Split 2 of the initial submission
pgbench: reduce and compensate throttling underestimation bias.
This is a consequence of relying on an integer random generator,
which allow to ensure that delays inserted stay reasonably in
range of the target average delay.
The bias was about 0.5% with 1000 distinct values. Multiplier added
to compensate the 0.05% bias with 10000 distinct integer values,
so there is no bias now.
--
Fabien.
diff --git a/contrib/pgbench/pgbench.c b/contrib/pgbench/pgbench.c
index ad8e272..572573a 100644
--- a/contrib/pgbench/pgbench.c
+++ b/contrib/pgbench/pgbench.c
@@ -929,13 +929,17 @@ top:
* that the series of delays will approximate a Poisson distribution
* centered on the throttle_delay time.
*
- * 1000 implies a 6.9 (-log(1/1000)) to 0.0 (log 1.0) delay multiplier.
+ * 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
*
* 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 * -log(getrand(thread, 1, 1000)/1000.0);
+ int64 wait = (int64) (throttle_delay *
+ 1.00055271703 * -log(getrand(thread, 1, 10000)/10000.0));
thread->throttle_trigger += wait;
--
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