From: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
---|---|
To: | PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Robert Haas <robertmhaas(at)gmail(dot)com> |
Subject: | strange parallel query behavior after OOM crashes |
Date: | 2017-03-30 18:38:55 |
Message-ID: | 6dd5675f-ef4c-fb3c-3b0c-c2a759fd631e@2ndquadrant.com |
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Lists: | pgsql-hackers |
Hi,
While doing some benchmarking, I've ran into a fairly strange issue with
OOM breaking LaunchParallelWorkers() after the restart. What I see
happening is this:
1) a query is executed, and at the end of LaunchParallelWorkers we get
nworkers=8 nworkers_launched=8
2) the query does a Hash Aggregate, but ends up eating much more memory
due to n_distinct underestimate (see [1] from 2015 for details), and
gets killed by OOM
3) the server restarts, the query is executed again, but this time we
get in LaunchParallelWorkers
nworkers=8 nworkers_launched=0
There's nothing else running on the server, and there definitely should
be free parallel workers.
4) The query gets killed again, and on the next execution we get
nworkers=8 nworkers_launched=8
again, although not always. I wonder whether the exact impact depends on
OOM killing the leader or worker, for example.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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