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: 2020-06-03T23:11:41Z
Lists: pgsql-bugs 
On Wed, Jun 3, 2020 at 2:35 PM Kyle Kingsbury <aphyr@jepsen.io> wrote:
> It looks like you're seeing a much higher txn success rate than I am--possibly due to your tuning? Might be worth adjusting --rate and/or --concurrency upwards

I can see what I assume is the same problem (a failure/table flip and
a huge graph) with "--concurrency 150 -r 10000", and with autovacuum
disabled on the Postgres side (this is the same relatively tuned
Postgres configuration that I used when Jepsen passed for me). It's
difficult to run the tests, so it's hard to isolate without it taking
a long time.

BTW, the tests are kind of flappy. The Linux OOM killer just killed
Java after 20 minutes or so, for example. I assume that this is to be
expected with the settings cranked up like this -- the analysis will
take longer and use more memory, too. Any tips on limiting that? Is
there any reason to think that running the same test twice will affect
the outcome of the second test?

I also see this sometimes, even though I thought I fixed it earlier --
it seems to happen at random:

Caused by: java.lang.AssertionError: Assert failed: No transaction wrote 8363 2
t2

The fact that Kyle saw such a high number of failed transactions,
which are difficult to reproduce here seems to suggest that the issue
is related to running out of shared memory for predicate locks and/or
bloat (which tends to have the side effect of increasing the need for
predicate locks). I continue to suspect that this is related to an
edge case with predicate locks. It could be related to running out of
predicate locks -- maybe an issue with the lock escalation? That would
tend to increase the number of failures by quite a lot.

-- 
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.