Why do pg_upgrade's test use the serial schedule?

Hi,

The test I'm most commonly waiting for when doing a parallel make check
is pg_upgrade. And to a significant degree that's because it uses the
serial installcheck rather than installcheck-parallel.

Is there a reason to not use installcheck-parallel?

serial:
19.42user 7.77system 1:53.23elapsed 24%CPU (0avgtext+0avgdata 111420maxresident)k
parallel:
20.76user 7.72system 1:27.05elapsed 32%CPU (0avgtext+0avgdata 112300maxresident)k

So, a saving of ~25s.


ISTM we also should disable fsyncs for the postmaster instances. Wins
another ~5s on my laptop, and I suspect it makes a larger difference on
some of the buildfarm animals.

Greetings,

Andres Freund