High CPU and IO load on a Plesk Server: kswapd0 is active

Symptoms

• Health Monitoring or other monitoring daemon tools show high CPU and IO load on the server.

• There is few or none free RAM left:

  # free -h
  total used free shared buffers cached
  Mem: 8.0G 7.9G 0.1G 316M 0B 0.1G

• A lot of CPU time is used by the kswapd0 daemon:

  # ps -eo pid,pcpu,comm --sort pcpu | tail -n1
  19796 99.9 kswapd0

Cause

The server run out of free memory and started bypassing disk cache trying to allocate RAM needs causing high CPU and IO usage.

Resolution

Apply one or more of the following alternatives:

• Reach the hosting provider to increase the installed system memory (RAM)

• Increase the Virtual Memory size (SWAP) using the OS support guides.

• Monitor the RAM usage in a period of time using atop and take the needed actions to kill or remove unneeded processes.

• Reconfigure PHP-FPM usage in Domains > example.com > PHP Settings and decrease memory_limit, and/or pm.max_children