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69 lines
2.6 KiB
Markdown
69 lines
2.6 KiB
Markdown
# Moonpool
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[](https://github.com/c-cube/moonpool/actions/workflows/main.yml)
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A pool within a bigger pool (ie the ocean). Here, we're talking about
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pools of [Thread.t] which live within a fixed pool of [Domain.t].
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## Usage
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The user can create several thread pools. These pools use regular posix threads,
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but the threads are spread across multiple domains (on OCaml 5), which enables
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parallelism.
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```ocaml
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# #require "moonpool";;
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# let pool = Moonpool.Pool.create ~min:4 ();;
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val pool : Moonpool.Pool.t = <abstr>
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# let rec fib x =
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if x <= 1 then 1 else fib (x-1) + fib (x-2);;
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val fib : int -> int = <fun>
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# List.init 10 fib;;
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- : int list = [1; 1; 2; 3; 5; 8; 13; 21; 34; 55]
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# let fibs = Array.init 30 (fun n -> Moonpool.Fut.spawn ~on:pool (fun () -> fib n));;
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val fibs : int Moonpool.Fut.t array =
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[|<abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>;
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<abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>;
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<abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>;
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<abstr>; <abstr>; <abstr>; <abstr>; <abstr>; <abstr>|]
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# Moonpool.Fut.join_array fibs |> Moonpool.Fut.wait_block;;
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- : int array Moonpool.or_error =
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Ok
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[|1; 1; 2; 3; 5; 8; 13; 21; 34; 55; 89; 144; 233; 377; 610; 987; 1597; 2584;
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4181; 6765; 10946; 17711; 28657; 46368; 75025; 121393; 196418; 317811;
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514229; 832040|]
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```
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## OCaml versions
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This works for OCaml >= 4.05.
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- On OCaml 4.xx, there are no domains, so this is just a library for regular thread pools
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with not actual parallelism (except for threads that call C code that releases the runtime lock, that is).
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- on OCaml 5.xx, there is a fixed pool of domains (using the recommended domain count).
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These domains do not do much by themselves, but we schedule new threads on them, and group
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threads from each domain into pools.
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Each domain might thus have multiple threads that belong to distinct pools (and several threads from
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the same pool, too — this is useful for threads blocking on IO).
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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.
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Multiple threads have to share a single core and do not run in parallel on it[^1].
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We can therefore build pools that spread their worker threads on multiple cores to enable parallelism within each pool.
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TODO: actually use https://github.com/haesbaert/ocaml-processor to pin domains to cores,
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possibly optionally using `select` in dune.
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## License
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MIT license.
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## Install
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```sh, skip
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$ opam install moonpool
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```
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[^1]: let's not talk about hyperthreading.
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