From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@enterprisedb.com>
cc: Noah Misch <noah@leadboat.com>, Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: DSM segment handle generation in background workers
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Comments: In-reply-to Thomas Munro <thomas.munro@enterprisedb.com>
	message dated "Mon, 12 Nov 2018 23:11:19 +1300"
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Date: Mon, 12 Nov 2018 09:39:01 -0500
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Thomas Munro <thomas.munro@enterprisedb.com> writes:
> On Mon, Nov 12, 2018 at 9:34 PM Noah Misch <noah@leadboat.com> wrote:
>> 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-chan=
ging
>> 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 bit=
s 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?

I doubt that's a good idea; to a first approximation, it would mean that
half the seed depends only on the PID and the other half only on the
timestamp.  Maybe we could improve matters a little by left-shifting the
PID four bits or so, but I think we still want it to mix with some
rapidly-changing time bits.

I'm not really sure that we need to do anything though.  Basically,
what we've got here is a tradeoff between how many bits change over
a given timespan and how unpredictable those bits are.  I don't see
that one of those is necessarily more important than the other.

			regards, tom lane