Date: Mon, 12 Nov 2018 08:33:58 +0000
From: Noah Misch <noah@leadboat.com>
To: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: DSM segment handle generation in background workers
Message-ID: <20181112083358.GA1073796@rfd.leadboat.com>
References: 
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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?