While using php-fpm sometimes we may face some pm.max_children related warnings inside error log file, or sometime php-fpm: pool consumes high cpu continuously.

The reason of the issue may raise because of not configuring pm.max_children properly.

What is pm.max_children ?

pm.max_children can be called process manager of php-fpm. By setting max_children value we can set up how many process php-fpm can create while serving requests.

We can check php-fpm error logs inside /var/log directory in ubuntu/debian system.

How to calculate pm.max_children for php-fpm ?

1) open the www.conf file

You can see the default assigned values

2) Now follow the following method to calculate the max_children number

[Total Available RAM] - [Reserved RAM] - [10% buffer] = [Available RAM for PHP]

Results:
[Available RAM for PHP] / [Average Process Size] = [max_children]

pm.max_children = [max_children]
pm.start_servers = [25% of max_children]
pm.min_spare_servers = [25% of max_children]
pm.max_spare_servers = [75% of max_children]

Also you can use the virtual calculator to calculate.