Resolving ERROR: out of shared memory in PostgreSQL

Introduction

Encountering the ERROR: out of shared memory in PostgreSQL can be frustrating and halt the execution of your queries. This error typically occurs when the server runs out of shared memory or semaphore slots required for locks and other operations. In this blog, we will explore the causes of this error and provide step-by-step solutions to resolve it.

Understanding the Cause

The out of shared memory error occurs when PostgreSQL exhausts the available shared memory or semaphore slots. This can happen due to various reasons such as:

• Running out of shared buffers or max_locks_per_transaction.
• Too many active connections or transactions.
• Inadequate configuration settings for PostgreSQL memory parameters.

Step-by-Step Solutions with Examples

To resolve the out of shared memory error, you need to adjust the PostgreSQL configuration settings and ensure efficient memory usage. Below are the steps to do this, along with examples:

1. Increase max_locks_per_transaction

The max_locks_per_transaction parameter controls the maximum number of locks that can be held by each transaction. Increasing this value can help resolve the error. By default, this parameter is set to 64. Depending on the complexity of your transactions, you might need to increase it:

-- Open your postgresql.conf file and increase the value of max_locks_per_transaction
max_locks_per_transaction = 128  -- Example value, adjust as necessary

-- After making changes, restart the PostgreSQL server
sudo systemctl restart postgresql

You can also set this parameter on the fly, but it will only apply to new connections:

ALTER SYSTEM SET max_locks_per_transaction = '128';

2. Increase shared_buffers

The shared_buffers parameter controls the amount of memory allocated for shared memory buffers. Increasing this value can help alleviate the error. The default value is often too low for production systems, so you may need to adjust it:

-- Open your postgresql.conf file and increase the value of shared_buffers
shared_buffers = 512MB  -- Example value, adjust as necessary

-- After making changes, restart the PostgreSQL server
sudo systemctl restart postgresql

Note that setting this too high can have negative effects if your system does not have sufficient RAM. The rule of thumb is to set it to 25% of the available memory on the server.

3. Monitor and Optimize Active Connections

High numbers of active connections can exhaust shared memory. Use the pg_stat_activity view to monitor active connections and terminate unnecessary ones:

-- View active connections
SELECT * FROM pg_stat_activity;

-- Terminate an unnecessary connection
SELECT pg_terminate_backend(pid) FROM pg_stat_activity WHERE pid = ;

Additionally, consider using connection pooling to manage your connections more efficiently. Tools like PgBouncer can help in maintaining an optimal number of active connections to the database.

4. Adjust Kernel Parameters (Linux)

On Linux systems, you may need to adjust kernel parameters to increase the available shared memory and semaphores:

-- Add the following lines to /etc/sysctl.conf
kernel.shmmax = 536870912  # Maximum shared segment size (bytes)
kernel.shmall = 2097152    # Total shared memory (pages)
kernel.sem = 250 32000 100 128  # Semaphore settings

-- Apply the changes
sudo sysctl -p

These changes will help ensure that your system can allocate enough shared memory for PostgreSQL operations.

Conclusion

Resolving the ERROR: out of shared memory in PostgreSQL involves understanding the underlying cause and adjusting the configuration settings accordingly. By following the steps and examples provided in this blog, you can ensure that your PostgreSQL server runs smoothly without encountering this error.

Always monitor your PostgreSQL server's performance and make necessary adjustments to the configuration settings to maintain optimal performance and reliability. Regular maintenance and performance tuning can prevent many common issues, including running out of shared memory.

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