# Performance Tuning (Linux)

Due to the typically large number of devices and data in IoT applications, EMQX, as an MQTT server, is responsible for handling and delivering messages generated by a massive number of devices. In this scenario, optimizing EMQX system performance becomes particularly crucial.

Optimization aims to maximize the following aspects of performance:

- **Message Processing Capability**: Enhancing EMQX's ability to process messages quickly and efficiently, ensuring it can rapidly receive, process, and forward device-generated messages.
- **Throughput**: Increasing throughput to ensure the system can handle and deliver messages from devices in a timely manner.
- **Stability**: Reducing latency under high loads, improving system responsiveness, and lowering the risk of system crashes or failures.

This page provides general tuning suggestions for benchmarking and deployment.

## Turn Off Swap

Linux swap partitions may cause nondeterministic memory latency to an Erlang virtual machine, significantly affecting the system stability.
It is recommended to turn off the swap permanently.

- To turn off swap immediately, execute the command `sudo swapoff -a`.

- To turn off swap permanently, comment out the `swap` line in `/etc/fstab` and reboot the host.

## Linux Kernel Tuning

The system-wide limit on max opened file handles:

```bash
# 2 millions 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 the current session:

```bash
ulimit -n 2097152
```

### `/etc/sysctl.conf`

Persist `fs.file-max` configuration to `/etc/sysctl.conf`:

```bash
fs.file-max = 2097152
```

Set the maximum number of file handles for the service in `/etc/systemd/system.conf`:

```bash
DefaultLimitNOFILE=2097152
```

### `emqx.service`

Set the maximum number of file handles for emqx service in one of the below paths depending on which Linux distribution is used.

- `/usr/lib/systemd/system/emqx.service`
- `/lib/systemd/system/emqx.service`

```bash
LimitNOFILE=2097152
```

### `/etc/security/limits.conf`

Persist the maximum number of opened file handles for users in `/etc/security/limits.conf`:

```bash
*      soft   nofile      2097152
*      hard   nofile      2097152
```

## TCP Network Tuning

Increase the number of incoming connections backlog:

```bash
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

```bash
sysctl -w net.ipv4.ip_local_port_range='1024 65535'
```

TCP Socket read/write buffer:

```bash
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'
```

TCP connection tracking:

```bash
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
```

TIME-WAIT Bucket Pool, Recycling, and Reuse:

```bash
sysctl -w net.ipv4.tcp_max_tw_buckets=1048576

# Enabling following option is not recommended. It could cause connection reset under NAT
# sysctl -w net.ipv4.tcp_tw_recycle=1
# sysctl -w net.ipv4.tcp_tw_reuse=1
```

Timeout for FIN-WAIT-2 Sockets:

```bash
sysctl -w net.ipv4.tcp_fin_timeout=15
```

## Erlang VM Tuning

Tune and optimize the Erlang VM in `etc/emqx.conf` file:

```bash
## Sets the maximum number of simultaneously existing ports for this system
node.max_ports = 2097152
```

## EMQX Tuning

### Listener Acceptor

Tune the acceptor pool size and `max_connections` limit in `etc/base.hocon`.

To optimize connection handling, you can adjust the acceptor pool size and the `max_connections` limit in the `etc/emqx.conf` configuration file.

For example, to configure TCP listeners:

```bash
## TCP Listener
listeners.tcp.$name.acceptors = 64
listeners.tcp.$name.max_connections = 1024000
```

`acceptors`: The number of acceptor processes handling incoming connections.

`max_connections`: The maximum number of concurrent connections allowed.

### Distribution Port Buffer Size

In large clusters with many replicant nodes, it is suggested to tune the distribution port buffer size by setting the `node.dist_buffer_size` parameter on core nodes.

```bash
# Buffer size in KB. The following sets the maximum to approximately 2 GB.
node.dist_buffer_size=2097151
```

This adjustment helps core nodes better handle traffic spikes caused by mass client reconnections.

Additionally, if you observe warning log messages like the following, increasing this buffer size can mitigate the issue:

```
[warning] msg: busy_dist_port ...
```


## Client Machine Tuning

Tune the client machine to benchmark EMQX:

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


### MQTT Benchmarking

Test tools for concurrent connections: [emqtt_bench](https://github.com/emqx/emqtt_bench).