How to Fix Out of Shared Memory Error in PostgreSQL

The out of shared memory error in PostgreSQL happens when the server runs out of memory allocated for internal locks and other shared resources. To fix it, increase the max_locks_per_transaction setting in postgresql.conf and restart the server to allocate more shared memory for locks.

Why This Happens

This error occurs because PostgreSQL uses a fixed amount of shared memory to manage locks and other internal structures. When many transactions hold locks or when many objects are locked, the allocated shared memory for locks is exhausted. This causes PostgreSQL to throw the out of shared memory error.

For example, if your application opens many tables or objects in a single transaction, the default lock memory may not be enough.

sql

BEGIN;
LOCK TABLE table1 IN ACCESS EXCLUSIVE MODE;
LOCK TABLE table2 IN ACCESS EXCLUSIVE MODE;
-- Imagine many more LOCK TABLE commands here
COMMIT;

Output

ERROR:  out of shared memory
DETAIL:  Failed on request for lock.
HINT:  You might need to increase max_locks_per_transaction.

The Fix

To fix this error, increase the max_locks_per_transaction setting in your postgresql.conf file. This setting controls how many locks can be held per transaction. Increasing it allocates more shared memory for locks.

After changing the setting, restart PostgreSQL for the change to take effect.

bash

# In postgresql.conf
max_locks_per_transaction = 128  # increase from default 64

# Then restart PostgreSQL server
# On Linux systems, for example:
sudo systemctl restart postgresql

Output

PostgreSQL server restarted successfully.

# Now the previous locking commands run without error.

Prevention

To avoid this error in the future:

• Monitor your transactions to keep them short and avoid locking many objects at once.
• Regularly review and tune max_locks_per_transaction based on your workload.
• Use connection pooling to reduce long-running transactions.
• Consider application design changes to reduce lock contention.

Related Errors

Similar errors you might see include:

• out of memory: General memory exhaustion, fixed by increasing system memory or PostgreSQL memory settings.
• too many connections: Too many client connections, fixed by lowering max_connections or using connection pooling.
• deadlock detected: Conflicting locks causing a deadlock, fixed by reviewing transaction logic.

Key Takeaways

Increase max_locks_per_transaction in postgresql.conf to fix out of shared memory errors.

Restart PostgreSQL after changing configuration for the fix to apply.

Keep transactions short and avoid locking many objects simultaneously.

Monitor and tune PostgreSQL settings based on your workload.

Use connection pooling to reduce lock and memory pressure.