Hi,
pondering
http://archives.postgresql.org/message-id/CA%2BTgmoZJdA6K7-17K4A48rVB0UPR98HVuaNcfNNLrGsdb1uChg%40mail.gmail.com
et al I was wondering why it's a good idea for pgbench to doINSTR_TIME_SET_CURRENT(start_time);srandom((unsigned int)
INSTR_TIME_GET_MICROSEC(start_time));
to initialize randomness and thenfor (i = 0; i < nthreads; i++) thread->random_state[0] = random();
thread->random_state[1]= random(); thread->random_state[2] = random();
to initialize the individual thread random state which is then used by
pg_erand48().
To me it seems better to instead initialize srandom() with a known value
(say, uh, 0). Or even better don't use random() at all, and fill a
global pg_erand48() with a known state; and use pg_erand48() to
initialize the thread states.
Obviously that doesn't make pgbench entirely reproducible, but it seems
a lot better than now. Individual threads would do work in a
reproducible order.
I see very little reason to have the current behaviour, or at the very
least not by default.
Greetings,
Andres Freund
