With the convergence between the multicore and standard runtime across
OCaml 4.10.0 to 4.13.0, the development of OCaml multicore has reached a point
where further integration into OCaml's main branch requires fully committing to
a switch to OCaml multicore.

The OCaml team has decided that the time has come for such a commitment.
The new major version, OCaml 5, will be a multicore version of OCaml.
Moreover, OCaml 4.14 will be the last minor release of the 4.x series of OCaml.

Multicore Minimum Viable Product (MVP)
------------------------------------------------------

The first version of OCaml multicore, code-named OCaml 5.0, will be
a Minimum Viable Product focused on:

- x86-64
- Linux, MacOS, Windows mingw-w64
- Parallelism through Domains [1]
- Concurrency through Effect Handlers [2] (without syntactic support and exposed as functions from the standard library)

Our plan is to integrate the multicore branch into the main branch during the
next 6 months. Hopefully, OCaml 5.0 will then be released between March and
April 2022.

Note that OCaml 5.0 focuses on minimal (solid) support for the multicore
runtime system, and will not provide stable user-facing concurrency and
parallelism libraries. There has been a lot of experimentation [3,4] in the last
few years, and some work remains to offer long-term, user-facing concurrent and
parallel programming abstractions. OCaml 5.0 will be a great time to start
adding concurrency and parallelism to your OCaml programs, but the libraries
will still be in flux.


Long term support for OCaml 4.14
----------------------------------------------

While OCaml 5 is stabilising, we plan to extend the support period for
OCaml 4.14 by publishing minor bugfix releases whenever needed. In particular,
OCaml 4.14 will be supported until all tier-1 architectures and operating systems
are available in OCaml 5, and OCaml 5 sequential performance is close enough to
that of OCaml 4.

The sequential glaciation
---------------------------------

To ensure that maintainers can concentrate on Multicore integration, and avoid
any rebase work for the Multicore developers, the trunk branch will be
feature-frozen starting from November 2021. All non-bugfix non-multicore
contributions will be delayed to after the Multicore integration.
We are calling this period the "sequential glaciation".

We understand that this may be frustrating for our contributors, and apologize
for the delay in getting your nice work reviewed and merged into the codebase.
We hope that the sequential glaciation will be a good opportunity to help with
the Multicore integration, review and testing, and/or focus on non-core-compiler
efforts and the rest of the OCaml ecosystem.

With this early feature-freeze, we also plan to release OCaml 4.14.0 in advance,
between January-February 2022, reducing the concurrency between the OCaml 5.0
and OCaml 4.14.0 releases.

References
---------------
[1] "Retrofitting Parallelism onto OCaml", ICFP 2020, https://arxiv.org/abs/2004.11663
[2] "Retrofitting Concurrency onto OCaml", PLDI 2021, https://arxiv.org/abs/2104.00250
[3] Domainslib -- Parallel Programming over Multicore OCaml, https://github.com/ocaml-multicore/domainslib
[4] eio -- Effects-based Parallel IO for OCaml, https://github.com/ocaml-multicore/eio

Happy hacking,
Florian Angeletti, for the OCaml team.