From: "Alfonso Moscato" <alfonso.moscato@merqurio.it>
To: <pgsql-admin@lists.postgresql.org>
Subject: repeated out of shared memory error - not related to
 max_locks_per_transaction
Date: Fri, 20 Jul 2018 13:14:53 +0200
Message-ID: <00a601d4201a$e2291a10$a67b4e30$@merqurio.it>
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We are getting crazy with "out of shared memory" errors, and we can't figure
the reason.
We have postgresql "PostgreSQL 9.6.9 on x86_64-pc-linux-gnu (Ubuntu
9.6.9-2.pgdg16.04+1), compiled by gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0
20160609, 64-bit", the server has 92gb of ram, it is a mixed environment
(mostly OLTP, with some DW), with 100 sessions constantly open (a CRM) and
some long queries run every half an hour.
Everything works fine, except that after 1 day and half we start receiving a
lot of "out of shared memory" messages.
I am sure it is not related with the usual max_locks_per_transaction issue,
because we have set max_locks_per_transaction to 384, and when we receive
these messages we have no more than 50/100 locks totally.
Restarting the server usually works fine for one day and hal more, and then
messages restart.
Looking at the log, we see that this error starts casually, sometimes on
very small queries, returning some kbytes of data.
We have tried a lot of different configurations. we have tried with pgtune
and pgconfig 2.0.

Currently, we have: 
max_connections = 200
shared_buffers = 23GB
effective_cache_size = 69GB
maintenance_work_mem = 2GB
checkpoint_completion_target = 0.9
wal_buffers = 16MB
default_statistics_target = 100
random_page_cost = 4
effective_io_concurrency = 2
work_mem = 60293kB
min_wal_size = 2GB
max_wal_size = 4GB
max_worker_processes = 4
max_parallel_workers_per_gather = 2
max_locks_per_transaction = 384
but we tried with work_mem to 130mb, shared_buffer to a maximum fo 40gb,
effective_cache to 4gb

shared memory limits are very big: 
max number of segments = 4096
max seg size (kbytes) = 18014398509465599
max total shared memory (kbytes) = 18014398442373116
min seg size (bytes) = 1

thanks