Horton

Re: Potential G2-item cycles under serializable isolation

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Kyle Kingsbury <aphyr@jepsen.io>
Cc: PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
Date: 2020-06-02T23:17:42Z
Lists: pgsql-bugs 
On Tue, Jun 2, 2020 at 10:24 AM Peter Geoghegan <pg@bowt.ie> wrote:
> I'd be surprised if your existing test cases needed any adjustment. My
> guess is that this won't take long.

You said it takes about a minute in your opening e-mail; how
consistent is this? I note from the Postgres logs you provided that
Postgres starts accepting connections at 2020-05-31 18:50:27.580, and
shows its last log message at  2020-05-31 18:51:29.781 PDT. So it's
suspiciously close to *exactly* one minute. Note that
autovacuum_naptime has as its default '1min'. Your workload probably
generates a lot of index bloat, which may tend to cause autovacuum to
want to delete whole B-Tree leaf pages, which impacts predicate
locking.

Could you check what happens when you reduce autovacuum_naptime to
(say) '5s' in postgresql.conf? Does that change make the G2-item cycle
issue manifest itself earlier? And can you discern any pattern like
that yourself?

It seems kind of inconvenient to run Jepsen -- I suppose I could use
Docker or something like that, but I don't have experience with it.
What do you think is the simplest workflow for somebody that just
wants to recreate your result on a Debian system?

-- 
Peter Geoghegan

Commits

  1. Doc: Add references for SI and SSI.

  2. Improve comments for [Heap]CheckForSerializableConflictOut().

  3. Avoid update conflict out serialization anomalies.

  4. Implement genuine serializable isolation level.