Re: xid_wraparound tests intermittent failure.

On Tue, Jul 23, 2024 at 3:49 AM Andrew Dunstan <andrew@dunslane.net> wrote:
>
>
> On 2024-07-22 Mo 9:29 PM, Masahiko Sawada wrote:
>
> On Mon, Jul 22, 2024 at 12:53 PM Andrew Dunstan <andrew@dunslane.net> wrote:
>
> On 2024-07-22 Mo 12:46 PM, Tom Lane wrote:
>
> Masahiko Sawada <sawada.mshk@gmail.com> writes:
>
> Looking at dodo's failures, it seems that while it passes
> module-xid_wraparound-check, all failures happened only during
> testmodules-install-check-C. Can we check the server logs written
> during xid_wraparound test in testmodules-install-check-C?
>
> Oooh, that is indeed an interesting observation.  There are enough
> examples now that it's hard to dismiss it as chance, but why would
> the two runs be different?
>
>
> It's not deterministic.
>
> I tested the theory that it was some other concurrent tests causing the issue, but that didn't wash. Here's what I
did:
>
>     for f in `seq 1 100`
>       do echo iteration = $f
>       meson test --suite xid_wraparound || break
>     done
>
> It took until iteration 6 to get an error. I don't think my Ubuntu instance is especially slow. e.g. "meson compile"
normallytakes a handful of seconds. Maybe concurrent tests make it more likely, but they can't be the only cause. 
>
> Could you provide server logs in both OK and NG tests? I want to see
> if there's a difference in the rate at which tables are vacuumed.
>
>
> See <https://bitbucket.org/adunstan/rotfang-fdw/downloads/xid-wraparound-result.tar.bz2>
>
>
> The failure logs are from a run where both tests 1 and 2 failed.
>

Thank you for sharing the logs.

I think that the problem seems to match what Alexander Lakhin
mentioned[1]. Probably we can fix such a race condition somehow but
I'm not sure it's worth it as setting autovacuum = off and
autovacuum_max_workers = 1 (or a low number) is an extremely rare
case. I think it would be better to stabilize these tests. One idea is
to turn the autovacuum GUC parameter on while setting
autovacuum_enabled = off for each table. That way, we can ensure that
autovacuum workers are launched. And I think it seems to align real
use cases.

Regards,

[1] https://www.postgresql.org/message-id/02373ec3-50c6-df5a-0d65-5b9b1c0c86d6%40gmail.com

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com