Re: DSM segment handle generation in background workers

On Mon, Nov 12, 2018 at 9:34 PM Noah Misch <noah(at)leadboat(dot)com> wrote:
> 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?

Hmm, right. How about applying pg_bswap32() to one of the terms, as
an easy approximation of reversing the bits?

--
Thomas Munro
http://www.enterprisedb.com