package domain_shims

README.md

This repository contains a self-contained implementation of the Domain module for OCaml 4.

Motivation

Domain is a new module of the OCaml 5 standard library that exposes the Multicore OCaml runtime's ability to run OCaml computations in parallel.

With domain-shims, you can compile run code that was written for OCaml 5 using the Domain module under OCaml 4. This could make it easier to gradually transition to OCaml 5, while preserving some sort of compatibility with previous versions, instead of forcing all your users to migrate in one go.

The domain-shims implementation of course does not provide true parallelism (as OCaml 5 does): its "fake domains" are implemented using the Thread module of OCaml 4, which never runs threads in parallel. In particular, domain-shims-using code will never get faster by spawning more domains.

We expect our implementation to be correct, in the sense that the observed behavior of domain-shims programs are also valid behaviors according to the OCaml 5 specification of domains. You can think of our non-parallel implementation as one extreme, unlikely and inefficient, but valid, interleaving of parallel computations.

Except for the lack of parallelism, the domain-shims implementation is otherwise pretty reasonable and fast enough: it should get you a valid result, in about the same time as OCaml 5 running on a single-core machine. (Usual time- and memory-performance variations between OCaml 4 and OCaml 5 of course apply here as well.)

Limitations

No Effect support

OCaml 5 also provides effect handlers as a new Effect module of the standard library. We do not provide an implementation of Effect, which is much harder to emulate using regular OCaml 4.

Someone could experiment with an implementation using the delimcc library for delimited control. (Please do this and report! It is probably fun.) We expect that the result would be much slower than the OCaml 5 implementation, probably by an order of magnitude. This would be okay for toy experiments. But it would not be good enough to seriously run libraries relying on effect handlers for performance-sensitive operations, such as Domainslib or Eio.

The other option would be to implement shim versions of Effect-using libraries. We are considering providing a shim for Domainslib in particular, which is very easy to do without emulating effects at all. This general approach sounds scary (no limit to the number of programs you need to provide shims for), but remember that OCaml 5 users are not supposed to use effect handlers too much yet, it was only made available for lightweight concurrent schedulers.

Testing

We tested this code by importing the parallel tests of the OCaml compiler distribution. (Except for a few tests also using Effect). They pass without changes, except for pingpong.ml which required a small tweak to terminate in reasonable time, as it assumes parallelism in practice -- see the comments in the code for more details.