package moonpool

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Pools of threads supported by a pool of domains


Dune Dependency







thread pool domain

Published: 17 Jun 2023



A pool within a bigger pool (ie the ocean). Here, we're talking about pools of Thread.t which live within a fixed pool of Domain.t.

This fixed pool of domains is shared between all the pools in moonpool. The rationale is that we should not have more domains than cores, so it's easier to pre-allocate exactly that many domains, and run more flexible thread pools on top.

In addition, Moonpool.Fut provides futures/promises that execute on these thread pools. The futures are thread safe.


The user can create several thread pools. These pools use regular posix threads, but the threads are spread across multiple domains (on OCaml 5), which enables parallelism.

# let pool = Moonpool.Pool.create ~min:4 ();;
val pool : Moonpool.Pool.t = <abstr>

# let rec fib x =
    if x <= 1 then 1 else fib (x-1) + fib (x-2);;
val fib : int -> int = <fun>

# List.init 10 fib;;
- : int list = [1; 1; 2; 3; 5; 8; 13; 21; 34; 55]

# let fibs = Array.init 35 (fun n -> Moonpool.Fut.spawn ~on:pool (fun () -> fib n));;
val fibs : int Moonpool.Fut.t array =
  [|<abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>;
    <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>;
    <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>;
    <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>;
    <abstr>; <abstr>; <abstr>|]

# Moonpool.Fut.join_array fibs |> Moonpool.Fut.wait_block;;
- : int array Moonpool.Fut.or_error =
 [|1; 1; 2; 3; 5; 8; 13; 21; 34; 55; 89; 144; 233; 377; 610; 987; 1597; 2584;
   4181; 6765; 10946; 17711; 28657; 46368; 75025; 121393; 196418; 317811;
   514229; 832040; 1346269; 2178309; 3524578; 5702887; 9227465|]

OCaml versions

This works for OCaml >= 4.08.

  • On OCaml 4.xx, there are no domains, so this is just a library for regular thread pools with not actual parallelism (except for threads that call C code that releases the runtime lock, that is).

  • on OCaml 5.xx, there is a fixed pool of domains (using the recommended domain count). These domains do not do much by themselves, but we schedule new threads on them, and group threads from each domain into pools. Each domain might thus have multiple threads that belong to distinct pools (and several threads from the same pool, too — this is useful for threads blocking on IO).

    A useful analogy is that each domain is a bit like a CPU core, and Thread.t is a logical thread running on a core. Multiple threads have to share a single core and do not run in parallel on it[^1]. We can therefore build pools that spread their worker threads on multiple cores to enable parallelism within each pool.

TODO: actually use to pin domains to cores, possibly optionally using select in dune.


MIT license.


$ opam install moonpool

[^1]: let's not talk about hyperthreading.

Dependencies (3)

  1. either >= "1.0"
  2. dune >= "3.0"
  3. ocaml >= "4.08"

Dev Dependencies (3)

  1. mdx >= "1.9.0" & with-test
  2. odoc with-doc
  3. trace with-test

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