domainslib VS RFCs

For nested parallel computations (think Scientific Programming, where one would use OpenMP, Rust Rayon, etc), we have domainslib [1]. Eio, a direct-style, effect-based IO library is pretty competitive against Rust Tokio [2]. The performance will only get better as we get closer to the 5.0 release.

[1] https://github.com/ocaml-multicore/domainslib

[2] See the http server performance graphs at https://tarides.com/blog/2022-03-01-segfault-systems-joins-t...

1. Domains are the unit of parallelism. A domain is essentially an OS thread with a bunch of extra runtime book-keeping data. You can use Domain.spawn (https://github.com/ocaml-multicore/ocaml-multicore/blob/5.00...) to spawn off a new domain which will run the supplied function and terminate when it finishes. This is heavyweight though, domains are expected to be long-running.

2. Domainslib is the library developed alongside multicore to aid users in exploiting parallelism. It supports nested parallelism and is pretty highly optimised (https://github.com/ocaml-multicore/domainslib/pull/29 for some graphs/numbers). The domainslib repo has some good examples: https://github.com/ocaml-multicore/domainslib/tree/master/te...

3. We've not tested against other forms of parallelism. There isn't anything stopping you exploiting SIMD in addition to parallelism from domains.

4. No, we've not compared performance by OS.

5. No plans for the multicore team to look at accelerator integration at the moment.

[3] Domainslib -- Parallel Programming over Multicore OCaml, https://github.com/ocaml-multicore/domainslib

Found while on my hunt for POPL23 preprints. I wonder how much it relates to the unboxed types project for OCaml.

A middle ground between uniform representation and specializing for every type is specializing for every data layout. See the unboxed types RFC for OCaml.

For both 2 and 3 you may find https://github.com/ocaml/RFCs/blob/unboxed-types/rfcs/unboxe... very interesting. I'm hoping that progresses beyond the RFC stage.