OCaml: measure execution time in toplevel

Question

How should we measure the execution time of a function in the OCaml toplevel?

Possible duplicate of [Running time in Ocaml](https://stackoverflow.com/questions/9061421/running-time-in-ocaml) — Mars, Aug 23 '17 at 02:59

score 11 · Accepted Answer · answered Dec 16 '14 at 18:07

11

As @user3075773 says, you can use Sys.time. However note that it returns processor time (CPU time). More often I want to know the wall clock time (elapsed time). You can get this from Unix.gettimeofday:

let time f x =
    let t = Unix.gettimeofday () in
    let fx = f x in
    Printf.printf "execution elapsed time: %f sec\n"
        (Unix.gettimeofday () -. t);
    fx

answered Dec 16 '14 at 18:07

Jeffrey Scofield

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This is a crucial distinction. Thanks for highlighting it. One might want either kind of time, but you definitely don't want to mix them up if they differ. – Mars Sep 07 '17 at 05:41

score 8 · Answer 2 · answered Dec 16 '14 at 17:47

8

let time f x =
    let t = Sys.time() in
    let fx = f x in
    Printf.printf "execution time: %fs\n" (Sys.time() -. t);
    fx

Works with any function, it will print how long did the function take to run in the toplevel.

answered Dec 16 '14 at 17:47

user3075773

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score 2 · Answer 3 · answered Sep 07 '17 at 06:17

Using gettimeofday like the accepted answer suggests is not a good idea as it is sensitive to calendar time operating system adjustements (e.g. via ntp).

If you just want CPU time then using Sys.time is fine. If you want wall-clock time, then a monotonic time source should be used. One is available for OCaml in the mtime package.

OCaml: measure execution time in toplevel

3 Answers3