Re: Intermittent buildfarm failures on wrasse

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: pgsql-hackers(at)lists(dot)postgresql(dot)org, Noah Misch <noah(at)leadboat(dot)com>, Peter Geoghegan <pg(at)bowt(dot)ie>, David Rowley <dgrowleyml(at)gmail(dot)com>
Subject: Re: Intermittent buildfarm failures on wrasse
Date: 2022-04-15 16:36:52
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Andres Freund <andres(at)anarazel(dot)de> writes:
> On April 15, 2022 11:23:40 AM EDT, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> The something is the logical replication launcher. In the failing runs,
>> it is advertising xmin = 724 (the post-initdb NextXID) and continues to
>> do so well past the point where tenk1 gets vacuumed.

> That explains it. Before shmstat autovac needed to wait for the stats collector to write out stats. Now it's near instantaneous. So the issue probably existed before, just unlikely to ever be reached.

Um, this is the logical replication launcher, not the autovac launcher.
Your observation that a sleep in get_database_list() reproduces it
confirms that, and I don't entirely see why the timing of the LR launcher
would have changed.

(On thinking about it, I suppose the AV launcher might trigger this
too, but that is not the PID I saw in testing.)

> We can't just ignore database less xmins for non-shared rels, because walsender propagates hot_standby_feedback that way. But we can probably add a flag somewhere indicating whether a database less PGPROC has to be accounted in the horizon for non-shared rels.

Yeah, I was also thinking about a flag in PGPROC being a more reliable
way to do this. Is there anything besides walsenders that should set
that flag?

regards, tom lane

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