On Tue, Oct 13, 2020 at 11:05 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Amit Kapila <amit.kapila16@gmail.com> writes:
> >> It is possible that MAXALIGN stuff is playing a role here and or the
> >> background transaction stuff. I think if we go with the idea of
> >> testing spill_txns and spill_count being positive then the results
> >> will be stable. I'll write a patch for that.
>
> Here's our first failure on a MAXALIGN-8 machine:
>
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=grison&dt=2020-10-13%2005%3A00%3A08
>
> So this is just plain not stable. It is odd though. I can
> easily think of mechanisms that would cause the WAL volume
> to occasionally be *more* than the "typical" case. What
> would cause it to be *less*, if MAXALIGN is ruled out?
>
The original theory I have given above [1] which is an interleaved
autovacumm transaction. Let me try to explain in a bit more detail.
Say when transaction T-1 is performing Insert ('INSERT INTO stats_test
SELECT 'serialize-topbig--1:'||g.i FROM generate_series(1, 5000)
g(i);') a parallel autovacuum transaction occurs. The problem as seen
in buildfarm will happen when autovacuum transaction happens after 80%
or more of the Insert is done.
In such a situation we will start decoding 'Insert' first and need to
spill multiple times due to the amount of changes (more than threshold
logical_decoding_work_mem) and then before we encounter Commit of
transaction that performed Insert (and probably some more changes from
that transaction) we will encounter a small transaction (autovacuum
transaction). The decode of that small transaction will send the
stats collected till now which will lead to the problem shown in
buildfarm.
[1] - https://www.postgresql.org/message-id/CAA4eK1Jo0U1oSJyxrdA7i-bOOTh0Hue-NQqdG-CEqwGtDZPjyw%40mail.gmail.com
--
With Regards,
Amit Kapila.