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https://github.com/ocaml-multicore/domainslib

Parallel Programming over Domains
https://github.com/ocaml-multicore/domainslib

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Parallel Programming over Domains

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# Domainslib - Nested-parallel programming



Domainslib provides support for nested-parallel programming. Domainslib provides async/await mechanism for spawning parallel tasks and awaiting their results. On top of this mechanism, domainslib provides parallel iteration functions. At its core, domainslib has an efficient implementation of work-stealing queue in order to efficiently share tasks with other domains.



Here is a _sequential_ program that computes nth Fibonacci number using recursion:



```ocaml

(* fib.ml *)

let n = try int_of_string Sys.argv.(1) with _ -> 1



let rec fib n = if n < 2 then 1 else fib (n - 1) + fib (n - 2)



let main () =

  let r = fib n in

  Printf.printf "fib(%d) = %d\n%!" n r



let _ = main ()

```



We can parallelise this program using Domainslib:



```ocaml

(* fib_par.ml *)

let num_domains = try int_of_string Sys.argv.(1) with _ -> 1

let n = try int_of_string Sys.argv.(2) with _ -> 1



(* Sequential Fibonacci *)

let rec fib n = 

  if n < 2 then 1 else fib (n - 1) + fib (n - 2)



module T = Domainslib.Task



let rec fib_par pool n =

  if n > 20 then begin

    let a = T.async pool (fun _ -> fib_par pool (n-1)) in

    let b = T.async pool (fun _ -> fib_par pool (n-2)) in

    T.await pool a + T.await pool b

  end else 

    (* Call sequential Fibonacci if the available work is small *)

    fib n



let main () =

  let pool = T.setup_pool ~num_domains:(num_domains - 1) () in

  let res = T.run pool (fun _ -> fib_par pool n) in

  T.teardown_pool pool;

  Printf.printf "fib(%d) = %d\n" n res



let _ = main ()

```



The parallel program scales nicely compared to the sequential version. The results presented below were obtained on a 2.3 GHz Quad-Core Intel Core i7 MacBook Pro with 4 cores and 8 hardware threads.



```bash

$ hyperfine './fib.exe 42' './fib_par.exe 2 42' \

            './fib_par.exe 4 42' './fib_par.exe 8 42'

Benchmark 1: ./fib.exe 42

  Time (mean ± sd):     1.217 s ±  0.018 s    [User: 1.203 s, System: 0.004 s]

  Range (min … max):    1.202 s …  1.261 s    10 runs



Benchmark 2: ./fib_par.exe 2 42

  Time (mean ± sd):    628.2 ms ±   2.9 ms    [User: 1243.1 ms, System: 4.9 ms]

  Range (min … max):   625.7 ms … 634.5 ms    10 runs



Benchmark 3: ./fib_par.exe 4 42

  Time (mean ± sd):    337.6 ms ±  23.4 ms    [User: 1321.8 ms, System: 8.4 ms]

  Range (min … max):   318.5 ms … 377.6 ms    10 runs



Benchmark 4: ./fib_par.exe 8 42

  Time (mean ± sd):    250.0 ms ±   9.4 ms    [User: 1877.1 ms, System: 12.6 ms]

  Range (min … max):   242.5 ms … 277.3 ms    11 runs



Summary

  './fib_par2.exe 8 42' ran

    1.35 ± 0.11 times faster than './fib_par.exe 4 42'

    2.51 ± 0.10 times faster than './fib_par.exe 2 42'

    4.87 ± 0.20 times faster than './fib.exe 42'

```



More example programs are available [here](https://github.com/ocaml-multicore/domainslib/tree/master/test).



## Installation



You can install this library using `OPAM`. 



```bash

$ opam switch create 5.1.0

$ opam install domainslib

```



## Development



If you are interested in hacking on the implementation, then `opam pin` this repository:



```bash

$ opam switch create 5.0.0+trunk --repo=default,alpha=git+https://github.com/kit-ty-kate/opam-alpha-repository.git

$ git clone https://github.com/ocaml-multicore/domainslib

$ cd domainslib

$ opam pin add domainslib file://`pwd`

```