The ulimit -Hn is the hard limit. The ulimit -Sn is the soft limit. We recommend you to define both soft and hard file descriptor limit to 65536. If the OS already defined the hard limit higher than 65536, then do not change the hard limit and set only the soft limit to 65536.
Issue/Introduction
The recommended value for the file descriptor limit for Unix/Linux System
Environment
Unix and Linux
Resolution
(As of writing this article, the hardware based EMS messaging appliance has hard limit of 20K and soft limit of 10K) For the software based Unix/Linux system, we recommend both soft and hard limit as 65536.