After running a while the benchee test I got the following error: no function clause matching in Task.async/1
I wrote a script with async/3 which works on Elixir 1.17.3-otp27.

defmodule Pmap do
  def pmap(collection,func) do
    collection
    |> Enum.map(&Task.async(Pmap, func, [&1]))
    |> Enum.map(&Task.await(&1))
  end

  def expensive(x) do
    x = x * x
    :timer.sleep(1000)
    x
  end

  def inexpensive(x) do
    x * x
  end
end

data = for x <- 1..100, do: x

 Benchee.run(
  %{
    "pmap, expensive" => fn -> Pmap.pmap(data, :expensive) end,
    "pmap, inexpensive" => fn -> Pmap.pmap(data, :inexpensive) end,
    "map, expensive" => fn -> Enum.map(data, &(Pmap.expensive(&1))) end,
    "map, inexpensive" => fn -> Enum.map(data, &(Pmap.inexpensive(&1))) end
  },
  memory_time: 2
 )

"""
Name                        ips        average  deviation         median         99th %
map, inexpensive       983.51 K        1.02 μs  ±3697.41%        0.89 μs        1.35 μs
pmap, inexpensive        2.46 K      406.45 μs    ±15.08%      403.66 μs      557.18 μs
pmap, expensive       0.00200 K   501196.83 μs     ±0.07%   501063.27 μs   501968.52 μs
map, expensive        0.00002 K 50099628.15 μs     ±0.00% 50099628.15 μs 50099628.15 μs

Comparison:
map, inexpensive       983.51 K
pmap, inexpensive        2.46 K - 399.74x slower +405.43 μs
pmap, expensive       0.00200 K - 492932.24x slower +501195.82 μs
map, expensive        0.00002 K - 49273499.77x slower +50099627.13 μs

Memory usage statistics:

Name                      average  deviation         median         99th %
map, inexpensive          1.56 KB     ±0.00%        1.56 KB        1.56 KB
pmap, inexpensive        39.62 KB     ±0.60%       39.60 KB       39.68 KB
pmap, expensive          40.21 KB     ±3.01%       39.60 KB       42.02 KB
map, expensive            1.56 KB     ±0.00%        1.56 KB        1.56 KB

Comparison:
map, inexpensive          1.56 KB
pmap, inexpensive        39.62 KB - 25.36x memory usage +38.06 KB
pmap, expensive          40.21 KB - 25.73x memory usage +38.64 KB
map, expensive            1.56 KB - 1.00x memory usage +0 KB


"""