Re: Improving connection scalability: GetSnapshotData()

From: Andres Freund <andres(at)anarazel(dot)de>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Daniel Gustafsson <daniel(at)yesql(dot)se>, "Jonathan S(dot) Katz" <jkatz(at)postgresql(dot)org>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Peter Geoghegan <pg(at)bowt(dot)ie>, Bruce Momjian <bruce(at)momjian(dot)us>, David Rowley <dgrowleyml(at)gmail(dot)com>
Subject: Re: Improving connection scalability: GetSnapshotData()
Date: 2020-08-16 20:52:58
Message-ID: 20200816205258.pjvmoljrk7ntqb3n@alap3.anarazel.de
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Hi,

On 2020-08-16 13:31:53 -0700, Andres Freund wrote:
> I now luckily have a rr trace of the problem, so I hope I can narrow it
> down to the original problem fairly quickly.

Gna, I think I see the problem. In at least one place I wrongly
accessed the 'dense' array of in-progress xids using the 'pgprocno',
instead of directly using the [0...procArray->numProcs) index.

Working on a fix, together with some improved asserts.

Greetings,

Andres Freund

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