Plesk interface is down or slow: server reached max_children setting

Symptoms

• Plesk is inaccessible with one of the following errors:

  DB query failed: SQLSTATE[HY000] [2002] Resource temporarily unavailable
  Type Plesk\Exception\Database 
  Message DB query failed: SQLSTATE[HY000] [2002] Resource temporarily unavailable 
  File Mysql.php 
  Line 60

  Server Error
  502 Bad Gateway
  Web server received an invalid response while acting as a gateway or proxy server.

• These errors are logged to /var/log/sw-cp-server/sw-engine.log:

  [pool plesk] server reached max_children setting (26), consider raising it

Cause

The Plesk panel reached its pm.max_children limit.

Resolution

Increase the limit.

1. Log in via SSH

2. Increase the pm.max_children value in /etc/sw-engine/pool.d/plesk.conf:

   # sed -i.bkp 's/max_children.*/max_children\ =\ 50/' /etc/sw-engine/pool.d/plesk.conf

   Note: The command above increases the limit to 50. Increase it further if the error continues.

3. Restart sw-engine in order to apply the changes:

   # service sw-engine restart