Subject: Re: Re: [COMMITTERS] pgsql: Tune GetSnapshotData()
	during Hot Standby by avoiding loop
From: Simon Riggs <simon@2ndQuadrant.com>
To: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>,
	pgsql-hackers@postgresql.org
In-Reply-To: <4BCC6C37.9090703@enterprisedb.com>
References: <20100418180608.16F067541D1@cvs.postgresql.org>
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	<4BCC1644.70008@enterprisedb.com> <1271667922.8305.17650.camel@ebony>
	<v2y603c8f071004190455gf18633fbx1a666c17c998b1a1@mail.gmail.com>
	<13439.1271687068@sss.pgh.pa.us> <1271687489.8305.18879.camel@ebony>
	<4BCC6C37.9090703@enterprisedb.com>
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Date: Mon, 19 Apr 2010 15:47:25 +0100
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On Mon, 2010-04-19 at 17:44 +0300, Heikki Linnakangas wrote:

> > Choices are
> > 
> > 1. Check RecoveryInProgress() once outside of lock, plus wild rumour of
> > Murphy
> > 
> > 2. Check RecoveryInProgress() before and after holding lock
> > 
> > 3. Check RecoveryInProgress() while holding lock
> 
> 4. Check RecoveryInProgress() once outside of lock, and scan the
> ProcArray anyway, just in case. That's what we did before this patch.
> Document that takenDuringRecovery == true means that the snapshot was
> most likely taken during recovery, but there is some race conditions
> where takenDuringRecovery is true even though the snapshot was taken
> just after recovery finished. AFAICS all of the other current uses of
> takenDuringRecovery work fine with that.

Checking RecoveryInProgress() is much cheaper than scanning the whole
ProcArray, so (4) is definitely worse than 1-3.

-- 
 Simon Riggs           www.2ndQuadrant.com