Erlang and Elixir make it very easy to send messages between processes even across the network, but there are a few pitfalls.

• Sending a message to many PIDs across the network also copies the message across the network that many times.
• Send calls cost about 70 µs/op so doing them in a loop eventually gets too expensive.

Discord runs a single GenServer per Discord server and some of these have over 30,000 PIDs connected to them from many different Erlang nodes. Increasingly we noticed some of them getting behind on processing their message queues and the culprit was the cost of 70 µs per send/2 call multiplied by connected sessions. How could we solve this?

Inspired by a blog post about boosting performance of message passing between nodes, Manifold was born. Manifold distributes the work of sending messages to the remote nodes of the PIDs, which guarantees that the sending processes at most only calls send/2 equal to the number of involved remote nodes. Manifold does this by first grouping PIDs by their remote node and then sending to Manifold.Partitioner on each of those nodes. The partitioner then consistently hashes the PIDs using :erlang.phash2/2, groups them by number of cores, sends to child workers, and finally those workers send to the actual PIDs. This ensures the partitioner does not get overloaded and still provides the linearizability guaranteed by send/2.

The results were great! We observed packets/sec drop by half immediately after deploying. The Discord servers in question also were finally able to keep up with their message queues.

There is an optional and experimental :offload send mode which offloads on the send side the send/2 calls to the receiving nodes to a pool of Manifold.Sender processes. To maintain the linearizability guaranteed by send/2, the same calling process always offloads the work to the same Manifold.Sender process. The size of the Manifold.Sender pool is configurable. This send mode is optional because its benefits are workload dependent. For some workloads, it might degrade overall performance. Use with caution.

Caution: To maintain the linearizability guaranteed by send/2, do not mix calls to Manifold with and without offloading. Mixed use of the two different send modes to the same set of receiving nodes would break the linearizability guarantee.

Add it to mix.exs

defp deps do
  [{:manifold, "~> 1.0"}]
end

Then just use it like the normal send/2 except it can also take a list of PIDs.

Manifold.send(self(), :hello)
Manifold.send([self(), self()], :hello)

To use the experimental :offload send mode, make sure the Manifold.Sender pool size is appropriate for the workload:

config :manifold, senders: <size>

Then:

Manifold.send(self(), :hello, send_mode: :offload)

Manifold takes a single configuration option, which sets the module it dispatches to actually call send. The default is GenServer. To set this variable, add the following to your config.exs:

config :manifold, gen_module: MyGenModule

In the above instance, MyGenModule must define a cast/2 function that matches the types of GenServer.cast.

Manifold is released under the MIT License. Check LICENSE file for more information.