/etc/sysctl.conf
net.ipv4.ip_local_port_range = 1024 65000

/etc/security/limits.conf
mongodb         hard    nofile          1048576
mongodb         soft    nofile          1048576
mongodb         soft    nproc           1048576
mongodb         hard    nproc           1048576
mongodb         hard    stack           16384
mongodb         soft    stack           16384
mongodb         hard    memlock         unlimited
mongodb         soft    memlock         unlimited

/lib/systemd/system/mongodb.service
# (file size)
LimitFSIZE=infinity
# (cpu time)
LimitCPU=infinity
# (virtual memory size)
LimitAS=infinity
# (open files)
LimitNOFILE=500000
# (processes/threads)
LimitNPROC=500000