Symptoms
- Websites on the server are experiencing 504 Gateway Time-out nginx error consistently after working without issues for a period of time.
- The Apache web server goes down periodically with the following error message in Apache error log:
-
on CentOS/RHEL-based distributions:
/var/log/httpd/error_log -
on Debian/Ubuntu-based distributions:
/var/log/apache2/error.logAH00484: server reached MaxRequestWorkers setting, consider raising the MaxRequestWorkers setting
server is within MinSpareThreads of MaxRequestWorkers, consider raising the MaxRequestWorkers setting
-
- The following error message is logged in domain's log at Plesk > Domains > Logs:
[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
MaxRequestWorkersusing the following formula: ServerLimit value x 25 =MaxRequestWorkersvalue. For example, ifServerLimitis set to 20, thenMaxRequestWorkerswill be 20 x 25 = 500. -
MPM Prefork: The default
MaxRequestWorkersvalue is 256. To increase it, you must also raiseServerLimit.
Note: The below values are examples. They must be adjusted according to the server capabilities and load.
-
Find which Multi-Processing Module (MPM) is currently in use:
# httpd -V | grep MPM
-
Open the
/etc/httpd/conf.modules.d/01-cgi.conffile 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
-
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.confin 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.conffile 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…