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The vm.args file is how one provides configuration for the Erlang VM itself. You can also use it to configure applications, but it is generally recommended to use config.exs or sys.config for that instead.

Distillery will generate a default vm.args file for you, which configures the VM for distribution, as shown below:

## Name of the node
-name <%= release_name %>@

## Cookie for distributed erlang
-setcookie <%= release.profile.cookie %>

## Heartbeat management; auto-restarts VM if it dies or becomes unresponsive
## (Disabled by default..use with caution!)

## Enable kernel poll and a few async threads
##+K true
##+A 5

## Increase number of concurrent ports/sockets
##-env ERL_MAX_PORTS 4096

## Tweak GC to run more often

# Enable SMP automatically based on availability
-smp auto

This is templated via EEx, so that we can dynamically set the name and cookie based on configuration.

Custom Args

You can easily provide your own vm.args file, either for the release as a whole, or for a specific environment, via the vm_args: "path/to/file" option in rel/config.exs. For example, perhaps you want to generate a vm.args which dynamically fetches the secret cookie or hostname from the environment, but uses the configured release name for the node name. You can do that like so:

## Node name
-name <%= release_name %>@${HOSTNAME}

## Node cookie, used for distribution
-setcookie ${NODE_COOKIE}

This will again be templated via EEx, so that release_name is replaced with the name of the release, and you can provide additional variables beyond the defaults via the :overlay_vars option in rel/config.exs.

The ${HOSTNAME} and ${NODE_COOKIE} parts will only be dynamically replaced at runtime if you export REPLACE_OS_VARS=true in the system environment prior to starting the release, so be sure you do so if you want to use this approach.

For more information on vm.args, please see the documentation on erl, specifically the Flags section.

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