===== rc.local ===== 
Edit /etc/rc.local: 
<code bash>
vi /etc/rc.local
</code>
Add:
<code bash>
#!/bin/sh -e

# ulimit
ulimit -n 51200
</code>

Add permission:
<code bash>
chmod +x /etc/rc.local
</code>

Enable
<code bash>
systemctl enable rc-local
</code>
Start:
<code bash>
systemctl start rc-local
</code>