503 Service Unavailable error with PHP-FPM

Symptoms

Your site encounters 503 errors, and an entry similar to the following can be found in the PHP-FPM error log.

CONFIG_TEXT: [01-Jan-1970 00:00:00] WARNING: [pool domain_tld] server reached max_children setting (5), consider raising it

Cause

The issue occurs when the PHP-FPM process becomes overloaded with requests. The max_children value can be increased system-wide or for an individual domain. Unless all domains encounter this issue, the value should be updated for an individual domain.

Resolution

1. Log into WHM as the root user
2. Navigate to Home / Software / MultiPHP Manager
3. Click the User Domain Settings tab
4. Click the PHP-FPM Settings button to the right of the user's domain
5. Increase the Max Requests value
6. Increase the Max Children value
7. Click the Update button at the bottom of the page

Additional Resources

PHP-FPM Limit Tuning Basics for Max Children

How to modify system PHP-FPM settings in WHM

How to modify PHP-FPM settings for a domain in WHM