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Re: testing HS/SR - 1 vs 2 performance

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Marko Kreen <markokr(at)gmail(dot)com>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Erik Rijkers <er(at)xs4all(dot)nl>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: testing HS/SR - 1 vs 2 performance
Date: 2010-04-23 17:38:48
Message-ID: 12382.1272044328@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Marko Kreen <markokr(at)gmail(dot)com> writes:
> Um, you have been burned by exactly this on x86 also:
>   http://archives.postgresql.org/pgsql-hackers/2009-03/msg01265.php

Yeah, we never did figure out exactly how come you were observing that
failure on Intel-ish hardware.  I was under the impression that Intel
machines didn't have weak-memory-ordering behavior.

I wonder whether your compiler had rearranged the code in ProcArrayAdd
so that the increment happened before the array element store at the
machine-code level.  I think it would be entitled to do that under
standard C semantics, since that ProcArrayStruct pointer isn't marked
volatile.

			regards, tom lane

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