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Not a tuning guide

General relation to OS configuration values

You need to know about and configure a couple of Operating System and Erlang VM configs to operate VerneMQ efficiently. First, make sure you have set appropriate OS file limits according to our guide here. Second, when you run into performance problems, don't forget to check the settings in the vernemq.conf file. (Can't open more than 10k connections? Well, is the listener configured to open more than 10k?)

TCP buffer sizes

This is the number one topic to look at, if you need to keep an eye on RAM usage.

Context: All network I/O in Erlang uses an internal driver. This driver will allocate and handle an internal application side buffer for every TCP connection. The default size of these buffers will determine your overall RAM use.

VerneMQ calculates the buffer size from the OS level TCP send and receive buffers:

val(buffer) >= max(val(sndbuf),val(recbuf))

Those values correspond to net.ipv4.tcp_wmem and net.ipv4.tcp_rmem in your OS's sysctl configuration. One way to minimize RAM usage is therefore to configure those settings (Debian example):

sudo sysctl -w net.ipv4.tcp_rmem="4096 16384 32768"
sudo sysctl -w net.ipv4.tcp_wmem="4096 16384 32768"

# Nope, these values are not recommendations!
# You really need to decide yourself.

This would result in a 32KB application buffer for every connection. On a multi-purpose server where you install VerneMQ as a test, you might not want to change your OS's TCP settings, of course. In that case, you can still configure the buffer sizes manually for VerneMQ by using the advanced.config file.

The advanced.config file

The advanced.config file is a supplementary configuration file that sits in the same directory as the vernemq.conf. You can set additional config values for any of the OTP applications that are part of a VerneMQ release. To just configure the TCP buffer size manually, you can create an advanced.config file:

[{vmq_server, [
          [{sndbuf, 4096},
           {recbuf, 4096}]}]}].

The vm.args file

For very advanced & custom configurations, you can add a vm.args file to the same directory where the vernemq.conf file is located. Its purpose is to configure parameters for the Erlang Virtual Machine. This will override any Erlang specific parameters your might have configured via the vernemq.conf. Normally, VerneMQ auto-generates a vm.args file for every boot in /var/lib/vernemq/generated.configs/ (Debian package example) from vernemq.conf and other potential configuration sources.

{% hint style="info" %} A manually generated vm.args is not supplementary, it is a full replacement of the auto-generated file! Keep that in mind. An easy way to go about this, is by copying and extending the auto-generated file. {% endhint %}

This is how a vm.args might look like:

+P 256000
-env ERL_MAX_ETS_TABLES 256000
-env ERL_CRASH_DUMP /erl_crash.dump
-env ERL_MAX_PORTS 262144
+A 64
-setcookie vmq  # Important: Use your own private cookie... 
-name VerneMQ@
+K true
+sbt db
+sbwt very_long
+swt very_low
+sub true
+Mulmbcs 32767
+Mumbcgs 1
+Musmbcs 2047
# Nope, these values are not recommendations!
# You really need to decide yourself, again ;)

A note on TLS

Using TLS will of course increase the CPU load during connection setup. Latencies in message delivery will be increased, and your overall message throughput per second will be lower.

TLS will require considerably more RAM. Instead of 2 Erlang processes per connection, TLS will use 3. You'll have a session process, a queue process, and a TLS handler process that can encapsulate quite a big state (> 30KB).

Erlang/OTP uses it's own TLS implementation, only using OpenSSL for crypto, but not connection handling. For situations with high connection setup rate or overall high connection churn rate, the Erlang TLS implementation might be too slow. On the other hand, Erlang TLS gives you great concurrency & fault isolation for long-lived connections.

Some Erlang deployments terminate SSL/TLS with an external component or with a load balancer component. Do some testing & try to find out what works best for you.

{% hint style="info" %} The Erlang TLS implementation is rather picky on certificate chains & formats. Don't give up, if you encounter errors first. On Linux, you can find out more with the openssl s_client command quickly. {% endhint %}