Thread: Out of shared memory while creating a backup with pg_dump

Hi,

while creating a backup with pg_dump and parallel write accesses to the database following error occured:
2014-09-24 07:46:46 ... ERROR:  out of shared memory
2014-09-24 07:46:46 ... HINT:  You might need to increase max_pred_locks_per_transaction.

I´m running postgresql 9.3.4 on Ubuntu 12.04 server. The server has 8GB RAM and 4 cores.
/proc/sys/kernel/shmmax: 2147483648
/proc/sys/kernel/shmall: 2097152

The original configuration was changed in only a few relevant points:
* shared_buffers = 128MB
* work_mem = 10MB

We run a hot standby slave, therefore the following changes were necessary:
* wal_level = hot_standby
* max_wal_senders = 1
* wal_keep_segments = 64
* hot_standby = on
* hot_standby_feedback = on


I didn´t understand the HINT:
You might need to increase max_pred_locks_per_transaction.

What is the reason for the error, that the shared memory ran out, or that the number of predicate locks is too small?
If i increase max_pred_locks_per_transaction i get probably even faster an "out of shared memory" error.

What is the best way to fix the problem?

The database consists of approximately 150 tables and is about 140GB in size. Where one table with 130 GB (~280000000
rows)requires the most space. 

Please help me to troubleshoot this severe problem. I will feedback any required information.

Thanks in advance
Christian

________________________________
manroland web systems GmbH -- Managing Director: Joern Gossé
Registered Office: Augsburg -- Trade Register: AG Augsburg -- HRB-No.: 26816 -- VAT: DE281389840

Confidentiality note:
This eMail and any files transmitted with it are confidential and intended solely for the use of the individual or
entityto whom they are addressed. If you are not the intended recipient, you are hereby notified that any use or
disseminationof this communication is strictly prohibited. If you have received this eMail in error, then please delete
thiseMail. 

! Please consider your environmental responsibility before printing this eMail !
________________________________

"christian.echerer@manroland-web.com" <christian.echerer@manroland-web.com> wrote:

> I didn´t understand the HINT:
> You might need to increase max_pred_locks_per_transaction.

That is a configuration setting in postgresql.conf which controls
how much shared memory is reserved at start-up to hold information
on the predicate locks required to make serializable transactions
work.

> What is the reason for the error, that the shared memory ran out,
> or that the number of predicate locks is too small?

The shared memory reserved at start-up for predicate lock
information was too small.

> If i increase max_pred_locks_per_transaction i get probably even
> faster an "out of shared memory" error.

No.

> What is the best way to fix the problem?

Increase max_pred_locks_per_transaction.  If you are using
serializable transactions this usually needs to be increased to see
optimal performance.  The optimal setting depends on data structure
and the nature of the workload.  Increases to 10 or 20 times the
default value are not unusual.

Be sure to review the "for optimal performance" suggestions at the
bottom of this section of the documentation:

http://www.postgresql.org/docs/9.3/interactive/transaction-iso.html#XACT-SERIALIZABLE

If you can declare some transactions which will not be modifying
data as READ ONLY you will probably reduce the number of predicate
locks required.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company