Re: DSM segment handle generation in background workers - Mailing list pgsql-hackers

On Sat, Oct 13, 2018 at 11:45:17PM +1300, Thomas Munro wrote:
> +    /* Set a different seed for random() in every backend. */
> +    srandom((unsigned int) MyProcPid ^ (unsigned int) MyStartTimestamp);

> -    TimestampDifference(0, port->SessionStartTime, &secs, &usecs);
> -    srandom((unsigned int) (MyProcPid ^ (usecs << 12) ^ secs));

Compared to the old code, the new code requires more wall time to visit every
possible seed value.  New code xor's the PID bits into the fastest-changing
timestamp bits, so only about twenty bits can vary within any given one-second
period.  (That assumes a PID space of twenty or fewer bits; fifteen bits is
the Linux default.)  Is that aspect of the change justified?