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

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Aleksei Khikhich
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kb: how-to Plesk for Linux

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 customize PHP settings as per the article.

Comments

3 comments

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  • Avatar
    adrian TNT

    For tips, I would add:

    You can progressivly increase the value, and know exactly if it was too low, inside the log:

    /var/log/plesk-php82-fpm/error.log (replace php82 with your own version)
    It will mention exact domain name that reached the value and what the value limit was at that time:
    WARNING: [pool example3.com] server reached max_children setting (10)
    In this case example3.com had max_children set to 10 which was too low.
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  • Avatar
    adrian TNT

    Note the first link at point 4 is "not found":

    https://plesk-new.zendesk.com/hc/en-us/articles/12377673921431-How-to-customize-PHP-FPM-pool-settings-for-a-domain-through-Plesk-graphical-interface-

    I think the right page is this:

    https://www.plesk.com/kb/support/how-to-customize-php-fpm-pool-settings-for-a-domain-through-plesk-graphical-interface-2/
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  • Avatar
    Permanently deleted user

    Hello Adrian,

    Thank you for the comments. The article was updated.

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