https://stackoverflow.com/questions/880557/socket-accept-too-many-open-files
    cat /proc/sys/fs/file-max
    ulimit -n
    in /etc/security/limits.conf - it's the nofile param.
        * soft nofile 16384
        * hard nofile 16384
    https://unix.stackexchange.com/questions/365622/how-to-raise-open-file-limit-etc-security-limits-d-vs-etc-security-limits-co
        /etc/security/limits.d/30-user-nofile.conf

2021
https://www.baeldung.com/linux/error-too-many-open-files
    lsof | wc -l
    sudo vi /etc/security/limits.d/30-user-nofile.conf
2019
https://winitpro.ru/index.php/2019/11/19/oshibka-too-many-open-files-v-linux/