Websites are unavailable with configured cgroups: 503 Service Unavailable

Symptoms

Websites are unavailable:

503 Service Unavailable

The following errors in /var/www/vhosts/system/example.com/logs/error_log

[proxy_fcgi:error] [pid 31123:tid 139667655595776] (70007)The timeout specified has expired: [client 203.0.113.2:57054] AH01075: Error dispatching request to :

PHP-FPM is enabled for the domain example.com .

In the /var/log/plesk-phpXX-fpm/error.log the following errors:

WARNING: [pool example.com] server reached max_children setting (35), consider raising it
ERROR: unable to read what child say: Bad file descriptor (9)

Tuning of FPM pool settings does not fix the issue and child processes permanently reach limits

CGroups feature is enabled on the server and limits are set for the domain.

Cause

CGroups limits result in significant time to process requests to websites and increasing of number of php-fpm child processes. As a result php-fpm stopped to respond.

Resolution

Increase CGroups limits for subscription example.com