How to calculate pm.max_children value?

Question

How to calculate pm.max_children value?

Answer

1. Connect to the server via SSH.
2. To calculate and change the value of the parameter pm.max_children based on the amount of RAM on the system, the following command will help to determine the memory used by each (PHP-FPM) child process:
   

   # ps -ylC php-fpm --sort:rss
   S UID  PID PPID C PRI NI     RSS      SZ WCHAN TTY   TIME     CMD
   S   0  931   1  0  80  0     87040    99039   ep_pol ? 00:00:00 php-fpm

   Note: The RSS column shows non-swapped physical memory usage by PHP-FPM processes in kiloBytes.

3. If on an average each PHP-FPM process takes ~85MB of RAM on the server, the appropriate value for pm.max_children can be calculated as:
   

   CONFIG_TEXT: pm.max_children = Total RAM dedicated to the web server / Max child process size

   For example, if the server has 8GB of RAM, then the pm.max_children value will be the following:

   CONFIG_TEXT: pm.max_children = 6144MB / 85MB = 72

   Note: The received number of children have to be distributed among all websites on the server. Moreover, it is required to keep in mind that most popular websites require more children while others, non-popular ones - less.

   For example, there are 3 websites on the server:

   • example.com

     A popular online shop. The number of visitors is very high.

   • example.net

     A home page of some person which number of visitors is almost zero.

   • example.org

     A little jewelry online shop which appeared 1 month ago. The number of visitors is not high.

   In this case, it is required to allocate children, for example, as follows:

   • example.com

     50 children.

   • example.net

     2 children.

   • example.org

     20 children.

4. To apply changes for pm.max_children value follow steps from the article: How to apply PHP-FPM pool settings for all the domains?

Additional Information

Websites on PHP-FPM are unavailable or loading slowly: "server reached max_children setting" OR "pool seems busy"