This article covers methods to resolve Plesk error pool seems busy. This error happens when the PHP-FPM pm.max_children has been reached by the website(s).
To Increasing pm.max_children for one domain via a command-line interface:
1. Create a php.ini file in domain's system directory /var/www/vhosts/system/example.com/conf/:
$ touch /var/www/vhosts/system/example.com/conf/php.ini
2. Open the php.ini file in any text editor (for example, vi editor) and add the following records:
pm.max_children = 20
3. Save the changes and close the file.
4. Run the following command to apply the changes: