Increase the maximum number of open files / file descriptors
If you are root, execute the command below
ulimit -SHn 65535
If you want to set it permanently, especially for a particular user, change the file /etc/security/limits.conf to have the following lines (assume user = www-data)
www-data hard nofile 65535 www-data soft nofile 65535
And make sure uncomment pam_limits.so from the different files under /etc/pam.d, e.g. sudo, login, sshd, ...
Finally restart your system.