Error on Plesk for Linux: Fork failed – Cannot allocate memory

Symptoms

• Many actions on a Plesk server cannot be performed with the following error:

  Cannot allocate memory

• Sites or random Plesk pages are not accessible with:

  502 Bad Gateway

  ---

  503 Service Unavailable

• The following error appears in the system log:

  • On RHEL-based OSes the following error appears in /var/log/messages:

    Out of memory in UB 69762: OOM killed process 22008 (php-cgi) score 0 vm:429108kB, rss:107332kB, swap:0kB
    Out of memory in UB 69762: OOM killed process 22028 (php-cgi) score 0 vm:429968kB, rss:108648kB, swap:0kB

  • Debian/Ubuntu based OSes the following error appears in /var/log/syslog:

    Nov 12 06:44:42 systemd[1]: mariadb.service: A process of this unit has been killed by the OOM killer.

• Unable to open Tools & Settings > IP Address Banning (Fail2Ban):

  Error: The IP Address Banning (Fail2Ban) service failed: f2bmng failed: Traceback (most recent call last): File "/usr/local/psa/admin/sbin/f2bmng",
  line 5, in <module> import f2bmng File "/usr/local/psa/lib/modules/python/f2bmng/f2bmng.py",
  line 6, in <module> import json File "/usr/lib/python2.6/json/init.py",
  line 108, in <module> from .decoder import JSONDecoder ImportError: No module named decoder

• The following error can appear during migration:

  Migration tools tried to perform operation in 3 attempts.
  ----------------------------------------------------------------------------
  Attempt #1 failed with the following error:
  Rsync failed to copy files from the source server 'source' (203.0.113.2) to target Plesk server: Command execution failed on the local server with non-zero exit code.
  ...
  exit code: 23
  stdout:
  stderr: rsync: recv_generator: failed to stat "/var/www/vhosts/example.com/httpdocs/cache/cachefs/2/2b7e2f329409023b850af3509758defe": Cannot allocate memory (12)

• The next error appears from time to time in an attempt to open the Plesk > Tools & Settings > Mail Server Settings > Mail Queue page:

  external error:
  mailqueuemng failed: Unable to prepare reading queue.
  Message mailqueuemng failed: Unable to prepare reading queue.
  File Agent.php
  Line 228
  Type PleskUtilException

Cause

Server is running out of physical RAM.

Resolution

Contact your hosting provider for more information on how to increase physical RAM on the server.

Additional information

What are the recommended values for SWAP, RAM and Disk space while using Plesk Obsidian?