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Re: testing HS/SR - 1 vs 2 performance

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Erik Rijkers <er(at)xs4all(dot)nl>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: testing HS/SR - 1 vs 2 performance
Date: 2010-04-27 21:45:53
Message-ID: 1272404753.4161.8355.camel@ebony (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Tue, 2010-04-27 at 17:24 -0400, Tom Lane wrote:

> Isn't the snapshotOldestActiveXid filter in
> RecordKnownAssignedTransactionIds completely wrong/useless/bogus?
> AFAICS, snapshotOldestActiveXid is only set once at the start of
> recovery.  This means it will soon be too old to provide any useful
> filtering.  But what's far worse is that the XID space will eventually
> wrap around, and that test will start filtering *everything*.
> I think we should just lose that test, as well as the variable.

Yes, though it looks like it is still necessary in creating a valid
initial state because otherwise we may have xids in KnownAssigned array
that are already complete. The comment there talks about wasting memory,
though it appears to be a correctness issue.

So perhaps a similar test is required in ProcArrayApplyRecoveryInfo()
but not in RecordKnownAssignedTransactionIds(). That way it is applied,
but only once at initialisation.

 Simon Riggs 

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