27 October, 2021

Plesk

liton

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Symptoms

Apache web server goes down periodically with the following error message in its error log:

  • on CentOS/RHEL-based distributions: /var/log/httpd/error_log

  • on Debian/Ubuntu-based distributions: /var/log/apache2/error.log

server reached MaxRequestWorkers setting, consider raising the MaxRequestWorkers setting

Cause

The MaxRequestWorkers limit 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.

Resolution

Adjust the MaxRequestWorkers settings for Apache. General formula looks like below: 

# MaxRequestWorkers = (Total RAM - Memory used for Linux, DB, etc.) / 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 any 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:


      LoadModule cgid_module modules/mod_cgid.so
      MaxRequestWorkers 400
      ServerLimit 16

    • For MPM Prefork:


      LoadModule cgi_module modules/mod_cgi.so
      MaxRequestWorkers 400
      ServerLimit 400

  3. Restart Apache to apply the changes:

    # service httpd restart