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Arrange to generate different random sequences in the different child

2009年12月11日 21:50:06 +0000 (21:50 +0000)

processes of a pgbench run, when we are using -j > 1 and are emulating
threads via fork(). Otherwise the children all inherit the same random
sequence state and produce the same random-number sequence.

In the threaded case the different threads will share one RNG state, so
they will produce different subsets of one sequence, which is maybe more
correlated than a purist would like but will not be "the same". So we
leave that case alone.

First noticed by Takahiro Itagaki, and is also part of the explanation
for the pgbench misbehavior recently reported by Jaime Casanova.

contrib/pgbench/pgbench.c patch | blob | blame | history

diff --git a/contrib/pgbench/pgbench.c b/contrib/pgbench/pgbench.c

index d41a87cfea85d0fbb5e72c4a4880715540f6c202..b5520754e4a479c6e91355dcd5cd84bdda9b34eb 100644 (file)

--- a/contrib/pgbench/pgbench.c

+++ b/contrib/pgbench/pgbench.c

@@ -4,7 +4,7 @@

* A simple benchmark program for PostgreSQL

* Originally written by Tatsuo Ishii and enhanced by many contributors.

- * $PostgreSQL: pgsql/contrib/pgbench/pgbench.c,v 1.91 2009年09月10日 13:59:57 ishii Exp $

+ * $PostgreSQL: pgsql/contrib/pgbench/pgbench.c,v 1.92 2009年12月11日 21:50:06 tgl Exp $

@@ -1916,7 +1916,7 @@ main(int argc, char **argv)

INSTR_TIME_SET_CURRENT(start_time);

srandom((unsigned int) INSTR_TIME_GET_MICROSEC(start_time));

- /* process bultin SQL scripts */

+ /* process builtin SQL scripts */

switch (ttype)

{

case 0:

@@ -2201,6 +2201,7 @@ pthread_create(pthread_t *thread,

{

fork_pthread *th;

void *ret;

+ instr_time start_time;

th = (fork_pthread *) malloc(sizeof(fork_pthread));

pipe(th->pipes);

@@ -2211,20 +2212,31 @@ pthread_create(pthread_t *thread,

free(th);

return errno;

}

- if (th->pid != 0) /* parent process */

+ if (th->pid != 0) /* in parent process */

{

close(th->pipes[1]);

*thread = th;

return 0;

}

- /* child process */

+ /* in child process */

close(th->pipes[0]);

/* set alarm again because the child does not inherit timers */

if (duration > 0)

setalarm(duration);

+ /*

+ * Set a different random seed in each child process. Otherwise they

+ * all inherit the parent's state and generate the same "random"

+ * sequence. (In the threaded case, the different threads will obtain

+ * subsets of the output of a single random() sequence, which should be

+ * okay for our purposes.)

+ */

+ INSTR_TIME_SET_CURRENT(start_time);

+ srandom(((unsigned int) INSTR_TIME_GET_MICROSEC(start_time)) +

+ ((unsigned int) getpid()));

ret = start_routine(arg);

write(th->pipes[1], ret, sizeof(TResult));

close(th->pipes[1]);

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