Increase Open File Limit 

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# Add the following line to /etc/security/limits.conf
# {user}/@{group} soft/hard/-(both) {item} {vaule}
* - nofile 999999

# make sure the file (/etc/pam.d/login for console, /etc/pam.d/lightdm for lightdm and so on) contains the following line
session required pam_limits.so

# login again

# Check
ulimit -Sn # soft limit; can be raised up to the hard limit
ulimit -Hn # hard limit

Reference

Increase the open files limit on Linux