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Commit26a76cb

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Restrict pgbench's zipfian parameter to ensure good performance.
Remove the code that supported zipfian distribution parameters lessthan 1.0, as it had undocumented performance hazards, and it's notclear that the case is useful enough to justify either fixing ordocumenting those hazards.Also, since the code path for parameter > 1.0 could perform badlyfor values very close to 1.0, establish a minimum allowed valueof 1.001. This solution seems superior to the previous vaguedocumentation warning about small values not performing well.Fabien Coelho, per a gripe from Tomas VondraDiscussion:https://postgr.es/m/b5e172e9-ad22-48a3-86a3-589afa20e8f7@2ndquadrant.com
1 parent4fd05bb commit26a76cb

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3 files changed

+32
-193
lines changed

3 files changed

+32
-193
lines changed

‎doc/src/sgml/ref/pgbench.sgml‎

Lines changed: 11 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -1543,29 +1543,17 @@ f(x) = PHI(2.0 * parameter * (x - mu) / (max - min + 1)) /
15431543
middle quarter (1.0 / 4.0) of the interval (i.e. from
15441544
<literal>3.0 / 8.0</literal> to <literal>5.0 / 8.0</literal>) and 95% from
15451545
the middle half (<literal>2.0 / 4.0</literal>) of the interval (second and third
1546-
quartiles). The minimum <replaceable>parameter</replaceable> is 2.0 for performance
1547-
of the Box-Muller transform.
1546+
quartiles). The minimumallowed<replaceable>parameter</replaceable>
1547+
value is 2.0.
15481548
</para>
15491549
</listitem>
15501550
<listitem>
15511551
<para>
1552-
<literal>random_zipfian</literal> generates an approximated bounded Zipfian
1553-
distribution. For <replaceable>parameter</replaceable> in (0, 1), an
1554-
approximated algorithm is taken from
1555-
"Quickly Generating Billion-Record Synthetic Databases",
1556-
Jim Gray et al, SIGMOD 1994. For <replaceable>parameter</replaceable>
1557-
in (1, 1000), a rejection method is used, based on
1558-
"Non-Uniform Random Variate Generation", Luc Devroye, p. 550-551,
1559-
Springer 1986. The distribution is not defined when the parameter's
1560-
value is 1.0. The function's performance is poor for parameter values
1561-
close and above 1.0 and on a small range.
1562-
</para>
1563-
<para>
1552+
<literal>random_zipfian</literal> generates a bounded Zipfian
1553+
distribution.
15641554
<replaceable>parameter</replaceable> defines how skewed the distribution
15651555
is. The larger the <replaceable>parameter</replaceable>, the more
15661556
frequently values closer to the beginning of the interval are drawn.
1567-
The closer to 0 <replaceable>parameter</replaceable> is,
1568-
the flatter (more uniform) the output distribution.
15691557
The distribution is such that, assuming the range starts from 1,
15701558
the ratio of the probability of drawing <replaceable>k</replaceable>
15711559
versus drawing <replaceable>k+1</replaceable> is
@@ -1576,6 +1564,13 @@ f(x) = PHI(2.0 * parameter * (x - mu) / (max - min + 1)) /
15761564
itself is produced <literal>(3/2)*2.5 = 2.76</literal> times more
15771565
frequently than <literal>3</literal>, and so on.
15781566
</para>
1567+
<para>
1568+
<application>pgbench</application>'s implementation is based on
1569+
"Non-Uniform Random Variate Generation", Luc Devroye, p. 550-551,
1570+
Springer 1986. Due to limitations of that algorithm,
1571+
the <replaceable>parameter</replaceable> value is restricted to
1572+
the range [1.001, 1000].
1573+
</para>
15791574
</listitem>
15801575
</itemizedlist>
15811576

‎src/bin/pgbench/pgbench.c‎

Lines changed: 19 additions & 148 deletions
Original file line numberDiff line numberDiff line change
@@ -135,10 +135,10 @@ static intpthread_join(pthread_t th, void **thread_return);
135135
#defineLOG_STEP_SECONDS5/* seconds between log messages */
136136
#defineDEFAULT_NXACTS10/* default nxacts */
137137

138-
#defineZIPF_CACHE_SIZE15/* cache cells number */
139-
140138
#defineMIN_GAUSSIAN_PARAM2.0/* minimum parameter for gauss */
141-
#defineMAX_ZIPFIAN_PARAM1000/* maximum parameter for zipfian */
139+
140+
#defineMIN_ZIPFIAN_PARAM1.001/* minimum parameter for zipfian */
141+
#defineMAX_ZIPFIAN_PARAM1000.0/* maximum parameter for zipfian */
142142

143143
intnxacts=0;/* number of transactions per client */
144144
intduration=0;/* duration in seconds */
@@ -410,35 +410,6 @@ typedef struct
410410
intecnt;/* error count */
411411
}CState;
412412

413-
/*
414-
* Cache cell for random_zipfian call
415-
*/
416-
typedefstruct
417-
{
418-
/* cell keys */
419-
doubles;/* s - parameter of random_zipfian function */
420-
int64n;/* number of elements in range (max - min + 1) */
421-
422-
doubleharmonicn;/* generalizedHarmonicNumber(n, s) */
423-
doublealpha;
424-
doublebeta;
425-
doubleeta;
426-
427-
uint64last_used;/* last used logical time */
428-
}ZipfCell;
429-
430-
/*
431-
* Zipf cache for zeta values
432-
*/
433-
typedefstruct
434-
{
435-
uint64current;/* counter for LRU cache replacement algorithm */
436-
437-
intnb_cells;/* number of filled cells */
438-
intoverflowCount;/* number of cache overflows */
439-
ZipfCellcells[ZIPF_CACHE_SIZE];
440-
}ZipfCache;
441-
442413
/*
443414
* Thread state
444415
*/
@@ -460,8 +431,6 @@ typedef struct
460431

461432
int64throttle_trigger;/* previous/next throttling (us) */
462433
FILE*logfile;/* where to log, or NULL */
463-
ZipfCachezipf_cache;/* for thread-safe zipfian random number
464-
* generation */
465434

466435
/* per thread collected stats */
467436
instr_timestart_time;/* thread start time */
@@ -975,77 +944,12 @@ getPoissonRand(RandomState *random_state, double center)
975944
return (int64) (-log(uniform)*center+0.5);
976945
}
977946

978-
/* helper function for getZipfianRand */
979-
staticdouble
980-
generalizedHarmonicNumber(int64n,doubles)
981-
{
982-
inti;
983-
doubleans=0.0;
984-
985-
for (i=n;i>1;i--)
986-
ans+=pow(i,-s);
987-
returnans+1.0;
988-
}
989-
990-
/* set harmonicn and other parameters to cache cell */
991-
staticvoid
992-
zipfSetCacheCell(ZipfCell*cell,int64n,doubles)
993-
{
994-
doubleharmonic2;
995-
996-
cell->n=n;
997-
cell->s=s;
998-
999-
harmonic2=generalizedHarmonicNumber(2,s);
1000-
cell->harmonicn=generalizedHarmonicNumber(n,s);
1001-
1002-
cell->alpha=1.0 / (1.0-s);
1003-
cell->beta=pow(0.5,s);
1004-
cell->eta= (1.0-pow(2.0 /n,1.0-s)) / (1.0-harmonic2 /cell->harmonicn);
1005-
}
1006-
1007-
/*
1008-
* search for cache cell with keys (n, s)
1009-
* and create new cell if it does not exist
1010-
*/
1011-
staticZipfCell*
1012-
zipfFindOrCreateCacheCell(ZipfCache*cache,int64n,doubles)
1013-
{
1014-
inti,
1015-
least_recently_used=0;
1016-
ZipfCell*cell;
1017-
1018-
/* search cached cell for given parameters */
1019-
for (i=0;i<cache->nb_cells;i++)
1020-
{
1021-
cell=&cache->cells[i];
1022-
if (cell->n==n&&cell->s==s)
1023-
return&cache->cells[i];
1024-
1025-
if (cell->last_used<cache->cells[least_recently_used].last_used)
1026-
least_recently_used=i;
1027-
}
1028-
1029-
/* create new one if it does not exist */
1030-
if (cache->nb_cells<ZIPF_CACHE_SIZE)
1031-
i=cache->nb_cells++;
1032-
else
1033-
{
1034-
/* replace LRU cell if cache is full */
1035-
i=least_recently_used;
1036-
cache->overflowCount++;
1037-
}
1038-
1039-
zipfSetCacheCell(&cache->cells[i],n,s);
1040-
1041-
cache->cells[i].last_used=cache->current++;
1042-
return&cache->cells[i];
1043-
}
1044-
1045947
/*
1046948
* Computing zipfian using rejection method, based on
1047949
* "Non-Uniform Random Variate Generation",
1048950
* Luc Devroye, p. 550-551, Springer 1986.
951+
*
952+
* This works for s > 1.0, but may perform badly for s very close to 1.0.
1049953
*/
1050954
staticint64
1051955
computeIterativeZipfian(RandomState*random_state,int64n,doubles)
@@ -1056,6 +960,10 @@ computeIterativeZipfian(RandomState *random_state, int64 n, double s)
1056960
u,
1057961
v;
1058962

963+
/* Ensure n is sane */
964+
if (n <=1)
965+
return1;
966+
1059967
while (true)
1060968
{
1061969
/* random variates */
@@ -1072,39 +980,16 @@ computeIterativeZipfian(RandomState *random_state, int64 n, double s)
1072980
return (int64)x;
1073981
}
1074982

1075-
/*
1076-
* Computing zipfian using harmonic numbers, based on algorithm described in
1077-
* "Quickly Generating Billion-Record Synthetic Databases",
1078-
* Jim Gray et al, SIGMOD 1994
1079-
*/
1080-
staticint64
1081-
computeHarmonicZipfian(ZipfCache*zipf_cache,RandomState*random_state,
1082-
int64n,doubles)
1083-
{
1084-
ZipfCell*cell=zipfFindOrCreateCacheCell(zipf_cache,n,s);
1085-
doubleuniform=pg_erand48(random_state->xseed);
1086-
doubleuz=uniform*cell->harmonicn;
1087-
1088-
if (uz<1.0)
1089-
return1;
1090-
if (uz<1.0+cell->beta)
1091-
return2;
1092-
return1+ (int64) (cell->n*pow(cell->eta*uniform-cell->eta+1.0,cell->alpha));
1093-
}
1094-
1095983
/* random number generator: zipfian distribution from min to max inclusive */
1096984
staticint64
1097-
getZipfianRand(ZipfCache*zipf_cache,RandomState*random_state,int64min,
1098-
int64max,doubles)
985+
getZipfianRand(RandomState*random_state,int64min,int64max,doubles)
1099986
{
1100987
int64n=max-min+1;
1101988

1102989
/* abort if parameter is invalid */
1103-
Assert(s>0.0&&s!=1.0&&s <=MAX_ZIPFIAN_PARAM);
990+
Assert(MIN_ZIPFIAN_PARAM <=s&&s <=MAX_ZIPFIAN_PARAM);
1104991

1105-
returnmin-1+ ((s>1)
1106-
?computeIterativeZipfian(random_state,n,s)
1107-
:computeHarmonicZipfian(zipf_cache,random_state,n,s));
992+
returnmin-1+computeIterativeZipfian(random_state,n,s);
1108993
}
1109994

1110995
/*
@@ -2426,25 +2311,25 @@ evalStandardFunc(TState *thread, CState *st,
24262311
}
24272312
elseif (func==PGBENCH_RANDOM_ZIPFIAN)
24282313
{
2429-
if (param <=0.0||param==1.0||param>MAX_ZIPFIAN_PARAM)
2314+
if (param<MIN_ZIPFIAN_PARAM||param>MAX_ZIPFIAN_PARAM)
24302315
{
24312316
fprintf(stderr,
2432-
"zipfian parameter must be in range (0, 1) U (1, %d]"
2433-
" (got %f)\n",MAX_ZIPFIAN_PARAM,param);
2317+
"zipfian parameter must be in range [%.3f, %.0f]"
2318+
" (not %f)\n",
2319+
MIN_ZIPFIAN_PARAM,MAX_ZIPFIAN_PARAM,param);
24342320
return false;
24352321
}
2322+
24362323
setIntValue(retval,
2437-
getZipfianRand(&thread->zipf_cache,
2438-
&st->cs_func_rs,
2439-
imin,imax,param));
2324+
getZipfianRand(&st->cs_func_rs,imin,imax,param));
24402325
}
24412326
else/* exponential */
24422327
{
24432328
if (param <=0.0)
24442329
{
24452330
fprintf(stderr,
24462331
"exponential parameter must be greater than zero"
2447-
" (got %f)\n",param);
2332+
" (not %f)\n",param);
24482333
return false;
24492334
}
24502335

@@ -4996,8 +4881,6 @@ printResults(TState *threads, StatsData *total, instr_time total_time,
49964881
tps_include,
49974882
tps_exclude;
49984883
int64ntx=total->cnt-total->skipped;
4999-
inti,
5000-
totalCacheOverflows=0;
50014884

50024885
time_include=INSTR_TIME_GET_DOUBLE(total_time);
50034886

@@ -5025,15 +4908,6 @@ printResults(TState *threads, StatsData *total, instr_time total_time,
50254908
printf("number of transactions actually processed: "INT64_FORMAT"\n",
50264909
ntx);
50274910
}
5028-
/* Report zipfian cache overflow */
5029-
for (i=0;i<nthreads;i++)
5030-
{
5031-
totalCacheOverflows+=threads[i].zipf_cache.overflowCount;
5032-
}
5033-
if (totalCacheOverflows>0)
5034-
{
5035-
printf("zipfian cache array overflowed %d time(s)\n",totalCacheOverflows);
5036-
}
50374911

50384912
/* Remaining stats are nonsensical if we failed to execute any xacts */
50394913
if (total->cnt <=0)
@@ -5916,9 +5790,6 @@ main(int argc, char **argv)
59165790
initRandomState(&thread->ts_sample_rs);
59175791
thread->logfile=NULL;/* filled in later */
59185792
thread->latency_late=0;
5919-
thread->zipf_cache.nb_cells=0;
5920-
thread->zipf_cache.current=0;
5921-
thread->zipf_cache.overflowCount=0;
59225793
initStats(&thread->stats,0);
59235794

59245795
nclients_dealt+=thread->nstate;

‎src/bin/pgbench/t/001_pgbench_with_server.pl‎

Lines changed: 2 additions & 29 deletions
Original file line numberDiff line numberDiff line change
@@ -666,13 +666,13 @@ sub pgbench
666666
[
667667
'set zipfian param to 1',
668668
2,
669-
[qr{zipfian parameter must be in range\(0, 1\) U\(1,\d+\]}],
669+
[qr{zipfian parameter must be in range\[1\.001, 1000\]}],
670670
q{\set i random_zipfian(0, 10, 1)}
671671
],
672672
[
673673
'set zipfian param too large',
674674
2,
675-
[qr{zipfian parameter must be in range\(0, 1\) U\(1,\d+\]}],
675+
[qr{zipfian parameter must be in range\[1\.001, 1000\]}],
676676
q{\set i random_zipfian(0, 10, 1000000)}
677677
],
678678
[
@@ -802,33 +802,6 @@ sub pgbench
802802
{$n=>$script });
803803
}
804804

805-
# zipfian cache array overflow
806-
pgbench(
807-
'-t 1', 0,
808-
[qr{processed: 1/1},qr{zipfian cache array overflowed 1 time\(s\)} ],
809-
[qr{^}],
810-
'pgbench zipfian array overflow on random_zipfian',
811-
{
812-
'001_pgbench_random_zipfian'=>q{
813-
\set i random_zipfian(1, 100, 0.5)
814-
\set i random_zipfian(2, 100, 0.5)
815-
\set i random_zipfian(3, 100, 0.5)
816-
\set i random_zipfian(4, 100, 0.5)
817-
\set i random_zipfian(5, 100, 0.5)
818-
\set i random_zipfian(6, 100, 0.5)
819-
\set i random_zipfian(7, 100, 0.5)
820-
\set i random_zipfian(8, 100, 0.5)
821-
\set i random_zipfian(9, 100, 0.5)
822-
\set i random_zipfian(10, 100, 0.5)
823-
\set i random_zipfian(11, 100, 0.5)
824-
\set i random_zipfian(12, 100, 0.5)
825-
\set i random_zipfian(13, 100, 0.5)
826-
\set i random_zipfian(14, 100, 0.5)
827-
\set i random_zipfian(15, 100, 0.5)
828-
\set i random_zipfian(16, 100, 0.5)
829-
}
830-
});
831-
832805
# throttling
833806
pgbench(
834807
'-t 100 -S --rate=100000 --latency-limit=1000000 -c 2 -n -r',

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