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Apache keeps going down on a Plesk server: server reached MaxRequestWorkers setting

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Daniel Yordanov
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Plesk for Linux kb: technical FR:PPM-2076

Applicable to:

  • Plesk for Linux

Symptoms

  • Websites on the server are experiencing 504 Gateway Time-out nginx errors consistently after working without issues for a period of time
  • The Apache web server goes down periodically with the following error message in its Apache error log:

    Note: On RHEL-based distributions, the Apache error is located at /var/log/httpd/error_log , while on Debian-based distributions, it is located at /var/log/apache2/error.log

    CONFIG_TEXT: [Fri Nov 17 10:26:21.896808 2023] [mpm_event:error] [pid 27010:tid 140326926068672] AH00484: server reached MaxRequestWorkers setting, consider raising the MaxRequestWorkers setting

    CONFIG_TEXT: [Fri Nov 18 10:06:21.125808 2023] [mpm_prefork:error] [pid 23090:tid 120336926067734] AH00484: server reached MaxRequestWorkers setting, consider raising the MaxRequestWorkers setting

  • Error similar to the following entries occur in the nginx error log for many domains on the server:

    CONFIG_TEXT: [error] 26927#0: *417 connect() failed (110: Connection timed out) while connecting to upstream, client: 203.0.113.2, server: example.com, request: "GET /?amp HTTP/1.1", upstream: "https://203.0.113.2/?amp", host: "example.com"

Cause

The MaxRequestWorkers limit set for the Apache web server has been reached and there are no free workers to serve new requests to Apache. As a result, requests are being served slowly or not served at all and therefore the websites become inaccessible, even though the server's CPU and RAM resources are not exhausted.

Resolution

Adjust the MaxRequestWorkers settings for Apache. The general formula for making the necessary calculation is the following: 

# MaxRequestWorkers = (Total RAM - Memory used for Linux, DB, etc.) / average Apache process size

  • MPM Event: The default ServerLimit value is 16. To increase it, you must also raise MaxRequestWorkers using the following formula: ServerLimit value x 25 = MaxRequestWorkers value. For example, if ServerLimit is set to 20, then MaxRequestWorkers will be 20 x 25 = 500.

  • MPM Prefork: The default MaxRequestWorkers value is 256. To increase it, you must also raise ServerLimit.

Note: The below values are examples. They must be adjusted according to the server capabilities and load.

For CentOS/RHEL-based distribution

 

  1. Find which Multi-Processing Module (MPM) is currently in use:

    # httpd -V | grep MPM

  2. Open the /etc/httpd/conf.modules.d/01-cgi.conf file in a text editor (for example, vi editor) and increase the values of the following directives (or add them if they are not present in the file):

    • For MPM Event:

      CONFIG_TEXT: <IfModule mpm_event_module>
      LoadModule cgid_module modules/mod_cgid.so
      MaxRequestWorkers 400
      ServerLimit 16
      </IfModule>

    • For MPM Prefork:

      CONFIG_TEXT: <IfModule mpm_prefork_module>
      LoadModule cgi_module modules/mod_cgi.so
      MaxRequestWorkers 400
      ServerLimit 400
      </IfModule>

  3. Save the changes and close the file.

  4. Restart Apache to apply the changes:

    # service httpd restart

 

For Debian/Ubuntu-based distributions

 

  1. Find which Multi-Processing Module (MPM) is currently in use:

    # apache2ctl -V | grep MPM

  2. Change the MPM configuration:

    • For MPM Event:

      Open the /etc/apache2/mods-enabled/mpm_event.conf in a text editor (for example, vi editor) and increase the values of the following directives (or add them if they are not present in the file):

      CONFIG_TEXT: MaxRequestWorkers 400
      ServerLimit 16

    • For MPM Prefork:

      Open the /etc/apache2/mods-enabled/mpm_prefork.conf file in a text editor (for example, vi editor) and increase the values of the following directives (or add them if they are not present in the file):

      CONFIG_TEXT: MaxRequestWorkers 400
      ServerLimit 400

  3. Save the changes and close the file.

  4. Restart Apache to apply the changes:

    # service apache2 restart

Note: if the issue persists, switch MPM to Prefork in Tools & Settings > Apache Web Server.

Comments

1 comment

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  • Avatar
    Ehud Ziegelman (Edited )

    Hi Kuzma Ivanov,

     

    Please note that the results of calculations for lots and lots of servers, of the below, to the best of my understanding totally does not suit, the default recommended values, as set in this current article. 

    Can you please advise?

     

    MaxRequestWorkers = (Total RAM - Memory used for Linux, DB, etc.) / average Apache process size

    • MPM Event: The default ServerLimit value is 16. To increase it, you must also raise MaxRequestWorkers using the following formula: ServerLimit value x 25 = MaxRequestWorkers value. For example, if ServerLimit is set to 20, then MaxRequestWorkers will be 20 x 25 = 500.

    • MPM Prefork: The default MaxRequestWorkers value is 256. To increase it, you must also raise ServerLimit.

     

    MPM EVENT

    MaxRequestWorkers 400
    ServerLimit 16
    0
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