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.

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)

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