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Commit5b7e036

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Avoid unnecessary precision loss for pgbench's --rate target.
It's fairly silly to truncate the throttle_delay to integer when the onlymath we ever do with it requires converting back to double. Furthermore,given that people are starting to complain about restrictions like onlysupporting 1K client connections, I don't think we're very far away fromsituations where the precision loss matters. As the code stood, forexample, there's no difference between --rate 100001 and --rate 111111;both get converted to throttle_delay = 9. Somebody trying to run 100threads and have each one dispatch around 1K TPS would find this lack ofprecision rather surprising, especially since the required per-threaddelays are around 1ms, well within the timing precision of modern systems.
1 parent64171b3 commit5b7e036

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‎src/bin/pgbench/pgbench.c

Lines changed: 13 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -169,7 +169,7 @@ doublesample_rate = 0.0;
169169
* When threads are throttled to a given rate limit, this is the target delay
170170
* to reach that rate in usec. 0 is the default and means no throttling.
171171
*/
172-
int64throttle_delay=0;
172+
doublethrottle_delay=0;
173173

174174
/*
175175
* Transactions which take longer than this limit (in usec) are counted as
@@ -826,9 +826,12 @@ getGaussianRand(TState *thread, int64 min, int64 max, double parameter)
826826
/*
827827
* random number generator: generate a value, such that the series of values
828828
* will approximate a Poisson distribution centered on the given value.
829+
*
830+
* Individual results are rounded to integers, though the center value need
831+
* not be one.
829832
*/
830833
staticint64
831-
getPoissonRand(TState*thread,int64center)
834+
getPoissonRand(TState*thread,doublecenter)
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{
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/*
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* Use inverse transform sampling to generate a value > 0, such that the
@@ -839,7 +842,7 @@ getPoissonRand(TState *thread, int64 center)
839842
/* erand in [0, 1), uniform in (0, 1] */
840843
uniform=1.0-pg_erand48(thread->random_state);
841844

842-
return (int64) (-log(uniform)*((double)center)+0.5);
845+
return (int64) (-log(uniform)*center+0.5);
843846
}
844847

845848
/* helper function for getZipfianRand */
@@ -5114,8 +5117,8 @@ main(int argc, char **argv)
51145117
fprintf(stderr,"invalid rate limit: \"%s\"\n",optarg);
51155118
exit(1);
51165119
}
5117-
/* Invert rate limit intoa time offset */
5118-
throttle_delay=(int64) (1000000.0 /throttle_value);
5120+
/* Invert rate limit intoper-transaction delay in usec */
5121+
throttle_delay=1000000.0 /throttle_value;
51195122
}
51205123
break;
51215124
case'L':
@@ -5239,7 +5242,11 @@ main(int argc, char **argv)
52395242
if (nthreads>nclients)
52405243
nthreads=nclients;
52415244

5242-
/* compute a per thread delay */
5245+
/*
5246+
* Convert throttle_delay to a per-thread delay time. Note that this
5247+
* might be a fractional number of usec, but that's OK, since it's just
5248+
* the center of a Poisson distribution of delays.
5249+
*/
52435250
throttle_delay *=nthreads;
52445251

52455252
if (argc>optind)

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