| From: | Noah Misch <noah(at)leadboat(dot)com> |
|---|---|
| To: | Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: DSM segment handle generation in background workers |
| Date: | 2018-11-12 08:33:58 |
| Message-ID: | 20181112083358.GA1073796@rfd.leadboat.com |
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| Lists: | 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?
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