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

• Cannot access Plesk or edit PHP settings:

  Error: The license key is invalid. To use Plesk, you must obtain and install a new, valid license key. The license key is for use only with Plesk within the following virtual environments: Virtuozzo, VMware, Microsoft Hyper-V, Xen, KVM, Virtuozzo Server, LXC, Docker.

• 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

• 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

• MagicSpam can not be started. It hangs on "loading" state.

Cause

Server is running out of physical RAM, some limits are exceeded on the server

Resolution

Increase amount of RAM or SWAP values on the server.

Note: This is a task that should be performed by the System Administrator

Note: In the case of a virtual environment (VPS or Virtual Machine), contact your service provider in order to increase server limits.

Additional information

What are recommended requirements of SWAP, RAM and Disk space for Plesk Onyx?