Eio.Flow.copy is very slow on mac OS

[jinwoo]

I ran one of the examples from the README file on my macbook.

let () =
  Eio_main.run @@ fun env ->
  Eio.Flow.copy
    (Eio.Stdenv.stdin env)
    (Eio.Stdenv.stdout env)

Compiling and running this code took ~38 sec against a 10G file while the standard library version only took ~2 sec.

Is this a limitation of the eio_luv backend?

[talex5]

Could you try it with the posix backend (#448) and see if that helps?

[patricoferris]

Took this for a spin on a mac (M1) and indeed see a noticeable improvement using the posix backend over libuv.

With Eio_luv:

truncate -s 2G dummy
_build/default/main.exe <dummy | pv >/dev/null
2.00GiB 0:00:13 [ 148MiB/s]

And then with Eio_posix:

truncate -s 2G dummy
_build/default/main.exe <dummy | pv >/dev/null
2.00GiB 0:00:03 [ 569MiB/s]

Whilst having a look I noticed the README still talks about this benchmark in terms of Eio_linux's use of splice but iiuc as of #327 this is not the case, perhaps we should comment that out for now?

[talex5]

Yeah, we should probably replace that section with a graph from https://github.com/ocaml-multicore/retro-httpaf-bench (there are some updates on my https://github.com/talex5/retro-httpaf-bench/tree/eio branch that need merging first though).

[talex5]

With Eio 0.9, macOS will be using the eio_posix backend now and the problem should be fixed.