RabbitMQ

RabbitMQ is a message broker written in Erlang. It is currently the default provider of message queues in Kolla Ansible deployments.

Passing arguments to RabbitMQ server’s Erlang VM

Erlang programs run in an Erlang VM (virtual machine) and use the Erlang runtime. The Erlang VM can be configured.

Kolla Ansible makes it possible to pass arguments to the Erlang VM via the usage of the rabbitmq_server_additional_erl_args variable. The contents of it are appended to the RABBITMQ_SERVER_ADDITIONAL_ERL_ARGS environment variable which is passed to the RabbitMQ server startup script. Kolla Ansible already configures RabbitMQ server for IPv6 (if necessary). Any argument can be passed there as documented in https://www.rabbitmq.com/runtime.html

The default value for rabbitmq_server_additional_erl_args is +S 2:2 +sbwt none +sbwtdcpu none +sbwtdio none.

By default RabbitMQ starts N schedulers where N is the number of CPU cores, including hyper-threaded cores. This is fine when you assume all CPUs are dedicated to RabbitMQ. Its not a good idea in a typical Kolla Ansible setup. Here we go for two scheduler threads (+S 2:2). More details can be found here: https://www.rabbitmq.com/runtime.html#scheduling and here: https://erlang.org/doc/man/erl.html#emulator-flags

The +sbwt none +sbwtdcpu none +sbwtdio none arguments prevent busy waiting of the scheduler, for more details see: https://www.rabbitmq.com/runtime.html#busy-waiting.