Deadlock between VACUUM and ALTER TABLE commands - Mailing list pgsql-general
| From | Kevin Burke |
|---|---|
| Subject | Deadlock between VACUUM and ALTER TABLE commands |
| Date | |
| Msg-id | CAEYV4pZzdb6B4gsxaAnAYO35ozkSA2=Le5bR-sF_dEi68a=X7Q@mail.gmail.com Whole thread Raw |
| Responses |
Re: Deadlock between VACUUM and ALTER TABLE commands
|
| List | pgsql-general |
Hello,
Occasionally our test suite gets into a deadlocked state. I was curious why this happens, and what our options are for dealing with it.
We run ~2900 tests on an Ubuntu machine in sequence against a Postgres 9.4.6 database. There are about 60 tables; each test runs ~3 queries, and the largest test probably inserts ~1000 rows. To ensure each test starts with a clean slate, we run the following query in between each test run:
ALTER TABLE table1 DISABLE TRIGGER ALL; DELETE FROM table1; ALTER TABLE table1 ENABLE TRIGGER ALL;
ALTER TABLE table2 DISABLE TRIGGER ALL; DELETE FROM table2; ALTER TABLE table2 ENABLE TRIGGER ALL;
...
ALTER TABLE table60 DISABLE TRIGGER ALL; DELETE FROM table60; ALTER TABLE table60 ENABLE TRIGGER ALL;
ALTER SEQUENCE <sequence> RESTART WITH 1;
--
We also have a check at the end of every test run that the test has returned all database connections to the pool, so I don't believe there are any other database queries in flight at this time.
Why mess around with DISABLE/ENABLE TRIGGER instead of TRUNCATE? We observed that TRUNCATE took about 200ms, but this method takes about 13ms. Over a our test suite TRUNCATE is going to more or less double the length of the suite.
We could resolve some kind of foreign key dependency tree, issue the DELETE FROM's in the right order, and skip DISABLE TRIGGER etc., but haven't invested the time to do this or figured out how to maintain it/update it as we add new tables.
We could also run each test in a transaction and issue a ROLLBACK at the end, unfortunately our ORM does not support transactions.
We observed that when we pushed tests to a third-party CI service, maybe 1 in 100 test runs fails with a mysterious timeout (set to 18 seconds). It doesn't seem to correlate with any one test.
I finally figured out how to enable the right logging and export our database logs from the remote machine. We saw the following results: https://gist.github.com/kevinburkeshyp/4f5eef58411b1c3c1160a6f6008135b8. I've also pasted relevant parts of the Postgres configuration at this URL.
Specifically we kick off the ALTER TABLE ... DELETE, a second later (our deadlock_timeout) a log message appears that the ALTER TABLE is waiting for an AccessExclusiveLock held by process 16936... which is later revealed to be an autovacuum process of the "events" table.
(The events table also gets cleared between every test. It has 5 columns that store less-than-60-byte integers, and one created_at timestamp. There are indexes on the primary key and on LOWER(object_id), another column. We need to put indexes on the LOWER() fields because of our ORM.)
Finally, the test timeout causes the application layer to start closing connections, at which point the ALTER TABLE gains the exclusive lock.
Here are my questions:
- An AccessExclusiveLock seems to indicate that the vacuum is running a VACUUM FULL, if I am reading the documentation correctly. Yet the documentation for autovacuum states that "The autovacuum daemon attempts to work this way, and in fact will never issue VACUUM FULL." The statements from the logs and from the documentation seem to be in conflict; am I missing something?
- Why does vacuum take so long/hang? I would expect it to be a fast operation.
- Should I just turn off autovacuum altogether? If not, how should I tune it? Our test run is 10k queries, 6 minutes, each of the ~3k tests runs that DELETE FROM query listed above, and maybe an average of 2 other queries.
- What other information can I/should I log here?
- Why don't we run into this more often?
kevin
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