Knowledge Base

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

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.) / 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

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

# httpd -V | grep MPM
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:
  
  <IfModule mpm_event_module>
  LoadModule cgid_module modules/mod_cgid.so
  MaxRequestWorkers 400
  ServerLimit 16
  </IfModule>
- For MPM Prefork:
  
  <IfModule mpm_prefork_module>
  LoadModule cgi_module modules/mod_cgi.so
  MaxRequestWorkers 400
  ServerLimit 400
  </IfModule>
Save the changes and close the file.
Restart Apache to apply the changes:

# service httpd restart

For Debian/Ubuntu-based distributions

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

# apache2ctl -V | grep MPM
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):
  
  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):
  
  MaxRequestWorkers 400
  ServerLimit 400
Save the changes and close the file.
Restart Apache to apply the changes:

# service apache2 restart