Applicable to:
- Plesk 12.5 for Linux
- Plesk Onyx for Linux
Symptoms
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Apache web server goes down periodically with the following error message in its error log
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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
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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]
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The server is not under DDoS attack:
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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
213 203.0.113.8If the number of connections is high (400+) for some IP addresses, follow the steps from this KB article.
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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
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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, ifServerLimitset 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 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.
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Find which Multi-Processing Module (MPM) is currently in use:
# httpd -V | grep MPM
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Open the
/etc/httpd/conf.modules.d/01-cgi.conffile 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>
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Restart Apache to apply the changes:
# service httpd restart
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Find which Multi-Processing Module (MPM) is currently in use:
# apache2ctl -V | grep MPM
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Change the MPM configuration:
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For MPM Event:
Open the
/etc/apache2/mods-enabled/mpm_event.confin 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.conffile 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
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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:
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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.
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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.
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A significant time to process MySQL queries can also be the reason. Check this KB article to track down slow SQL queries.
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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.
Comments
9 comments
I am tired of plesk, i inscreased this and it is still happening
Still having the same issue on two servers despite raising
MaxRequestWorkersto 1000@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.
for mpm_event where i can change MaxRequestWorkers value?
@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.
This issue continues to be a problem, despite having these settings already (which I just verified following the instructions above). Everything has been working fine on my server for past few months since I fixed some problems after a previous Plesk update. Lo and behold, Plesk autoinstaller updated last night around 3am and I have been dealing with this problem since I got up. Frustrating that Plesk updates keep causing problems.
@Ryan Yzquierdo,
If you still experience any issues after the update, don't hesitate to contact our Support Team so they can thoroughly check the issue.
I got the same error.
To ward off url attacks, I wrote in php in case of evil Url
sleep(99999);
exit;
I dropped sleep and changed it into
exit;
That was my problem. Now I get no maxRequestErro error any more.
Hello Björn
Thank you for sharing your user experience.
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