Step One: In system level, set the limitation of the amount of the files which can be opened by the user who running the Linux Program
# vi /etc/security/limites.conf
Add these lines:
......
rmt - nofile 1024
rmt - nproc 1024
(Add: Setting the rmt user can open 8192 files and 8192 processes in this Linux as an example here)
Step Two: In program level, check the setting of the amount of the files which can be opened by Linux program
# /bin/systemctl show 'rmt-server-mirror.service' | sort
......
LimitNOFILE=524288
......
(Add: The amount of opening the file by program rmt-server-mirror.service is limited at 8096 as an example here)