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Commitc8b2499

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Simplify calculation of Poisson distributed delays in pgbench --rate mode.
The previous coding first generated a uniform random value between 0.0 and1.0, then converted that to an integer between 1 and 10000, and divided thatagain by 10000. Those conversions are unnecessary; we can use the doublevalue that pg_erand48() returns directly. While we're at it, put the logicinto a helper function, getPoissonRand().The largest delay generated by the old coding was about 9.2 times theaverage, because of the way the uniformly distributed value used for thecalculation was truncated to 1/10000 granularity. The new coding doesn'thave such clamping. With my laptop's DBL_MIN value, the maximum delay withthe new coding is about 700x the average. That seems acceptable - anyreasonable pgbench session should last long enough to average that out.Backpatch to 9.4.
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‎contrib/pgbench/pgbench.c

Lines changed: 22 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -476,6 +476,25 @@ getrand(TState *thread, int64 min, int64 max)
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returnmin+ (int64) ((max-min+1)*pg_erand48(thread->random_state));
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}
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/*
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* random number generator: generate a value, such that the series of values
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* will approximate a Poisson distribution centered on the given value.
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*/
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staticint64
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getPoissonRand(TState*thread,int64center)
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{
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/*
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* Use inverse transform sampling to generate a value > 0, such that the
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* expected (i.e. average) value is the given argument.
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*/
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doubleuniform;
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/* erand in [0, 1), uniform in (0, 1] */
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uniform=1.0-pg_erand48(thread->random_state);
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return (int64) (-log(uniform)* ((double)center)+0.5);
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}
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/* call PQexec() and exit() on failure */
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staticvoid
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executeStatement(PGconn*con,constchar*sql)
@@ -933,21 +952,13 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile, AggVa
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if (throttle_delay&& !st->is_throttled)
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{
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/*
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* Use inverse transform sampling to randomly generate a delay, such
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* that the series of delays will approximate a Poisson distribution
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* centered on the throttle_delay time.
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*
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* 10000 implies a 9.2 (-log(1/10000)) to 0.0 (log 1) delay
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* multiplier, and results in a 0.055 % target underestimation bias:
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*
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* SELECT 1.0/AVG(-LN(i/10000.0)) FROM generate_series(1,10000) AS i;
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* = 1.000552717032611116335474
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* Generate a delay such that the series of delays will approximate a
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* Poisson distribution centered on the throttle_delay time.
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*
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* If transactions are too slow or a given wait is shorter than a
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* transaction, the next transaction will start right away.
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*/
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int64wait= (int64) (throttle_delay*
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1.00055271703*-log(getrand(thread,1,10000) /10000.0));
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int64wait=getPoissonRand(thread,throttle_delay);
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thread->throttle_trigger+=wait;
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