PHP-FPM (FastCGI Process Manager) manages PHP processes that handle web server requests through FastCGI. It is one way to execute Joomla server-side PHP code.
Pools and handlers
A pool groups workers with shared configuration, an operating system user and permissions. The web server connects through a socket or TCP port. Different sites can use different pools and PHP versions.
Worker limits affect concurrent requests and memory use. Increasing them without measuring memory can make an incident worse. Slow requests can occupy every available worker.
Joomla checks
The command-line PHP version may differ from the website handler. For Joomla, inspect the actual web PHP version, loaded extensions, settings and file permissions.
When Joomla fails, correlate web server, PHP-FPM and application logs. A running service does not prove that the site connects to the correct pool. After a fix, test login, forms and saving data as well as the home page.
Conclusion
PHP-FPM connects the web server to PHP execution. Diagnosis requires checks of the specific pool, its resources and the actual site journey.