ftp download using elixir and erlang interop

ftp.ex

defmodule Reporter do
  def progress(last_data, file_name, {size_type, chunk_size}) do
    IO.inspect({last_data[:downloaded], "#{last_data[:rate]} MB/s", file_name, size_type, chunk_size})

    new_timestamp = :os.system_time(:milli_seconds)
    timestamp_diff = new_timestamp - last_data[:last_timestamp]

    case chunk_size do
      :unknown ->
        downloaded = 0
        rate = 0
      _ ->
        downloaded = last_data[:downloaded] + chunk_size
        # Kilobytes per second
        cond do
          timestamp_diff > 0 ->
            rate = (chunk_size/1000000)/(timestamp_diff/1000)
          true ->
            rate = last_data[:rate]
        end
    end

    %{downloaded: downloaded, last_timestamp: new_timestamp, rate: rate}
  end
end

cfg = [
  host: 'ftp.iinet.net.au',
  progress: {
    Reporter,
    :progress,
    %{
      downloaded: 0,
      last_timestamp: :os.system_time(:milli_seconds),
      rate: 0
    }
  }
]

:inets.start
{:ok, pid} = :inets.start(:ftpc, cfg)
:ftp.user(pid, 'anonymous', 'anonymous')
:ftp.type(pid, :binary)
:ftp.ls(pid)
a = :os.system_time(:milli_seconds)
:ftp.recv(pid, '1mb.dat')
b = :os.system_time(:milli_seconds)
IO.puts "Time spent: #{(b-a)/1000} seconds"

{1040680, "1.46 MB/s", '1mb.dat', :transfer_size, 1460}
{1042140, "1.46 MB/s", '1mb.dat', :transfer_size, 1460}
{1043600, "1.46 MB/s", '1mb.dat', :transfer_size, 1460}
{1045060, "1.46 MB/s", '1mb.dat', :transfer_size, 1460}
{1046520, "0.73 MB/s", '1mb.dat', :transfer_size, 1460}
{1047980, "0.73 MB/s", '1mb.dat', :transfer_size, 596}
{1048576, "0.596 MB/s", '1mb.dat', :transfer_size, 0}
:ok
iex(12)> b = :os.system_time(:milli_seconds)
1470772308509
iex(13)> IO.puts "Time spent: #{(b-a)/1000} seconds"
Time spent: 9.922 seconds

Calculating the transfer rate within the progress block doesn't seem feasible. Maybe it's better for each download to start a different process that watches the progress each 1-2 seconds.