Tuning Guide

Tuning the Linux Kernel, Networking, Erlang VM and the EMQ broker for one million concurrent MQTT connections.

Linux Kernel Tuning

The system-wide limit on max opened file handles:

# 2 million system-wide
sysctl -w fs.file-max=2097152
sysctl -w fs.nr_open=2097152
echo 2097152 > /proc/sys/fs/nr_open

The limit on opened file handles for current session:

ulimit -n 1048576


Add the ‘fs.file-max’ to /etc/sysctl.conf, make the changes permanent:

fs.file-max = 1048576


Persist the limits on opened file handles for users in /etc/security/limits.conf:

*      soft   nofile      1048576
*      hard   nofile      1048576

Network Tuning

Increase number of incoming connections backlog:

sysctl -w net.core.somaxconn=32768
sysctl -w net.ipv4.tcp_max_syn_backlog=16384
sysctl -w net.core.netdev_max_backlog=16384

Local Port Range:

sysctl -w net.ipv4.ip_local_port_range="1000 65535"

Read/Write Buffer for TCP connections:

sysctl -w net.core.rmem_default=262144
sysctl -w net.core.wmem_default=262144
sysctl -w net.core.rmem_max=16777216
sysctl -w net.core.wmem_max=16777216
sysctl -w net.core.optmem_max=16777216

#sysctl -w net.ipv4.tcp_mem='16777216 16777216 16777216'
sysctl -w net.ipv4.tcp_rmem='1024 4096 16777216'
sysctl -w net.ipv4.tcp_wmem='1024 4096 16777216'

Connection Tracking:

sysctl -w net.nf_conntrack_max=1000000
sysctl -w net.netfilter.nf_conntrack_max=1000000
sysctl -w net.netfilter.nf_conntrack_tcp_timeout_time_wait=30

The TIME-WAIT Buckets Pool, Recycling and Reuse:


# Enable fast recycling of TIME_WAIT sockets.  Enabling this
# option is not recommended for devices communicating with the
# general Internet or using NAT (Network Address Translation).
# Since some NAT gateways pass through IP timestamp values, one
# IP can appear to have non-increasing timestamps.
# net.ipv4.tcp_tw_recycle = 1
# net.ipv4.tcp_tw_reuse = 1

Timeout for FIN-WAIT-2 sockets:

net.ipv4.tcp_fin_timeout = 15

Erlang VM Tuning

Tuning and optimize the Erlang VM in etc/emq.conf file:

## Erlang Process Limit
node.process_limit = 2097152

## Sets the maximum number of simultaneously existing ports for this system
node.max_ports = 1048576

The EMQ Broker

Tune the acceptor pool, max_clients limit and sockopts for TCP listener in etc/emqttd.config:

## TCP Listener
listener.tcp.external =
listener.tcp.external.acceptors = 64
listener.tcp.external.max_clients = 1000000

Client Machine

Tune the client machine to benchmark emqttd broker:

sysctl -w net.ipv4.ip_local_port_range="500 65535"
sysctl -w fs.file-max=1000000
echo 1000000 > /proc/sys/fs/nr_open
ulimit -n 100000


Test tool for concurrent connections: https://github.com/emqtt/emqtt_benchmark