wait_for_processes.ex

defmodule App do

  def bench do
    :timer.tc(fn -> start() end)
  end

  defp start do
    run()
    wait()
  end

  defp run do
    for _ <- 1..900 do
      P.start_link(self())
    end
  end

  defp wait(900), do: :ok

  defp wait(counter \\ 1) do
    IO.inspect(counter)
    receive do
      :done ->
        wait(counter + 1)
    end
  end
end

defmodule P do
  use GenServer

  def start_link(parent) do
    GenServer.start_link(__MODULE__, parent)
  end

  def init(parent) do
    send(self(), {:do_work, parent})
    {:ok, []}
  end

  def handle_info({:do_work, parent}, state) do
    Process.sleep(10)
    send(parent, :done)
    {:noreply, state}
  end
end

{elapsed_time, _} = App.bench()