Multicore OCaml: May 2020 update

Multicore OCaml: May 2020

Welcome to the May 2020 update from the Multicore OCaml team! As with previous updates, many thanks to @shakthimaan and @kayceesrk for help assembling this month’s roundup.

A major milestone in May 2020 has been the completion of rebasing of Multicore OCaml all the way from 4.06 to 4.10! The Parallel Minor GC variant that performs stop-the-world parallel minor collection is the default branch for the compiler, which means that compatibility with C bindings is now much simpler than with the older minor GC design.

I’ve received many questions asking if this means that multicore OCaml will “just work” with the opam ecosystem now. Not quite yet: we estimate that we are now one PR away from this working, which requires that the existing Threads module is backported to multicore OCaml to support the older (non-parallel-in-the-runtime but concurrent) uses of threading that existing OCaml supports. This effort was begun a year ago by @jhw in #240 and now rebased and being reviewed by @engil in #342. Once that is merged and tested by us on a bunch of packages and bulk builds, we should be good to start using Multicore OCaml with opam. Stay tuned for more on that next month!

The ongoing and completed tasks for the Multicore OCaml are listed first, which are then followed by improvements to the Sandmark benchmarking project. Finally, the status of the contributions to upstream OCaml are mentioned for your reference. This month has also seen a meeting of the core OCaml runtime developers to assign post-rebasing tasks (such as also porting statmemprof, how to handle non-x86 architectures, Windows support, etc) to ensure a more complete view of the upstreaming tasks ahead. The task list is long, but steadily decreasing in length.

As to how to contribute currently, there is an incredibly exciting seam of work that has now started on the appropriate programming abstractions to support parallel algorithms in OCaml. See this thread for more on that, and also on the Domainslib repository for more low-level examples of traditional parallel algorithms. In a month or so, we expect that the multicore switch will also be more suitable for use with opam, but don’t let that stop you from porting your favourite parallel benchmark to Domainslib today.

Multicore OCaml


  • ocaml-multicore/ocaml-multicore#339
    Proposal for domain-local storage

    A new proposal for implementing a domain-local storage in Multicore
    OCaml has been created.

  • ocaml-multicore/domainslib#8
    Task library slowdown if the number of domains is greater than 8

    This is an ongoing investigation on why there is a slowdown with
    domainslib version 0.2 for the Game of Life benchmark when the
    number of domains is greater than eight.

  • ocaml-multicore/ocaml-multicore#340
    Fix in concurrent_minor_gc

    An implementation is provided for using
    Atomic.get and Atomic.compare_and_set to obtain the correct
    semantics to handle assertion failure in interp.c.

  • ocaml-multicore/ocaml-multicore#338
    Introduce Lazy.try_force and Lazy.try_force_val

    The Lazy.try_force and Lazy.try_force_val functions are
    implemented for concurrent lazy abstractions to handle the RacyLazy

  • ocaml-multicore/ocaml-multicore#333
    Random module functions slowdown on multiple cores

    There is an observed slowdown for the Random module on multiple
    cores, and the issue is being analysed in detail.

  • ocaml-multicore/ocaml-multicore#343
    Fix extcall noalloc DWARF

    The patch provides a fix for the emitted DWARF information for
    extcall noalloc. This PR is currently under review.


  • ocaml-multicore/ocaml-multicore#337
    Update opam file to 4.10.0+multicore

    The rebasing of Multicore OCaml to 4.11 branch
    (parallel_minor_gc_4_11) point is now complete! The
    file for 4.10.0+multicore has been made the default in the
    multicore-opam repository.

  • ocaml-multicore/ocaml-multicore#335
    Add byte_domain_state.tbl to install files

    A patch to install byte_domain_state.tbl and caml/*.h files has
    now been included in the runtime/Makefile which is required for
    parallel_minor_gc_4_10 branch.

  • The Multicore OCaml major GC implementation verification using the
    SPIN model checker is available at the following GitHub repository



  • ocaml-bench/sandmark#115
    Task API Port: LU-Decomposition, Floyd Warshall, Mandelbrot, N-body

    Porting of the following programs - LU-Decomposition, Floyd
    Warshall, Mandelbrot and N-body to use the Task API.

  • ocaml-bench/sandmark#37
    Make benchmark wrapper user configurable

    The ability to dynamically specify the input commands and their
    respective arguments to the benchmark scripts is currently being

  • ocaml-bench/sandmark#106
    Promote dune > 2.0

    Sandmark works with dune 1.11.4 and we need to support dune greater
    than 2.0 moving forward. The upgrade path with the necessary package
    builds is being tested.


  • ocaml-bench/sandmark#109
    Added sequential-interactive.ipynb

    An interactive notebook to run and analyse sequential benchmarks has
    been included. Given an artifacts directory with the benchmark
    files, the notebook prompts you in the GUI to select different
    commit and compiler variants for analysis. A sample screenshot of
    the UI is shown below:

    The PR adds error handling, user input validation and the project
    README has also been updated.

  • ocaml-bench/sandmark#111
    Add parallel initialisation and parallel copy to LU decomposition benchmark

    The parallel initialisation is now added to LU decomposition
    numerical benchmark

  • ocaml-bench/sandmark#113
    Use --format=columns with pip3 list in Makefile

    A fix for the “DEPRECATION: The default format will switch to
    columns in the future” warning when using pip3 list has now been
    added to the Makefile with the use of the --format=columns option.

  • ocaml-bench/sandmark#116
    Use sudo for parallel benchmark builds

    The Makefile has been updated with the right combination of sudo
    and OPAM environment variables so that we can now run parallel
    benchmarks in Sandmark. The sudo command is required exclusively for
    using the chrt command. We can now perform nightly builds for both
    serial and parallel benchmarks!

  • ocaml-bench/sandmark#118
    Refactored README and added JupyterHub info

    The Sandmark README file has now been updated to include information
    on configuration, usage of JupyterHub, benchmarking and a quick
    start guide!



  • ocaml/ocaml#9541
    Add manual page for the instrumented runtime

    A draft manual for the instrumented runtime eventlog tracing has
    been created. Please feel free to review the document and share your
    valuable feedback.

  • ocaml/dune#3500
    Support building executables against OCaml 4.11 instrumented runtime

    OCaml 4.11.0 has built-in support for the instrumented runtime, and
    it will be useful to have dune generate instrumented targets.


  • ocaml/ocaml#9082
    Eventlog tracing system

    The Eventlog tracing proposal for the OCaml runtime that uses the
    Binary Trace Format (CTF) is now merged with upstream OCaml

  • ocaml/ocaml#9534
    [RFC] Dynamic check for naked pointers

    An RFC for adding the ability to dynamically identify naked pointers
    in the 4.10.0 compiler.

  • ocaml/ocaml#9573
    Reimplement Unix.create_process and related functions without Unix.fork

    The use of process creation functions in the Unix module is not
    suitable for Multicore OCaml, for both behaviour and efficiency. The
    patch provides an implementation that uses posix_spawn.

  • ocaml/ocaml#9564
    Add a macro for out-of-heap block header

    This PR adds a macro definition to construct a out-of-heap block
    header in runtime/caml/mlvalues.h. The objective is to use the
    header for out of heap objects.

As always, we would like to thank all the OCaml developers and users for their continued support and contribution to the project. Stay safe out there.


  • API: Application Programming Interface
  • CTF: Common Trace Format
  • DWARF: Debugging With Attributed Record Formats
  • GC: Garbage Collector
  • GUI: Graphical User Interface
  • LU: Lower-Upper
  • OPAM: OCaml Package Manager
  • PIP: Pip Installs Python
  • PR: Pull Request
  • RFC: Request for Comments
  • UI: User Interface