Unhandled Exception with OCaml 5.0.0~beta1 - ocaml

Question

I'm inconsistently getting this error in a first experiment with OCaml 5.0.0~beta1:
Fatal error: exception Stdlib.Effect.Unhandled(Domainslib__Task.Wait(_, _))
My setup:
Processor: Intel(R) Core(TM) i7-8750H CPU # 2.20GHz
Debian 10 (buster)
opam version 2.1.3 installed as binary from this script
opam switch: "→ 5.0.0~beta1 ocaml-base-compiler.5.0.0~beta1 5.0.0~beta1"
After a quick read of this tutorial, I copied the parallel_matrix_multiply function and added some code in the end just to use it:
open Domainslib
let parallel_matrix_multiply pool a b =
let i_n = Array.length a in
let j_n = Array.length b.(0) in
let k_n = Array.length b in
let res = Array.make_matrix i_n j_n 0 in
Task.parallel_for pool ~start:0 ~finish:(i_n - 1) ~body:(fun i ->
for j = 0 to j_n - 1 do
for k = 0 to k_n - 1 do
res.(i).(j) <- res.(i).(j) + a.(i).(k) * b.(k).(j)
done
done);
res ;;
let pool = Task.setup_pool ~num_domains:3 () in
let a = Array.make_matrix 2 2 1 in
let b = Array.make_matrix 2 2 2 in
let c = parallel_matrix_multiply pool a b in
for i = 0 to 1 do
for j = 0 to 1 do
Printf.printf "%d " c.(i).(j)
done;
print_char '\n'
done;;
I then compile it with no errors with
ocamlfind ocamlopt -linkpkg -package domainslib parallel_for.ml
and then comes the problem: executing the generated a.out file sometimes (rarely) prints the expected output
4 4
4 4
but usually ends with the error mentioned earlier:
Fatal error: exception Stdlib.Effect.Unhandled(Domainslib__Task.Wait(_, _))
Sorry if I am making some trivial mistake, but I can't understand what is going on, especially given that the error happens inconsistently.

Answer 1

The parallel_matrix_multiply computation is running outside of the Domainslib scheduler, thus whenever a task yields to the scheduler, the Wait effect is unhandled and transformed into a Effect.Unhandled exception.
The solution is to run the parallel computation within Task.run:
...
let c = Task.run pool (fun () -> parallel_matrix_multiply pool a b) in
...