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

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Kuzma Ivanov
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Applicable to:

  • Plesk 12.5 for Linux
  • Plesk Onyx for Linux

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

    CONFIG_TEXT: server reached MaxRequestWorkers setting, consider raising the MaxRequestWorkers setting

  • If watchdog is installed on the server, the following message may appear in /usr/local/psa/var/modules/watchdog/log/monit.log:

    CONFIG_TEXT: file: hosting monit: GENERIC: error receiving data -- Resource temporarily unavailable hosting monit: 'apache' failed protocol test [generic] at INET[127.0.0.1:7080]

  • The server is not under DDoS attack:

    • This command shows the list of source IP addresses and number of their current connections:

      # netstat -an | egrep ':80|:443' | grep ESTABLISHED | awk '{print $5}' | grep -o -E "([0-9]{1,3}[\.]){3}[0-9]{1,3}" | sort -n | uniq -c | sort -nr

      499 203.0.113.2
      318 203.0.113.5
      213 203.0.113.8

      If the number of connections is high (400+) for some IP addresses, follow the steps from this KB article.

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.

 

  • 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 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 should be adjusted according to the server capabilities and load.

Warning: Be careful, too high values of MaxRequestWorkers and ServerLimit directives may cause Apache to become unresponsive.

 

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:

      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. 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 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):

      CONFIG_TEXT: MaxRequestWorkers 400
      ServerLimit 16

    • For MPM Prefork:

      Open the /etc/apache2/mods-enabled/mpm_prefork.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):

      CONFIG_TEXT: MaxRequestWorkers 400
      ServerLimit 400

  3. Restart Apache to apply the changes:

    # service apache2 restart

 

Additional Information

 

Identifying a possible cause

We recommend to identify the cause of the MaxRequestWorkers limit being reached. Click on the "See how" section for details:

See how

 

  1. Find websites that slow down Apache:

    The other reason why the number of worker processes can increase is how long it takes to process a request.

    • Enable Apache server statistics according to this KB article and monitor Apache web-server performance.

      Check the SS, Req fields and check what websites and pages have higher values. If it requires a significant time to process a .php script, the code of this script should be reviewed and optimized.

    • A significant time to process MySQL queries can also be the reason. Check this KB article to track down slow SQL queries.

  2. Switch websites with high traffic load to nginx-only hosting:

    If there are websites with high traffic load, switching websites to nginx-only hosting allows decreasing the load on Apache. To do this:

    2.1. Install and enable the nginx web-server.

    2.2. Once installed, switch on nginx-only hosting for a domain.

 

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Comments

5 comments

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  • Avatar
    Kingsley Felix

    I am tired of plesk, i inscreased this and it is still happening

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  • Avatar
    Dan Neuhaus

    Still having the same issue on two servers despite raising MaxRequestWorkers to 1000

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  • Avatar
    Bulat Tsydenov

    @Dan, Am I right that even after changing 'MaxRequestWorkers' to 1000 and restarting apache you still see 'server reached MaxRequestWorkers setting' errors in apache logs? Additionally, you may check this article which contains troubleshooting steps.

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  • Avatar
    Luis Zubeldia

    for mpm_event where i can change MaxRequestWorkers value?

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  • Avatar
    Alexandr Tumanov

    @Luis, the path is almost the same:

     

    Debian/Ubuntu /etc/apache2/mods-enabled/mpm_event.conf 

    CentOS the same file but <IfModule mpm_event_module> section.

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