Measure execution time of a function in milliseconds at runtime

bench.ml

(** [bench work] runs a function [work] and returns the amount of milliseconds
    it took to finish. Uses [Sys.time] to mark the start and end of execution. *)
let bench work =
  let start_time = Sys.time () in
  work ();
  let end_time = Sys.time () in
  (end_time -. start_time) *. 1000.0
;;

(* Let's give it a test run! *)

Printf.printf "100M item list construction took %f ms.\n"
@@ bench @@ fun () ->
  let xs : int list ref = ref [] in
  for i = 1 to 100_000_000 do
    xs := i :: !xs
  done;
  (*> Computing the 10th fibonacci sequence number took 5260.875000 ms. *)
;;

(* Or testing an existing function *)

let rec fib = function
  | 0 -> 0
  | 1 -> 1
  | n -> fib (n - 1) + fib (n - 2)
;;

Printf.printf "Computing the 41th fibonacci sequence number took %f ms.\n"
@@ bench @@ fun () -> fib 10
(* Computing the 10th fibonacci sequence number took 10452.497000 ms. *)
;;