Knowledge Base

Apache web-server uses all free RAM and becomes unresponsive on a Plesk server: Unable to fork new process

Symptoms

Apache web-server uses all free RAM and becomes unresponsive. The following error message appears in its error logfile:

on CentOS/RHEL-based distributions: /var/log/httpd/error_log
on Debian/Ubuntu-based distributions: /var/log/apache2/error.log

[mpm_prefork:error] [pid 12526] (12)Cannot allocate memory: AH00159: fork: Unable to fork new process

Cause

Too high values are set for the Apache MaxRequestWorkers and ServerLimit directives, due to which all RAM is being used by Apache and new processes cannot be spawned.

Resolution

Decrease the values of the MaxRequestWorkers and ServerLimit directives:

Note: The default value of MaxRequestWorkers and ServerLimit is 256.

For CentOS/RHEL-based distribution

Connect to the Plesk server via SSH.
Find which Multi-Processing Module (MPM) is currently in use:

# httpd -V | grep MPM
Open the file /etc/httpd/conf.modules.d/01-cgi.conf in any text editor (For example, vi editor) and decrease the values of the following directives:
- For MPM Event:
  
  <IfModule mpm_event_module>
  LoadModule cgid_module modules/mod_cgid.so
  MaxRequestWorkers 256
  ServerLimit 256
  </IfModule>
- For MPM Prefork:
  
  <IfModule mpm_prefork_module>
  LoadModule cgi_module modules/mod_cgi.so
  MaxRequestWorkers 256
  ServerLimit 256
  </IfModule>
Save the changes and close the file.
Restart the Apache service to apply the changes:

# service httpd restart

For Debian/Ubuntu-based distributions

Connect to the Plesk server via SSH.
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 any text editor (For example, vi editor) and decrease the values of the following directives:
  
  MaxRequestWorkers 256
  ServerLimit 256
- For MPM Prefork:
  
  Open the /etc/apache2/mods-enabled/mpm_prefork.conf file in any text editor (For example, vi editor) and decrease the values of the following directives:
  
  MaxRequestWorkers 256
  ServerLimit 256
Save the changes and close the file.
Restart the Apache service to apply the changes:

# service apache2 restart