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How to calculate pm.max_children value on a Plesk server?

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Anastasia Zyrianova
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Plesk for Linux kb: how-to

Applicable to:

  • Plesk for Linux

Question

How to calculate pm.max_children value on a Plesk server?

Answer

  1. Connect to the server via SSH.

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

    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 and 6GB of RAM is planned to be allocated to the web server, then the pm.max_children value will be the following:

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

    Note: The received number of children has to be distributed among all websites on the server. Keep in mind that most popular websites require more children while others, non-popular ones - less.

    Example of how to divide the result between websites

    There are 3 domains on the server:

    • example.com

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

    • example.net

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

    • example.org

      A little jewelry online shop that was created a month ago. The number of visitors is not high.

    Is the appropriate pm.max_children value was calculated to be 72, in this case, it is required to allocate children, for example, as follows:

    • For example.com:

      50 children.

    • For example.net:

      2 children.

    • For example.org:

      20 children.

  4. To apply changes for pm.max_children value follow steps from the articles:

Comments

2 comments

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    Marcus Dressler

    can you please explain how you calculate "if the server has 8GB of RAM" to get 6144MB  ?

    Did you subtract some MB for operation system or is is a mistake ?

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    Anastasia Zyrianova

    Hello Marcus Dressler,

    Some memory should be left for the system to breathe. You need to take into account any other services running on the machine while calculating memory usage.
    The value of 6 GB was taken as an example here. It may be adjusted in accordance with your needs.

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