Horton

Shared memory leak on DSM slot exhaustion

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2020-01-30T09:53:38Z
Lists: pgsql-hackers

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Hello,

As reported over on pgsql-general[1], we leak shared memory when we
run out of DSM slots.  To see this, add the random-run-out-of-slots
hack I showed in that thread, create and analyze a table t(i) with a
million integers, run with dynamic_shared_memory_type=mmap, and try
SELECT COUNT(*) FROM t t1 JOIN t t2 USING (i) a few times and you'll
see that pgbase/pg_dynshmem fills up with leaked memory segments each
time an out-of-slots errors is raised.  (It happens with all DSM
types, but then the way to list the segments varies or there isn't
one, depending on type and OS.)  Here's a draft patch to fix that.

[1] https://www.postgresql.org/message-id/CA%2BhUKG%2Bzw87b70yJp%2BOzz6LqS6s9QvdO4%2BhQuZc%3DDWLMi6Od6A%40mail.gmail.com

Commits

  1. Fix memory leak on DSM slot exhaustion.