From 54a2d5b37b790cbfaf568fbbe6d2cc89650551d1 Mon Sep 17 00:00:00 2001 From: Heikki Linnakangas Date: Thu, 11 Sep 2014 13:00:48 +0300 Subject: [PATCH] Simplify calculation of Poisson distributed delays in pgbench --rate mode. The previous coding first generated a uniform random value between 0.0 and 1.0, then converted that to an integer between 1 and 10000, and divided that again by 10000. Those conversions are unnecessary; we can use the double value that pg_erand48() returns directly. While we're at it, put the logic into a helper function, getPoissonRand(). The largest delay generated by the old coding was about 9.2 times the average, because of the way the uniformly distributed value used for the calculation was truncated to 1/10000 granularity. The new coding doesn't have such clamping. With my laptop's DBL_MIN value, the maximum delay with the new coding is about 700x the average. That seems acceptable - any reasonable pgbench session should last long enough to average that out. Backpatch to 9.4. --- contrib/pgbench/pgbench.c | 33 ++++++++++++++++++++++----------- 1 file changed, 22 insertions(+), 11 deletions(-) diff --git a/contrib/pgbench/pgbench.c b/contrib/pgbench/pgbench.c index c5c082c938..4001a98f34 100644 --- a/contrib/pgbench/pgbench.c +++ b/contrib/pgbench/pgbench.c @@ -552,6 +552,25 @@ getGaussianRand(TState *thread, int64 min, int64 max, double threshold) return min + (int64)((max - min + 1) * rand); } +/* + * random number generator: generate a value, such that the series of values + * will approximate a Poisson distribution centered on the given value. + */ +static int64 +getPoissonRand(TState *thread, int64 center) +{ + /* + * Use inverse transform sampling to generate a value > 0, such that the + * expected (i.e. average) value is the given argument. + */ + double uniform; + + /* erand in [0, 1), uniform in (0, 1] */ + uniform = 1.0 - pg_erand48(thread->random_state); + + return (int64) (-log(uniform) * ((double) center) + 0.5); +} + /* call PQexec() and exit() on failure */ static void executeStatement(PGconn *con, const char *sql) @@ -1009,21 +1028,13 @@ top: if (throttle_delay && !st->is_throttled) { /* - * Use inverse transform sampling to randomly generate a delay, such - * that the series of delays will approximate a Poisson distribution - * centered on the throttle_delay time. - * - * 10000 implies a 9.2 (-log(1/10000)) to 0.0 (log 1) delay - * multiplier, and results in a 0.055 % target underestimation bias: - * - * SELECT 1.0/AVG(-LN(i/10000.0)) FROM generate_series(1,10000) AS i; - * = 1.000552717032611116335474 + * Generate a delay such that the series of delays will approximate a + * Poisson distribution centered on the throttle_delay time. * * If transactions are too slow or a given wait is shorter than a * transaction, the next transaction will start right away. */ - int64 wait = (int64) (throttle_delay * - 1.00055271703 * -log(getrand(thread, 1, 10000) / 10000.0)); + int64 wait = getPoissonRand(thread, throttle_delay); thread->throttle_trigger += wait; -- 2.40.0