Re: New instability in stats regression test

On Mon, Nov 27, 2023 at 02:01:51PM -0500, Tom Lane wrote:
> The problem as I see it is that this test:
> 
> SELECT :io_stats_post_reset < :io_stats_pre_reset;
> 
> requires an assumption that less I/O has happened since the commanded
> reset action than happened before it (extending back to the previous
> reset, or cluster start).  Since concurrent processes might be doing
> I/O, this has a race condition.  If we are slow enough about obtaining
> :io_stats_post_reset, the test *will* fail eventually.  But the shorter
> the distance back to the previous reset, the bigger the odds of
> observable trouble; thus Michael's concern that adding more reset
> tests in future would increase the risk of failure.

The new reset added just before checking the contents of pg_stat_io
reduces :io_stats_pre_reset from 7M to 50k.  That's a threshold easy
to reach if you have a checkpoint or an autovacuum running in
parallel.  I have not checked the buildfarm logs in details, but I'd
put a coin on a checkpoint triggered by time if the issue happened on
a slow machine.
--
Michael