Troubles with MySQL open_files_limit
It depends on your db architecture. If you have many tables in separate files in the MyISAM, ARIA, or Innodb with files per table you will want a higher open file limit. If you have everything in the Innodb / xtradb in a single tablespace then upping your file limits will not benefit.

Check current system limits

ulimit -a


Temporary limit increase 

ulimit -n 65535


Permanent update /etc/security/limits.conf

The mysql open file limit can't exceed system limits.

* soft nofile 65535
* hard nofile 65535