Symptoms

The following error is being displayed in Plesk:

SQLSTATE[HY000] [1135] Can't create a new thread (errno 11); if you are not out of available memory, you can consult the manual for a possible OS-dependent bug.

Cause

MySQL service is used actively, which causes it to overuse the maximum available limits for process treads.

Resolution

For Ubuntu\Debian\Centos below 6:

Modify the '/etc/security/limits.conf' directly, and configure limits for MySQL as below:

mysql  soft  nproc  4096

For Centos 6 and above:

Create a file '/etc/security/limits.d/91-mysql.conf' and include the same configuration as described above. The additional files is needed to override the global limit specified in the '/etc/security/limits.d/90-nproc.conf' .