Re: Improving connection scalability: GetSnapshotData()

From: Andres Freund <andres(at)anarazel(dot)de>
To: Ian Barwick <ian(dot)barwick(at)2ndquadrant(dot)com>
Cc: Michael Paquier <michael(at)paquier(dot)xyz>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, 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: 2020-09-14 23:17:18
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On 2020-09-09 17:02:58 +0900, Ian Barwick wrote:
> Attached, though bear in mind I'm not very familiar with parts of this,
> particularly 2PC stuff, so consider it educated guesswork.

Thanks for this, and the test case!

Your commit fixes the issues, but not quite correctly. Multixacts
shouldn't matter, so we don't need to do anything there. And for the
increases, I think they should be inside the already existing
ProcArrayLock acquisition, as in the attached.

I've also included a quite heavily revised version of your test. Instead
of using dblink I went for having a long-running psql that I feed over
stdin. The main reason for not liking the previous version is that it
seems fragile, with the sleep and everything. I expanded it to cover
2PC is as well.

The test probably needs a bit of cleanup, wrapping some of the
redundancy around the pump_until calls.

I think the approach of having a long running psql session is really
useful, and probably would speed up some tests. Does anybody have a good
idea for how to best, and without undue effort, to integrate this into I don't really have a great idea, so I think I'd leave
it with a local helper in the new test?



Attachment Content-Type Size
v2-0001-WIP-fix-and-test-snapshot-behaviour-on-standby.patch text/x-diff 7.6 KB

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