Add --latency-limit option to pgbench.

This allows transactions that take longer than specified limit to be counted
separately. With --rate, transactions that are already late by the time we
get to execute them are skipped altogether. Using --latency-limit with
--rate allows you to "catch up" more quickly, if there's a hickup in the
server causing a lot of transactions to stall momentarily.

Fabien COELHO, reviewed by Rukh Meski and heavily refactored by me.

2 files changed, 235 insertions, 66 deletions

diff --git a/contrib/pgbench/pgbench.c b/contrib/pgbench/pgbench.c
index e9431ee7860..3453a1f7a6b 100644
--- a/contrib/pgbench/pgbench.c
+++ b/contrib/pgbench/pgbench.c
@@ -141,6 +141,14 @@ double		sample_rate = 0.0;
 int64		throttle_delay = 0;
 
 /*
+ * Transactions which take longer than this limit (in usec) are counted as
+ * late, and reported as such, although they are completed anyway. When
+ * throttling is enabled, execution time slots that are more than this late
+ * are skipped altogether, and counted separately.
+ */
+int64		latency_limit = 0;
+
+/*
  * tablespace selection
  */
 char	   *tablespace = NULL;
@@ -238,6 +246,8 @@ typedef struct
 	int64		throttle_trigger;		/* previous/next throttling (us) */
 	int64		throttle_lag;	/* total transaction lag behind throttling */
 	int64		throttle_lag_max;		/* max transaction lag */
+	int64		throttle_latency_skipped; /* lagging transactions skipped */
+	int64		latency_late;	/* late transactions */
 } TState;
 
 #define INVALID_THREAD		((pthread_t) 0)
@@ -250,6 +260,8 @@ typedef struct
 	int64		sqlats;
 	int64		throttle_lag;
 	int64		throttle_lag_max;
+	int64		throttle_latency_skipped;
+	int64		latency_late;
 } TResult;
 
 /*
@@ -284,6 +296,8 @@ typedef struct
 
 	long		start_time;		/* when does the interval start */
 	int			cnt;			/* number of transactions */
+	int			skipped;		/* number of transactions skipped under
+								 * --rate and --latency-limit */
 
 	double		min_latency;	/* min/max latencies */
 	double		max_latency;
@@ -348,7 +362,7 @@ static void setalarm(int seconds);
 static void *threadRun(void *arg);
 
 static void doLog(TState *thread, CState *st, FILE *logfile, instr_time *now,
-	  AggVals *agg);
+	  AggVals *agg, bool skipped);
 
 static void
 usage(void)
@@ -375,6 +389,8 @@ usage(void)
 		 "  -f, --file=FILENAME      read transaction script from FILENAME\n"
 		   "  -j, --jobs=NUM           number of threads (default: 1)\n"
 		   "  -l, --log                write transaction times to log file\n"
+		   "  -L, --latency-limit=NUM  count transactions lasting more than NUM ms\n"
+		   "                           as late.\n"
 		   "  -M, --protocol=simple|extended|prepared\n"
 		   "                           protocol for submitting queries (default: simple)\n"
 		   "  -n, --no-vacuum          do not run VACUUM before tests\n"
@@ -994,7 +1010,9 @@ void
 agg_vals_init(AggVals *aggs, instr_time start)
 {
 	/* basic counters */
-	aggs->cnt = 0;				/* number of transactions */
+	aggs->cnt = 0;				/* number of transactions (includes skipped) */
+	aggs->skipped = 0;			/* xacts skipped under --rate --latency-limit */
+
 	aggs->sum_latency = 0;		/* SUM(latency) */
 	aggs->sum2_latency = 0;				/* SUM(latency*latency) */
 
@@ -1050,8 +1068,34 @@ top:
 		int64		wait = getPoissonRand(thread, throttle_delay);
 
 		thread->throttle_trigger += wait;
-
 		st->txn_scheduled = thread->throttle_trigger;
+
+		/*
+		 * If this --latency-limit is used, and this slot is already late so
+		 * that the transaction will miss the latency limit even if it
+		 * completed immediately, we skip this time slot and iterate till the
+		 * next slot that isn't late yet.
+		 */
+		if (latency_limit)
+		{
+			int64		now_us;
+
+			if (INSTR_TIME_IS_ZERO(now))
+				INSTR_TIME_SET_CURRENT(now);
+			now_us = INSTR_TIME_GET_MICROSEC(now);
+			while (thread->throttle_trigger < now_us - latency_limit)
+			{
+				thread->throttle_latency_skipped++;
+
+				if (logfile)
+					doLog(thread, st, logfile, &now, agg, true);
+
+				wait = getPoissonRand(thread, throttle_delay);
+				thread->throttle_trigger += wait;
+				st->txn_scheduled = thread->throttle_trigger;
+			}
+		}
+
 		st->sleeping = 1;
 		st->throttling = true;
 		st->is_throttled = true;
@@ -1119,12 +1163,13 @@ top:
 		if (commands[st->state + 1] == NULL)
 		{
 			/* only calculate latency if an option is used that needs it */
-			if (progress || throttle_delay)
+			if (progress || throttle_delay || latency_limit)
 			{
 				int64		latency;
 
 				if (INSTR_TIME_IS_ZERO(now))
 					INSTR_TIME_SET_CURRENT(now);
+
 				latency = INSTR_TIME_GET_MICROSEC(now) - st->txn_scheduled;
 
 				st->txn_latencies += latency;
@@ -1137,11 +1182,15 @@ top:
 				 * transactions, overflow would take 256 hours.
 				 */
 				st->txn_sqlats += latency * latency;
+
+				/* record over the limit transactions if needed. */
+				if (latency_limit && latency > latency_limit)
+					thread->latency_late++;
 			}
 
 			/* record the time it took in the log */
 			if (logfile)
-				doLog(thread, st, logfile, &now, agg);
+				doLog(thread, st, logfile, &now, agg, false);
 		}
 
 		if (commands[st->state]->type == SQL_COMMAND)
@@ -1227,7 +1276,7 @@ top:
 	}
 
 	/* Record transaction start time under logging, progress or throttling */
-	if ((logfile || progress || throttle_delay) && st->state == 0)
+	if ((logfile || progress || throttle_delay || latency_limit) && st->state == 0)
 	{
 		INSTR_TIME_SET_CURRENT(st->txn_begin);
 
@@ -1605,7 +1654,8 @@ top:
  * print log entry after completing one transaction.
  */
 static void
-doLog(TState *thread, CState *st, FILE *logfile, instr_time *now, AggVals *agg)
+doLog(TState *thread, CState *st, FILE *logfile, instr_time *now, AggVals *agg,
+	  bool skipped)
 {
 	double		lag;
 	double		latency;
@@ -1622,7 +1672,10 @@ doLog(TState *thread, CState *st, FILE *logfile, instr_time *now, AggVals *agg)
 		INSTR_TIME_SET_CURRENT(*now);
 
 	latency = (double) (INSTR_TIME_GET_MICROSEC(*now) - st->txn_scheduled);
-	lag = (double) (INSTR_TIME_GET_MICROSEC(st->txn_begin) - st->txn_scheduled);
+	if (skipped)
+		lag = latency;
+	else
+		lag = (double) (INSTR_TIME_GET_MICROSEC(st->txn_begin) - st->txn_scheduled);
 
 	/* should we aggregate the results or not? */
 	if (agg_interval > 0)
@@ -1634,26 +1687,34 @@ doLog(TState *thread, CState *st, FILE *logfile, instr_time *now, AggVals *agg)
 		if (agg->start_time + agg_interval >= INSTR_TIME_GET_DOUBLE(*now))
 		{
 			agg->cnt += 1;
-			agg->sum_latency += latency;
-			agg->sum2_latency += latency * latency;
+			if (skipped)
+			{
+				/* there is no latency to record if the transaction was skipped */
+				agg->skipped += 1;
+			}
+			else
+			{
+				agg->sum_latency += latency;
+				agg->sum2_latency += latency * latency;
 
-			/* first in this aggregation interval */
-			if ((agg->cnt == 1) || (latency < agg->min_latency))
-				agg->min_latency = latency;
+				/* first in this aggregation interval */
+				if ((agg->cnt == 1) || (latency < agg->min_latency))
+					agg->min_latency = latency;
 
-			if ((agg->cnt == 1) || (latency > agg->max_latency))
-				agg->max_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;
+				/* and the same for schedule lag */
+				if (throttle_delay)
+				{
+					agg->sum_lag += lag;
+					agg->sum2_lag += lag * lag;
 
-				if ((agg->cnt == 1) || (lag < agg->min_lag))
-					agg->min_lag = lag;
-				if ((agg->cnt == 1) || (lag > agg->max_lag))
-					agg->max_lag = lag;
+					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
@@ -1677,11 +1738,15 @@ doLog(TState *thread, CState *st, FILE *logfile, instr_time *now, AggVals *agg)
 						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);
+					if (latency_limit)
+						fprintf(logfile, " %d", agg->skipped);
+				}
 				fputc('\n', logfile);
 
 				/* move to the next inteval */
@@ -1689,6 +1754,7 @@ doLog(TState *thread, CState *st, FILE *logfile, instr_time *now, AggVals *agg)
 
 				/* reset for "no transaction" intervals */
 				agg->cnt = 0;
+				agg->skipped = 0;
 				agg->min_latency = 0;
 				agg->max_latency = 0;
 				agg->sum_latency = 0;
@@ -1701,10 +1767,11 @@ doLog(TState *thread, CState *st, FILE *logfile, instr_time *now, AggVals *agg)
 
 			/* reset the values to include only the current transaction. */
 			agg->cnt = 1;
+			agg->skipped = skipped ? 1 : 0;
 			agg->min_latency = latency;
 			agg->max_latency = latency;
-			agg->sum_latency = latency;
-			agg->sum2_latency = latency * latency;
+			agg->sum_latency = skipped ? 0.0 : latency;
+			agg->sum2_latency = skipped ? 0.0 : latency * latency;
 			agg->min_lag = lag;
 			agg->max_lag = lag;
 			agg->sum_lag = lag;
@@ -1717,14 +1784,23 @@ doLog(TState *thread, CState *st, FILE *logfile, instr_time *now, AggVals *agg)
 #ifndef WIN32
 
 		/* This is more than we really ought to know about instr_time */
-		fprintf(logfile, "%d %d %.0f %d %ld %ld",
-				st->id, st->cnt, latency, st->use_file,
-				(long) now->tv_sec, (long) now->tv_usec);
+		if (skipped)
+			fprintf(logfile, "%d %d skipped %d %ld %ld",
+					st->id, st->cnt, st->use_file,
+					(long) now->tv_sec, (long) now->tv_usec);
+		else
+			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",
-				st->id, st->cnt, latency, st->use_file);
+		if (skipped)
+			fprintf(logfile, "%d %d skipped %d 0 0",
+					st->id, st->cnt, st->use_file);
+		else
+			fprintf(logfile, "%d %d %.0f %d 0 0",
+					st->id, st->cnt, latency, st->use_file);
 #endif
 		if (throttle_delay)
 			fprintf(logfile, " %.0f", lag);
@@ -2424,7 +2500,8 @@ printResults(int ttype, int64 normal_xacts, int nclients,
 			 TState *threads, int nthreads,
 			 instr_time total_time, instr_time conn_total_time,
 			 int64 total_latencies, int64 total_sqlats,
-			 int64 throttle_lag, int64 throttle_lag_max)
+			 int64 throttle_lag, int64 throttle_lag_max,
+			 int64 throttle_latency_skipped, int64 latency_late)
 {
 	double		time_include,
 				tps_include,
@@ -2463,7 +2540,17 @@ printResults(int ttype, int64 normal_xacts, int nclients,
 			   normal_xacts);
 	}
 
-	if (throttle_delay || progress)
+	if (throttle_delay && latency_limit)
+		printf("number of transactions skipped: " INT64_FORMAT " (%.3f %%)\n",
+			   throttle_latency_skipped,
+			   100.0 * throttle_latency_skipped / (throttle_latency_skipped + normal_xacts));
+
+	if (latency_limit)
+		printf("number of transactions above the %.1f ms latency limit: " INT64_FORMAT " (%.3f %%)\n",
+			   latency_limit / 1000.0, latency_late,
+			   100.0 * latency_late / (throttle_latency_skipped + normal_xacts));
+
+	if (throttle_delay || progress || latency_limit)
 	{
 		/* compute and show latency average and standard deviation */
 		double		latency = 0.001 * total_latencies / normal_xacts;
@@ -2578,6 +2665,7 @@ main(int argc, char **argv)
 		{"sampling-rate", required_argument, NULL, 4},
 		{"aggregate-interval", required_argument, NULL, 5},
 		{"rate", required_argument, NULL, 'R'},
+		{"latency-limit", required_argument, NULL, 'L'},
 		{NULL, 0, NULL, 0}
 	};
 
@@ -2607,6 +2695,8 @@ main(int argc, char **argv)
 	int64		total_sqlats = 0;
 	int64		throttle_lag = 0;
 	int64		throttle_lag_max = 0;
+	int64		throttle_latency_skipped = 0;
+	int64		latency_late = 0;
 
 	int			i;
 
@@ -2651,7 +2741,7 @@ main(int argc, char **argv)
 	state = (CState *) pg_malloc(sizeof(CState));
 	memset(state, 0, sizeof(CState));
 
-	while ((c = getopt_long(argc, argv, "ih:nvp:dqSNc:j:Crs:t:T:U:lf:D:F:M:P:R:", long_options, &optindex)) != -1)
+	while ((c = getopt_long(argc, argv, "ih:nvp:dqSNc:j:Crs:t:T:U:lf:D:F:M:P:R:L:", long_options, &optindex)) != -1)
 	{
 		switch (c)
 		{
@@ -2848,6 +2938,18 @@ main(int argc, char **argv)
 					throttle_delay = (int64) (1000000.0 / throttle_value);
 				}
 				break;
+			case 'L':
+				{
+					double limit_ms = atof(optarg);
+					if (limit_ms <= 0.0)
+					{
+						fprintf(stderr, "invalid latency limit: %s\n", optarg);
+						exit(1);
+					}
+					benchmarking_option_set = true;
+					latency_limit = (int64) (limit_ms * 1000);
+				}
+				break;
 			case 0:
 				/* This covers long options which take no argument. */
 				if (foreign_keys || unlogged_tables)
@@ -3143,6 +3245,8 @@ main(int argc, char **argv)
 		thread->random_state[0] = random();
 		thread->random_state[1] = random();
 		thread->random_state[2] = random();
+		thread->throttle_latency_skipped = 0;
+		thread->latency_late = 0;
 
 		if (is_latencies)
 		{
@@ -3217,6 +3321,8 @@ main(int argc, char **argv)
 			total_latencies += r->latencies;
 			total_sqlats += r->sqlats;
 			throttle_lag += r->throttle_lag;
+			throttle_latency_skipped += r->throttle_latency_skipped;
+			latency_late += r->latency_late;
 			if (r->throttle_lag_max > throttle_lag_max)
 				throttle_lag_max = r->throttle_lag_max;
 			INSTR_TIME_ADD(conn_total_time, r->conn_time);
@@ -3239,7 +3345,8 @@ main(int argc, char **argv)
 	INSTR_TIME_SUBTRACT(total_time, start_time);
 	printResults(ttype, total_xacts, nclients, threads, nthreads,
 				 total_time, conn_total_time, total_latencies, total_sqlats,
-				 throttle_lag, throttle_lag_max);
+				 throttle_lag, throttle_lag_max, throttle_latency_skipped,
+				 latency_late);
 
 	return 0;
 }
@@ -3264,7 +3371,8 @@ threadRun(void *arg)
 	int64		last_count = 0,
 				last_lats = 0,
 				last_sqlats = 0,
-				last_lags = 0;
+				last_lags = 0,
+				last_skipped = 0;
 
 	AggVals		aggs;
 
@@ -3467,7 +3575,8 @@ threadRun(void *arg)
 				/* generate and show report */
 				int64		count = 0,
 							lats = 0,
-							sqlats = 0;
+							sqlats = 0,
+							skipped = 0;
 				int64		lags = thread->throttle_lag;
 				int64		run = now - last_report;
 				double		tps,
@@ -3490,23 +3599,26 @@ threadRun(void *arg)
 				sqlat = 1.0 * (sqlats - last_sqlats) / (count - last_count);
 				stdev = 0.001 * sqrt(sqlat - 1000000.0 * latency * latency);
 				lag = 0.001 * (lags - last_lags) / (count - last_count);
+				skipped = thread->throttle_latency_skipped - last_skipped;
 
+				fprintf(stderr,
+						"progress %d: %.1f s, %.1f tps, "
+						"lat %.3f ms stddev %.3f",
+						thread->tid, total_run, tps, latency, stdev);
 				if (throttle_delay)
-					fprintf(stderr,
-							"progress %d: %.1f s, %.1f tps, "
-							"lat %.3f ms stddev %.3f, lag %.3f ms\n",
-							thread->tid, total_run, tps, latency, stdev, lag);
-				else
-					fprintf(stderr,
-							"progress %d: %.1f s, %.1f tps, "
-							"lat %.3f ms stddev %.3f\n",
-							thread->tid, total_run, tps, latency, stdev);
+				{
+					fprintf(stderr, ", lag %.3f ms", lag);
+					if (latency_limit)
+						fprintf(stderr, ", skipped " INT64_FORMAT, skipped);
+				}
+				fprintf(stderr, "\n");
 
 				last_count = count;
 				last_lats = lats;
 				last_sqlats = sqlats;
 				last_lags = lags;
 				last_report = now;
+				last_skipped = thread->throttle_latency_skipped;
 				next_report += (int64) progress *1000000;
 			}
 		}
@@ -3525,7 +3637,8 @@ threadRun(void *arg)
 				int64		count = 0,
 							lats = 0,
 							sqlats = 0,
-							lags = 0;
+							lags = 0,
+							skipped = 0;
 				int64		run = now - last_report;
 				double		tps,
 							total_run,
@@ -3550,23 +3663,26 @@ threadRun(void *arg)
 				sqlat = 1.0 * (sqlats - last_sqlats) / (count - last_count);
 				stdev = 0.001 * sqrt(sqlat - 1000000.0 * latency * latency);
 				lag = 0.001 * (lags - last_lags) / (count - last_count);
+				skipped = thread->throttle_latency_skipped - last_skipped;
 
+				fprintf(stderr,
+						"progress: %.1f s, %.1f tps, "
+						"lat %.3f ms stddev %.3f",
+						total_run, tps, latency, stdev);
 				if (throttle_delay)
-					fprintf(stderr,
-							"progress: %.1f s, %.1f tps, "
-							"lat %.3f ms stddev %.3f, lag %.3f ms\n",
-							total_run, tps, latency, stdev, lag);
-				else
-					fprintf(stderr,
-							"progress: %.1f s, %.1f tps, "
-							"lat %.3f ms stddev %.3f\n",
-							total_run, tps, latency, stdev);
+				{
+					fprintf(stderr, ", lag %.3f ms", lag);
+					if (latency_limit)
+						fprintf(stderr, ", " INT64_FORMAT " skipped", skipped);
+				}
+				fprintf(stderr, "\n");
 
 				last_count = count;
 				last_lats = lats;
 				last_sqlats = sqlats;
 				last_lags = lags;
 				last_report = now;
+				last_skipped = thread->throttle_latency_skipped;
 				next_report += (int64) progress *1000000;
 			}
 		}
@@ -3587,6 +3703,9 @@ done:
 	}
 	result->throttle_lag = thread->throttle_lag;
 	result->throttle_lag_max = thread->throttle_lag_max;
+	result->throttle_latency_skipped = thread->throttle_latency_skipped;
+	result->latency_late = thread->latency_late;
+
 	INSTR_TIME_SET_CURRENT(end);
 	INSTR_TIME_ACCUM_DIFF(result->conn_time, end, start);
 	if (logfile)
diff --git a/doc/src/sgml/pgbench.sgml b/doc/src/sgml/pgbench.sgml
index c4e0cbd79ae..7d203cda84c 100644
--- a/doc/src/sgml/pgbench.sgml
+++ b/doc/src/sgml/pgbench.sgml
@@ -345,6 +345,24 @@ pgbench <optional> <replaceable>options</> </optional> <replaceable>dbname</>
      </varlistentry>
 
      <varlistentry>
+      <term><option>-L</option> <replaceable>limit</></term>
+      <term><option>--latency-limit=</option><replaceable>limit</></term>
+      <listitem>
+       <para>
+        Transaction which last more than <replaceable>limit</> milliseconds
+        are counted and reported separately, as <firstterm>late</>.
+       </para>
+       <para>
+        When throttling is used (<option>--rate=...</>), transactions that
+        lag behind schedule by more than <replaceable>limit</> ms, and thus
+        have no hope of meeting the latency limit, are not sent to the server
+        at all. They are counted and reported separately as
+        <firstterm>skipped</>.
+       </para>
+       </listitem>
+     </varlistentry>
+
+     <varlistentry>
       <term><option>-M</option> <replaceable>querymode</></term>
       <term><option>--protocol=</option><replaceable>querymode</></term>
       <listitem>
@@ -454,6 +472,15 @@ pgbench <optional> <replaceable>options</> </optional> <replaceable>dbname</>
        </para>
 
        <para>
+        If <option>--latency-limit</> is used together with <option>--rate</>,
+        a transaction can lag behind so much that it is already over the
+        latency limit when the previous transaction ends, because the latency
+        is calculated from the scheduled start time. Such transactions are
+        not sent to the server, but are skipped altogether and counted
+        separately.
+       </para>
+
+       <para>
         A high schedule lag time is an indication that the system cannot
         process transactions at the specified rate, with the chosen number of
         clients and threads. When the average transaction execution time is
@@ -940,7 +967,7 @@ END;
    The format of the log is:
 
 <synopsis>
-<replaceable>client_id</> <replaceable>transaction_no</> <replaceable>time</> <replaceable>file_no</> <replaceable>time_epoch</> <replaceable>time_us</> [<replaceable>schedule_lag</replaceable>]
+<replaceable>client_id</> <replaceable>transaction_no</> <replaceable>time</> <replaceable>file_no</> <replaceable>time_epoch</> <replaceable>time_us</> <optional><replaceable>schedule_lag</replaceable></optional>
 </synopsis>
 
    where <replaceable>time</> is the total elapsed transaction time in microseconds,
@@ -950,20 +977,40 @@ END;
    UNIX epoch format timestamp and an offset
    in microseconds (suitable for creating an ISO 8601
    timestamp with fractional seconds) showing when
-   the transaction completed. The last field, <replaceable>schedule_lag</>, is
-   the difference between the transaction's scheduled start time, and the
-   time it actually started, in microseconds. It is only present when the
-   <option>--rate</> option is used.
+   the transaction completed.
+   Field <replaceable>schedule_lag</> is the difference between the
+   transaction's scheduled start time, and the time it actually started, in
+   microseconds. It is only present when the <option>--rate</> option is used.
+   The last field <replaceable>skipped_transactions</> reports the number of
+   transactions skipped because they were too far behind schedule. It is only
+   present when both options <option>--rate</> and <option>--latency-limit</>
+   are used.
   </para>
 
   <para>
-   Here are example outputs:
+   Here is a snippet of the log file generated:
 <screen>
  0 199 2241 0 1175850568 995598
  0 200 2465 0 1175850568 998079
  0 201 2513 0 1175850569 608
  0 202 2038 0 1175850569 2663
-</screen></para>
+</screen>
+
+   Another example with --rate=100 and --latency-limit=5 (note the additional
+   <replaceable>schedule_lag</> column):
+<screen>
+ 0 81 4621 0 1412881037 912698 3005
+ 0 82 6173 0 1412881037 914578 4304
+ 0 83 skipped 0 1412881037 914578 5217
+ 0 83 skipped 0 1412881037 914578 5099
+ 0 83 4722 0 1412881037 916203 3108
+ 0 84 4142 0 1412881037 918023 2333
+ 0 85 2465 0 1412881037 919759 740
+</screen>
+   In this example, transaction 82 was late, because it's latency (6.173 ms) was
+   over the 5 ms limit. The next two transactions were skipped, because they
+   were already late before they were even started.
+  </para>
 
   <para>
    When running a long test on hardware that can handle a lot of transactions,
@@ -979,7 +1026,7 @@ END;
    With the <option>--aggregate-interval</option> option, the logs use a bit different format:
 
 <synopsis>
-<replaceable>interval_start</> <replaceable>num_of_transactions</> <replaceable>latency_sum</> <replaceable>latency_2_sum</> <replaceable>min_latency</> <replaceable>max_latency</> [<replaceable>lag_sum</> <replaceable>lag_2_sum</> <replaceable>min_lag</> <replaceable>max_lag</>]
+<replaceable>interval_start</> <replaceable>num_of_transactions</> <replaceable>latency_sum</> <replaceable>latency_2_sum</> <replaceable>min_latency</> <replaceable>max_latency</> <optional><replaceable>lag_sum</> <replaceable>lag_2_sum</> <replaceable>min_lag</> <replaceable>max_lag</> <optional><replaceable>skipped_transactions</></optional></optional>
 </synopsis>
 
    where <replaceable>interval_start</> is the start of the interval (UNIX epoch
@@ -990,8 +1037,11 @@ END;
    <replaceable>latency_2_sum</> is a sum of 2nd powers of latencies. The last two
    fields are <replaceable>min_latency</> - a minimum latency within the interval, and
    <replaceable>max_latency</> - maximum latency within the interval. A transaction is
-   counted into the interval when it was committed. The last four fields, 
-   <replaceable>lag_sum</>, <replaceable>lag_2_sum</>, <replaceable>min_lag</>, and <replaceable>max_lag</>, are only present if the --rate option is used.
+   counted into the interval when it was committed. The fields in the end,
+   <replaceable>lag_sum</>, <replaceable>lag_2_sum</>, <replaceable>min_lag</>,
+   and <replaceable>max_lag</>, are only present if the <option>--rate</>
+   option is used. The very last one, <replaceable>skipped_transactions</>,
+   is only present if the option <option>--latency-limit</> is present, too.
    They are calculated from the time each transaction had to wait for the
    previous one to finish, i.e. the difference between each transaction's
    scheduled start time and the time it actually started.