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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: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: 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: 2010-04-25 23:18:48
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Lists: pgsql-hackers
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> [ v2 patch ]

I've been studying this some more while making notes for improved
comments, and I've about come to the conclusion that having readers
move the tail pointer (at the end of KnownAssignedXidsGetAndSetXmin)
is overly tricky and probably not a performance improvement anyway.
The code is in fact wrong as it stands: it's off-by-one about setting
the new tail value.  And there's potential for contention with multiple
readers all wanting to move the tail pointer at once.  And most
importantly, KnownAssignedXidsSearch can't move the tail pointer so
we might expend many inefficient searches while never moving the tail

I think we should get rid of that and just have the two functions that
can mark entries invalid (which they must do with exclusive lock)
advance the tail pointer when they invalidate the current tail element.
Then we have the very simple rule that only the startup process ever
changes this data structure.

			regards, tom lane

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