool		throttling;		/* whether nap is for throttling */
-	int64		until;			/* napping until (usec) */
 	Variable   *variables;		/* array of variable definitions */
 	int			nvariables;
-	instr_time	txn_begin;		/* used for measuring transaction latencies */
+	int64		txn_scheduled;	/* scheduled start time of transaction (usec) */
+	instr_time	txn_begin;		/* used for measuring schedule lag times */
 	instr_time	stmt_begin;		/* used for measuring statement latencies */
 	int64		txn_latencies;	/* cumulated latencies */
 	int64		txn_sqlats;		/* cumulated square latencies */
@@ -284,12 +284,17 @@ typedef struct
 
 	long		start_time;		/* when does the interval start */
 	int			cnt;			/* number of transactions */
-	double		min_duration;	/* min/max durations */
-	double		max_duration;
-	double		sum;			/* sum(duration), sum(duration^2) - for
+
+	double		min_latency;	/* min/max latencies */
+	double		max_latency;
+	double		sum_latency;	/* sum(latency), sum(latency^2) - for
 								 * estimates */
-	double		sum2;
+	double		sum2_latency;
 
+	double		min_lag;
+	double		max_lag;
+	double		sum_lag;		/* sum(lag) */
+	double		sum2_lag;		/* sum(lag*lag) */
 } AggVals;
 
 static Command **sql_files[MAX_FILES];	/* SQL script files */
@@ -968,12 +973,18 @@ agg_vals_init(AggVals *aggs, instr_time start)
 {
 	/* basic counters */
 	aggs->cnt = 0;				/* number of transactions */
-	aggs->sum = 0;				/* SUM(duration) */
-	aggs->sum2 = 0;				/* SUM(duration*duration) */
+	aggs->sum_latency = 0;		/* SUM(latency) */
+	aggs->sum2_latency = 0;				/* SUM(latency*latency) */
 
 	/* min and max transaction duration */
-	aggs->min_duration = 0;
-	aggs->max_duration = 0;
+	aggs->min_latency = 0;
+	aggs->max_latency = 0;
+
+	/* schedule lag counters */
+	aggs->sum_lag = 0;
+	aggs->sum2_lag = 0;
+	aggs->min_lag = 0;
+	aggs->max_lag = 0;
 
 	/* start of the current interval */
 	aggs->start_time = INSTR_TIME_GET_DOUBLE(start);
@@ -1016,7 +1027,7 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile, AggVa
 
 		thread->throttle_trigger += wait;
 
-		st->until = thread->throttle_trigger;
+		st->txn_scheduled = thread->throttle_trigger;
 		st->sleeping = 1;
 		st->throttling = true;
 		st->is_throttled = true;
@@ -1032,13 +1043,13 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile, AggVa
 
 		INSTR_TIME_SET_CURRENT(now);
 		now_us = INSTR_TIME_GET_MICROSEC(now);
-		if (st->until <= now_us)
+		if (st->txn_scheduled <= now_us)
 		{
 			st->sleeping = 0;	/* Done sleeping, go ahead with next command */
 			if (st->throttling)
 			{
 				/* Measure lag of throttled transaction relative to target */
-				int64		lag = now_us - st->until;
+				int64		lag = now_us - st->txn_scheduled;
 
 				thread->throttle_lag += lag;
 				if (lag > thread->throttle_lag_max)
@@ -1052,6 +1063,11 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile, AggVa
 
 	if (st->listen)
 	{							/* are we receiver? */
+		instr_time	now;
+		bool		now_valid = false;
+
+		INSTR_TIME_SET_ZERO(now); /* initialize to keep compiler quiet */
+
 		if (commands[st->state]->type == SQL_COMMAND)
 		{
 			if (debug)
@@ -1071,10 +1087,13 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile, AggVa
 		 */
 		if (is_latencies)
 		{
-			instr_time	now;
 			int			cnum = commands[st->state]->command_num;
 
-			INSTR_TIME_SET_CURRENT(now);
+			if (!now_valid)
+			{
+				INSTR_TIME_SET_CURRENT(now);
+				now_valid = true;
+			}
 			INSTR_TIME_ACCUM_DIFF(thread->exec_elapsed[cnum],
 								  now, st->stmt_begin);
 			thread->exec_count[cnum]++;
@@ -1083,12 +1102,16 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile, AggVa
 		/* transaction finished: record latency under progress or throttling */
 		if ((progress || throttle_delay) && commands[st->state + 1] == NULL)
 		{
-			instr_time	diff;
 			int64		latency;
 
-			INSTR_TIME_SET_CURRENT(diff);
-			INSTR_TIME_SUBTRACT(diff, st->txn_begin);
-			latency = INSTR_TIME_GET_MICROSEC(diff);
+			if (!now_valid)
+			{
+				INSTR_TIME_SET_CURRENT(now);
+				now_valid = true;
+			}
+
+			latency = INSTR_TIME_GET_MICROSEC(now) - st->txn_scheduled;
+
 			st->txn_latencies += latency;
 
 			/*
@@ -1106,9 +1129,8 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile, AggVa
 		 */
 		if (logfile && commands[st->state + 1] == NULL)
 		{
-			instr_time	now;
-			instr_time	diff;
-			double		usec;
+			double		lag;
+			double		latency;
 
 			/*
 			 * write the log entry if this row belongs to the random sample,
@@ -1117,10 +1139,13 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile, AggVa
 			if (sample_rate == 0.0 ||
 				pg_erand48(thread->random_state) <= sample_rate)
 			{
-				INSTR_TIME_SET_CURRENT(now);
-				diff = now;
-				INSTR_TIME_SUBTRACT(diff, st->txn_begin);
-				usec = (double) INSTR_TIME_GET_MICROSEC(diff);
+				if (!now_valid)
+				{
+					INSTR_TIME_SET_CURRENT(now);
+					now_valid = true;
+				}
+				latency = (double) (INSTR_TIME_GET_MICROSEC(now) - st->txn_scheduled);
+				lag = (double) (INSTR_TIME_GET_MICROSEC(st->txn_begin) - st->txn_scheduled);
 
 				/* should we aggregate the results or not? */
 				if (agg_interval > 0)
@@ -1132,15 +1157,27 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile, AggVa
 					if (agg->start_time + agg_interval >= INSTR_TIME_GET_DOUBLE(now))
 					{
 						agg->cnt += 1;
-						agg->sum += usec;
-						agg->sum2 += usec * usec;
+						agg->sum_latency += latency;
+						agg->sum2_latency += latency * latency;
 
 						/* first in this aggregation interval */
-						if ((agg->cnt == 1) || (usec < agg->min_duration))
-							agg->min_duration = usec;
+						if ((agg->cnt == 1) || (latency < agg->min_latency))
+							agg->min_latency = latency;
+
+						if ((agg->cnt == 1) || (latency > agg->max_latency))
+							agg->max_latency = latency;
+
+						/* and the same for schedule lag */
+						if (throttle_delay)
+						{
+							agg->sum_lag += lag;
+							agg->sum2_lag += lag * lag;
 
-						if ((agg->cnt == 1) || (usec > agg->max_duration))
-							agg->max_duration = usec;
+							if ((agg->cnt == 1) || (lag < agg->min_lag))
+								agg->min_lag = lag;
+							if ((agg->cnt == 1) || (lag > agg->max_lag))
+								agg->max_lag = lag;
+						}
 					}
 					else
 					{
@@ -1156,34 +1193,49 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile, AggVa
 							 * ifdef in usage), so we don't need to handle
 							 * this in a special way (see below).
 							 */
-							fprintf(logfile, "%ld %d %.0f %.0f %.0f %.0f\n",
+							fprintf(logfile, "%ld %d %.0f %.0f %.0f %.0f",
 									agg->start_time,
 									agg->cnt,
-									agg->sum,
-									agg->sum2,
-									agg->min_duration,
-									agg->max_duration);
+									agg->sum_latency,
+									agg->sum2_latency,
+									agg->min_latency,
+									agg->max_latency);
+							if (throttle_delay)
+								fprintf(logfile, " %.0f %.0f %.0f %.0f",
+										agg->sum_lag,
+										agg->sum2_lag,
+										agg->min_lag,
+										agg->max_lag);
+							fputc('\n', logfile);
 
 							/* move to the next inteval */
 							agg->start_time = agg->start_time + agg_interval;
 
 							/* reset for "no transaction" intervals */
 							agg->cnt = 0;
-							agg->min_duration = 0;
-							agg->max_duration = 0;
-							agg->sum = 0;
-							agg->sum2 = 0;
+							agg->min_latency = 0;
+							agg->max_latency = 0;
+							agg->sum_latency = 0;
+							agg->sum2_latency = 0;
+							agg->min_lag = 0;
+							agg->max_lag = 0;
+							agg->sum_lag = 0;
+							agg->sum2_lag = 0;
 						}
 
 						/*
 						 * and now update the reset values (include the
 						 * current)
 						 */
 						agg->cnt = 1;
-						agg->min_duration = usec;
-						agg->max_duration = usec;
-						agg->sum = usec;
-						agg->sum2 = usec * usec;
+						agg->min_latency = latency;
+						agg->max_latency = latency;
+						agg->sum_latency = latency;
+						agg->sum2_latency = latency * latency;
+						agg->min_lag = lag;
+						agg->max_lag = lag;
+						agg->sum_lag = lag;
+						agg->sum2_lag = lag * lag;
 					}
 				}
 				else
@@ -1195,18 +1247,21 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile, AggVa
 					 * This is more than we really ought to know about
 					 * instr_time
 					 */
-					fprintf(logfile, "%d %d %.0f %d %ld %ld\n",
-							st->id, st->cnt, usec, st->use_file,
+					fprintf(logfile, "%d %d %.0f %d %ld %ld",
+							st->id, st->cnt, latency, st->use_file,
 							(long) now.tv_sec, (long) now.tv_usec);
 #else
 
 					/*
 					 * On Windows, instr_time doesn't provide a timestamp
 					 * anyway
 					 */
-					fprintf(logfile, "%d %d %.0f %d 0 0\n",
+					fprintf(logfile, "%d %d %.0f %d 0 0",
 							st->id, st->cnt, usec, st->use_file);
 #endif
+					if (throttle_delay)
+						fprintf(logfile, " %.0f", lag);
+					fputc('\n', logfile);
 				}
 			}
 		}
@@ -1295,8 +1350,17 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile, AggVa
 
 	/* Record transaction start time under logging, progress or throttling */
 	if ((logfile || progress || throttle_delay) && st->state == 0)
+	{
 		INSTR_TIME_SET_CURRENT(st->txn_begin);
 
+		/*
+		 * When not throttling, this is also the transaction's scheduled start
+		 * time.
+		 */
+		if (!throttle_delay)
+			st->txn_scheduled = INSTR_TIME_GET_MICROSEC(st->txn_begin);
+	}
+
 	/* Record statement start time if per-command latencies are requested */
 	if (is_latencies)
 		INSTR_TIME_SET_CURRENT(st->stmt_begin);
@@ -1620,7 +1684,7 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile, AggVa
 				usec *= 1000000;
 
 			INSTR_TIME_SET_CURRENT(now);
-			st->until = INSTR_TIME_GET_MICROSEC(now) + usec;
+			st->txn_scheduled = INSTR_TIME_GET_MICROSEC(now) + usec;
 			st->sleeping = 1;
 
 			st->listen = 1;
@@ -3309,7 +3373,7 @@ threadRun(void *arg)
 						now_usec = INSTR_TIME_GET_MICROSEC(now);
 					}
 
-					this_usec = st->until - now_usec;
+					this_usec = st->txn_scheduled - now_usec;
 					if (min_usec > this_usec)
 						min_usec = this_usec;
 				}