Sample usage of OCaml parmap library

parmap_sample.ml

(**************************************************************************)
(* Sample usage of OCaml parmap library                                   *)
(* https://github.com/rdicosmo/parmap                                     *)
(*                                                                        *)
(* Computes pi using a simple approximation                               *)
(* \frac{\pi}{4} = \sum_{n=0}^{\infty} \frac{(-1)^n}{2n + 1}              *)
(*                                                                        *)
(* Compile with the following command:                                    *)
(* ocamlfind ocamlopt -linkpkg -package parmap parmap_sample.ml           *)
(**************************************************************************)


let compute_pi start stop =
  let rec loop n sum =
    if n >= stop then sum
    else
      let v = float (1 - (n mod 2) * 2) /. float (2 * n + 1) in
      loop (n + 1) (sum +. v)
  in
  4. *. (loop start 0.)

let () =
  let max_val = 10000000000 in
  let nproc = Parmap.get_default_ncores () in
  let step = max_val / nproc in
  let parts = Array.mapi (fun i v -> i * step) (Array.init nproc (fun x -> x)) in
  let parts_seq = Parmap.A parts in
  let results = Parmap.parmap (fun v -> compute_pi v (v + step - 1)) parts_seq in
  let pi = List.fold_left ((+.)) 0. results in
  print_endline (string_of_float pi)