Multicore OCaml: June 2020

Welcome to the June 2020 Multicore OCaml report! As with previous updates, many thanks to @shakthimaan and @kayceesrk for collating the updates for the month of June 2020. This is an incremental update; new readers may find it helpful to flick through the previous posts first.

This month has seen a tremendous surge of activity on the upstream OCaml project to prepare for multicore integration, as @xavierleroy and the core team have driven a number of initiatives to prepare the OCaml project for the full multicore featureset. To reflect this, from next month we will have a status page on the ocaml-multicore wiki with the current status of both our multicore branch and the upstream OCaml project itself.

Why not from this month? Well, there’s good news and bad news. Last month, I observed that we are a PR away from most of the opam ecosystem working with the multicore branch. The good news is that we are still a single PR away from it working, but it’s a different one :slight_smile: The retrofitting of the Threads library has brought up some design complexities, and so rather than putting in a “bandaid” fix, we are integrating a comprehensive solution that will work with system threads, domains and (eventually) fibres. That work has taken some time to get right, and I hope to be able to update you all on an opam-friendly OCaml 4.10.0+multicore in a few weeks.

Aside from this, there have been a number of other improvements going into the multicore branches: mingw Windows support, callstack improvements, fixing the Unix module and so on. The full list is in the detailed report later in this update.

Sandmark benchmarks

A major milestone in this month has been the upgrade to the latest dune.2.6.0 to build Multicore OCaml 4.10.0 for the Sandmark benchmarking project. A number of new OPAM packages have been added, and the existing packages have been upgraded to their latest versions. The Multicore OCaml code base has seen continuous performance improvements and enhancements which can be observed from the various PRs mentioned in the report.

We would like to thank:

  • @xavierleroy for working on a number of multicore-prequisite PRs to
    make stock OCaml ready for Multicore OCaml.
  • @camlspotter has reviewed and accepted the camlimages changes
    and made a release of camlimages.5.0.3 required for Sandmark.
  • @dinosaure for updating the decompress test benchmarks for Sandmark
    to build and run with dune.2.6.0 for Multicore OCaml 4.10.0.

A chapter on Parallel Programming in Multicore OCaml with topics on task pool, channels section, profiling with code examples is being written. We shall provide an early draft version of the document to the community for your valuable feedback.


Our “Retrofitting Parallism onto OCaml” paper has been officially accepted at ICFP 2020 which will be held virtually between August 23-28, 2020. A preprint of the paper was made available earlier, and will be updated in a few days with the camera-ready version for ICFP. Please do feel free to send on comments and queries even after the paper is published, of course.

Excitingly, another multicore-related paper on Cosmo: A Concurrent Separation Logic for Multicore OCaml will also be presented at the same conference.

The Multicore OCaml updates are first listed in our report, which are followed by improvements to the Sandmark benchmarking project. Finally, the changes made to upstream OCaml which include both the ongoing and completed tasks are mentioned for your reference.

Multicore OCaml


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

    An RFC proposal to implement a domain-local storage in Multicore
    OCaml. Kindly review the idea and share your feedback!

  • ocaml-multicore/ocaml-multicore#342
    Implementing the threads library with Domains

    An effort to rebase @jhwoodyatt’s implementation of the Thread
    library for Domains.

  • ocaml-multicore/ocaml-multicore#357
    Implementation of systhreads with pthreads

    Exploring the possibilty of implementing systhreads with pthreads,
    while still maintaining compatibility with the existing solution.

  • ocaml/dune#3548
    Dune fails to pick up secondary compiler

    The ocaml-secondary-compiler fails to install with dune.2.6.0. This
    is required as Multicore OCaml cannot build the latest dune without
    systhreads support.


  • ocaml-multicore/multicore-opam#22
    Update dune to 2.6.0

    The dune version in the Multicore OPAM repository is now updated to
    use the latest 2.6.0.

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

    An implementation of Lazy.try_force and Lazy.try_force_val
    functions to implement concurrent lazy abstractions.

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

    A patch that introduces through Atomic.get that
    provides the appropriate read barrier for correct exchange
    semantics for caml_atomic_exchange in memory.c.

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

    The DWARF information emitted for extcall noalloc had broken
    backtraces and this PR fixes the same.

  • ocaml-multicore/ocaml-multicore#345
    Absolute exception stack

    The representation of the exception stack is changed from relative
    addressing to absolute addressing and the results are promising. The
    Sandmark serial benchmark results after the change is illustrated in
    the following graph:

  • ocaml-multicore/ocaml-multicore#347
    Turn on -Werror by default

    Adds a --enable-warn-error option to configure to treat C
    compiler warnings as errors.

  • ocaml-multicore/ocaml-multicore#353
    Poll for interrupts in cpu_relax without locking

    Use Caml_check_gc_interrupt first to poll for interrupts without
    locking, and then proceeding to handle the interrupt with the lock.

  • ocaml-multicore/ocaml-multicore#354
    Add Caml_state_field to domain_state.h

    The Caml_state_field macro definition in domain_state.h is
    required for base-v0.14.0 to build for Multicore OCaml 4.10.0 with

  • ocaml-multicore/ocaml-multicore#355
    One more location to poll for interrupts without lock

    Another use of Caml_check_gc_interrupt first to poll for
    interrupts without lock, similar to

  • ocaml-multicore/ocaml-multicore#356
    Backup threads for domain

    Introduces backup threads to perform GC and handle service
    interrupts when the domain is blocked in the kernel.

  • ocaml-multicore/ocaml-multicore#358
    Fix up bad CFI information in amd64.S

    Add missing CFI_ADJUST directives in runtime/amd64.S for
    caml_call_poll and caml_allocN.

  • ocaml-multicore/ocaml-multicore#359
    Inline caml_domain_alone

    The PR makes caml_domain_alone an inline function to improve
    performance for caml_atomic_cas_field and other atomics in

  • ocaml-multicore/ocaml-multicore#360
    Parallel minor GC inline mask rework

    The inline mask rework for the promotion path to the
    parallel_minor_gc branch gives a 3-5% performance improvement for
    test_decompress sandmark benchmark, and a decrease in the executed
    instructions for all other benchmarks.

  • ocaml-multicore/ocaml-multicore#361
    Mark stack push work credit

    The PR improves the Multicore mark work accounting to be in line
    with stock OCaml.

  • ocaml-multicore/ocaml-multicore#362
    Iloadmut does not clobber rax and rdx when we do not have a read barrier

    A code clean-up to free the registers rax and rdx for OCaml code
    when Iloadmut is used.



  • ocaml-bench/sandmark#8
    Ability to run compiler variants in Sandmark

    A feature to specify configure options when building compiler
    variants such as flambda is useful for development and
    testing. This feature is being worked upon.

  • ocaml-bench/sandmark#107
    Add Coq benchmarks

    We are continuing to add more benchmarks to Sandmark for Multicore
    OCaml and investigating adding the Coq
    benchmarks to our repertoire!

  • ocaml-bench/sandmark#124
    User configurable paramwrapper added to Makefile

    A PARAMWRAPPER environment variable can be passed as an argument
    by specifying the --cpu-list to be used for parallel benchmark

  • ocaml-bench/sandmark#131
    Update decompress benchmarks

    Thanks to @dinosaure for updating the decompress benchmarks in order
    to run them with dune.2.6.0 for Multicore OCaml 4.10.0.

  • ocaml-bench/sandmark#132
    Update dependency packages to use dune.2.6.0 and Multicore OCaml 4.10.0

    Sandmark has been running with dune.1.11.4, and we need to move to
    the latest dune.2.6.0 for using Multicore OCaml 4.10.0 and beyond,
    as mentioned in Promote dune to >
    . The PR
    updates over 30 dependency packages and successfully builds both
    serial and parallel benchmarks!


  • camlspotter/camlimages#1
    Use dune-configurator instead of configurator for camlimages

    A new release of camlimages.5.0.3 was made by @camlspotter after
    accepting the changes to camlimages.opam in order to build with

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

    The changes to use the Domainslib.Task API for the listed benchmarks
    have been merged.

  • ocaml-bench/sandmark#121
    Mention sudo access for script

    The file has been updated with the necessary sudo
    configuration steps to execute the script for
    nightly builds.

  • ocaml-bench/sandmark#125
    Add cubicle benchmarks

    The German PFS and Szymanski's mutual exclusion algorithm cubicle
    benchmarks have been included in Sandmark.

  • ocaml-bench/sandmark#126
    Update ocaml-versions README to reflect 4.10.0+multicore

    The README has now been updated to reflect the latest 4.10.0
    Multicore OCaml compiler and its variants.

  • ocaml-bench/sandmark#129
    Add target to run parallel benchmarks in the CI

    The .drone.yml file used by the CI has been updated to run both the
    serial and parallel benchmarks.

  • ocaml-bench/sandmark#130
    Add missing dependencies in multicore-numerical

    The domainslib library has been added to the dune file for the
    multicore-numerical benchmark.



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

    The instrumented runtime
    has been merged to OCaml 4.11.0. A manual for the same has been
    created and is under review.


  • ocaml/ocaml#9619
    A self-describing representation for function closures

    The PR provides a way to record the position of the environment for
    each entry point for function closures.

  • ocaml/ocaml#9649
    Marshaling for the new closure representation

    The output_value marshaler has been updated to use the new closure
    representation. There is no change required for the input_value

  • ocaml/ocaml#9655
    Introduce type Obj.raw_data and functions Obj.raw_field,
    Obj.set_raw_field to manipulate out-of-heap pointers

    The PR introduces a type Obj.bits, and functions Obj.field_bits
    and Obj.set_field_bits to read and write bit representation of
    block fields to support the no-naked-pointer operation.

  • ocaml/ocaml#9678
    Reimplement Obj.reachable_word using a hash table to detect sharing

    The caml_obj_reachable_words now uses a hash table instead of
    modifying the mark bits of block headers to detect sharing. This is
    required for compatibility with Multicore OCaml.

  • ocaml/ocaml#9680
    Naked pointers and the bytecode interpreter

    The bytecode interpreter implementation is updated to support the
    no-naked-pointers mode operation as required by Multicore OCaml.

  • ocaml/ocaml#9682
    Signal handling in native code without the page table

    The patch uses the code fragment table instead of a page table
    lookup for signal handlers to know whether the signal came from
    ocamlopt-generated code.

  • ocaml/ocaml#9683
    globroots.c: adapt to no-naked-pointers mode

    The patch considers out-of-heap pointers as major-heap pointers in
    no-naked-pointers mode for global roots management.

  • ocaml/ocaml#9689
    Generic hashing for the new closure representation

    The hashing functions have been updated to use the latest closure
    representation from
    ocaml/ocaml#9619 for the
    no-naked-pointers mode.

  • ocaml/ocaml#9698
    The end of the page table is near

    The PR eliminates some of the use of the page tables in the runtime
    system when built with no-naked-pointers mode.

Our thanks to all the OCaml developers and users in the community for
their continued support and contribution to the project. Stay safe!


  • API: Application Programming Interface
  • CFI: Call Frame Information
  • CI: Continuous Integration
  • DWARF: Debugging With Attributed Record Formats
  • GC: Garbage Collector
  • ICFP: International Conference on Functional Programming
  • OPAM: OCaml Package Manager
  • PR: Pull Request
  • RFC: Request for Comments

Thanks for the awesome work!

Approximately, how much time is needed to finish that?

Has anyone successfully installed and used domainslib?

Looks like it requires a special version of dune from the multicore repo. However, is there a way to force installation of dune from a specific repo?
Or there’s no way to use both the multicore and the ordinary opam repositories for now and I have to make a multicore setup in a clean environment and install every package by hand there if it’s not in the multicore repo yet?

For dune specifically, the multicore opam repository has a dune package definition that uses a ocaml-secondary-compiler trick to install dune.

If you installed the switch using the instructions at
you can install a relatively recent dune from its repo. I think dune 2.6.2 isn’t published yet, but 2.6.1 is available. You can install it via opam install dune.2.6.1

That’s what I’ve been trying. Straight opam install domainslib in a fresh switch tries to pull dune 2.6.2—obviously from the official opam repo rather than the multicore one.

Installing dune 2.6.1 by hand does work (though I’m not sure if it comes from the multicore or the official repo). Then weird issues start:

utop # #require "domainslib";;
Cannot find file /home/dmbaturin/.opam/4.10.0+multicore/lib/domainslib/domainslib.cma.

utop # #require "domainslib";;
(* No error second time, hhm *)

utop # Domainslib.Task.parallel_for ;;
Line 1:
Error: Reference to undefined global `Domainslib__Task'

If I’m doing something wrong, please let me know.

I think when setting up the multicore switch the instructions add both the multicore repo and the regular opam repo to the switch. I’m guessing that’s why it tries to fetch dune 2.6.2 (when no version is specified) since that’s the highest version available that should fit the constraint specified by domainslib.

Hmm i remember having some issues when trying to run domainslib examples under the toplevel but I’m not sure about what’s the problem here. The actual library seems fine. I had just moved onto trying domainslib in regular programs, but this might be worth bringing up as an issue for the multicore project.

Thanks for your reply. I’ll experiment with it in compiled code and report the findings.

Also, is there an easy way to produce a +multicore+static+musl variant so that experimental Linux binaries can be easily distributed to people for testing?

The issue with using domainslib in utop has been fixed here. This update is not yet released to multicore-opam.
If you pin domainslib to dev-repo using opam pin add domainslib --dev-repo you should be able to use domainslib in utop.


Thanks @sudha! This makes the experience for just exploring the library much nicer :smile:

I can confirm I can use domainslib from the toplevel with the dev version. Thanks for the tip!

See the latest update – dune will soon work out of the box with multicore OCaml once the DEC PR is merged.