Re: [COMMITTERS] pgsql: Fix freezing of a dead HOT-updated tuple

On Fri, Oct 6, 2017 at 10:49 AM, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> I can tell that, in 9.6, REINDEX still reports the error we saw in
> earlier releases, after some of the runs of my reproducer scripts. I'm
> unable to reproduce it anymore in 9.3 to 9.5. I can't see the one Dan
> originally reported anywhere, either.

You mean the enhanced stress-test that varied fillfactor, added filler
columns, and so on [1]? Can you post that to the list, please? I think
that several of us would like to have a reproducible test case.

> I don't know if it's really the freeze map at fault or something else.

Ideally, it would be possible to effectively disable the new freeze
map stuff in a minimal way, for testing purposes. Perhaps the authors
of that patch, CC'd, can suggest a way to do that.

If I had to guess, I'd say that it's just as likely that the issue is
only reproducible on 9.6 because of the enhancements added in that
release that improved buffer pinning (the use of atomic ops to pin
buffers, moving buffer content locks into buffer descriptors, etc). It
was already a bit tricky to get the problem that remained after
20b6552 but before today's a5736bf to reproduce with Dan's script. It
often took me 4 or 5 attempts. (I wonder what it looks like with your
enhanced version of that script -- the one that I just asked about.)

It seems possible that we've merely reduced the window for the race to
the point that it's practically (though not theoretically) impossible
to reproduce the problem on versions < 9.6, though not on 9.6+.
Applying Occam's razor, the problem doesn't seem particularly likely
to be in the freeze map stuff, which isn't actually all that closely
related.

[1] https://postgr.es/m/20171005162402.jahqflf3mekileqm@alvherre.pgsql
--
Peter Geoghegan