Multicore OCaml: April 2021

Multicore OCaml: April 2021

Welcome to the April 2021 Multicore OCaml monthly report! My friends and colleagues on the project in India are going through a terrible second wave of the Covid pandemic, but continue to work to deliver all the updates from the Multicore OCaml project. This month’s update along with the previous updates have been compiled by myself, @kayceesrk and @shakthimaan.

Upstream OCaml 4.13 development

GC safepoints continues to be the focus of the OCaml 4.13 release development for multicore. While it might seem quiet with only one PR being worked on, you can also look at the compiler fork where an intrepid team of adventurous compiler backend hackers have been refining the design. You can also find more details of ongoing upstream work in the first core compiler development newsletter. To quote @xavierleroy from there, “it’s a nontrivial change involving a new static analysis and a number of tweaks in every code emitter, but things are starting to look good here.”.

Multicore OCaml trees

The switch to using OCaml 4.12 has now completed, and all of the development PRs are now working against that version. We’ve put a lot of focus into establishing whether or not Domain Local Allocation Buffers (ocaml-multicore#508) should go into the initial 5.0 patches or not.

What are DLABs? When testing multicore on larger core counts (up to 128), we observed that there was a lot of early promotion of values from the minor GCs (which are per-domain). DLABs were introduced in order to encourage domains to have more values that remained heap-local, and this should have increased our scalability. But computers being computers, we noticed the opposite effect – although the number of early promotions dropped with DLABs active, the overall performance was either flat or even lower! We’re still working on profiling to figure out the root cause – modern architectures have complex non-uniform and hierarchical memory and cache topologies that interact in unexpected ways. Stay tuned to next month’s monthly about the decision, or follow ocaml-multicore#508 directly!

The multicore ecosystem

Aside from this, the test suite coverage for the Multicore OCaml project has had significant improvement, and we continue to add more and more tests to the project. Please do continue with your contribution of parallel benchmarks. With respect to benchmarking, we have been able to build the Sandmark-2.0 benchmarks with the current-bench continuous benchmarking framework, which provides a GitHub frontend and PostgreSQL database to store the results. Some other projects such as Dune have also started also using current-bench, which is nice to see – it would be great to establish it on the core OCaml project once it is a bit more mature.

We are also rolling out a multicore-specific CI that can do differential testing against opam packages (for example, to help isolate if something is a multicore-specific failure or a general compilation error on upstream OCaml). We’re pushing this live at the moment, and it means that we are in a position to begin accepting projects that might benefit from multicore. If you do have a project on opam that would benefit from being tested with multicore OCaml, and if it compiles on 4.12, then please do get in touch. We’re initially folding in codebases we’re familiar with, but we need a diversity of sources to get good coverage. The only thing we’ll need is a responsive contact within the project that can work with us on the integration. We’ll start reporting on project statuses if we get a good response to this call.

As always, we begin with the Multicore OCaml ongoing and completed tasks. This is followed by the Sandmark benchmarking project updates and the relevant Multicore OCaml feature requests in the current-bench project. Finally, upstream OCaml work is mentioned for your reference.

Multicore OCaml



  • ocaml-multicore/domainslib#23
    Running tests: moving to dune runtest from manual commands in run_test target

    At present, the tests are executed with explicit exec commands in
    the Makefile, and the objective is to move to using the dune runtest command.

  • ocaml-multicore/ocaml-multicore#522
    Building the runtime with -O0 rather than -O2 causes testsuite to fail

    The use of -O0 optimization fails the runtime tests, while -O2
    optimization succeeds. This needs to be investigated further.

  • ocaml-multicore/ocaml-multicore#526
    weak-ephe-final issue468 can fail with really small minor heaps

    The failure of issue468 test is currently being looked into for the
    weak-ephe-final tests with a small minor heap (4096 words).

  • ocaml-multicore/ocaml-multicore#528
    Expand CI runs

    The PR implements parallel “callback” “gc-roots” “effects”
    “lib-threads” “lib-systhreads” tests, with taskset -c 0 option,
    and using a small minor heap. The CI coverage needs to be enhanced
    to add more variants and optimization flags.

  • ocaml-multicore/ocaml-multicore#542
    Add ephemeron lazy test

    Addition of tests to cover ephemerons, lazy values and domain
    lifecycle with GC.

  • ocaml-multicore/ocaml-multicore#545
    ephetest6 fails with more number of domains

    The test fails when more number of domains are
    spawned, and also deadlocks at times.

  • ocaml-multicore/ocaml-multicore#547
    Investigate failure

    The is disabled in the test suite and it is
    failing. This needs to be investigated further.

  • ocaml-multicore/ocaml-multicore#549
    zmq-lwt test failure

    An opam-ci bug that has reported a failure in the zmq-lwt test. It
    is throwing a Zmq.ZMQ_exception with a Context was terminated
    error message.


  • ocaml-multicore/ocaml-multicore#508
    Domain Local Allocation Buffers

    The code review and the respective changes for the Domain Local
    Allocation Buffer implementation is actively being worked upon.

  • ocaml-multicore/ocaml-multicore#514
    Update instructions in ocaml-variants.opam

    The ocaml-variants.opam and have been updated to
    now use the Multicore OCaml repository. We want different version
    strings for +domains and +domains+effects for the branches.

  • ocaml-multicore/ocaml-multicore#527
    Port eventlog to CTF

    The code review on the porting of the eventlog implementation to
    the Common Trace Format is in progress. The relevant code changes
    have been made and the tests pass.

  • ocaml-multicore/ocaml-multicore#529
    Fiber size control and statistics

    A feature request to set the maximum stack size for fibers, and to
    obtain memory statistics for the same.



  • ocaml-multicore/ocaml-multicore#533
    Systhreads synchronization use pthread functions

    The pthread_* functions are now used directly instead of
    caml_plat_* functions to be in-line with OCaml trunk. The
    Sys_error is raised now instead of Fatal error.

  • ocaml-multicore/ocaml-multicore#535
    Remove Multicore stats collection

    The configurable stats collection functionality is now removed from
    Multicore OCaml. This greatly reduces the diff with trunk and makes
    it easy for upstreaming.

  • ocaml-multicore/ocaml-multicore#536
    Remove emit_block_header_for_closure

    The emit_block_header_for_closure is no longer used and hence
    removed from asmcomp sources.

  • ocaml-multicore/ocaml-multicore#537
    Port @stedolan “Micro-optimise allocations on amd64 to save a register”

    The upstream micro-optimise allocations on amd64 to save a register
    have now been ported to Multicore OCaml. This greatly brings down
    the diff on amd64’s emit.mlp.


  • ocaml-multicore/ocaml-multicore#531
    Make native stack size limit configurable (and fix Gc.set)

    The stack size limit for fibers in native made is now made
    configurable through the Gc.set interface.

  • ocaml-multicore/ocaml-multicore#534
    Move allocation size information to frame descriptors

    The allocation size information is now propagated using the frame
    descriptors so that they can be tracked by statmemprof.

  • ocaml-multicore/ocaml-multicore#548
    Multicore implementation of Mutex, Condition and Semaphore

    The Mutex, Condition and Semaphore modules are now fully
    compatible with stdlib features and can be used with Domain.


  • ocaml-multicore/ocaml-multicore#532
    Addition of test for finaliser callback with major cycle

    Update to code that adds a test wherein a
    finaliser is run with a root in a register.

  • ocaml-multicore/ocaml-multicore#541
    Addition of a parallel tak testcase

    Parallel test cases to stress the minor heap and also enter the
    minor GC organically without calling a Gc function or a domain
    termination have now been added to the repository.

  • ocaml-multicore/ocaml-multicore#543
    Parallel version of weaklifetime test

    The parallel implementation of the test has now
    been added to the test suite, where the Weak structures are accessed
    by multiple domains.

  • ocaml-multicore/ocaml-multicore#546
    Coverage of domain life-cycle in domain_dls and ephetest_par tests

    Improvement to and for better
    coverage for domain lifecycle testing.


  • ocaml-multicore/ocaml-multicore#530
    Fix off-by-1 with gc_regs buckets

    An off-by-1 bug is now fixed when scanning the stack for the
    location of the previous gc_regs bucket.

  • ocaml-multicore/ocaml-multicore#540
    Fix small alloc retry

    The Alloc_small macro was not handling the case when the GC
    function does not return a minor heap with enough size, and this PR
    fixes the same along with code clean-ups.


  • ocaml-multicore/retro-httpaf-bench#3
    Add cohttp-lwt-unix to the benchmark

    A cohttp-lwt-unix benchmark is now added to the
    retro-httpaf-bench package along with the update to the

  • ocaml-multicore/domainslib#22
    Move the CI to 4.12 Multicore and Github Actions

    The CI has been switched to using GitHub Actions instead of
    Travis. The version of Multicore OCaml used in the CI is now

  • ocaml-multicore/mulicore-opam#51
    Update merlin and ocaml-lsp installation instructions for 4.12 variants

    The has been updated with instructions to use merlin and
    ocaml-lisp for 4.12+domains and 4.12+domains+effects branches.

  • dwarf_validator
    DWARF validation tool

    The DWARF validation tool in is now made
    available in a public repository. It single steps through the binary
    as it executes, and unwinds the stack using the DWARF directives.





  • We now have the frontend showing the graph results for Sandmark 2.0 builds
    with current-bench for
    CI. A raw output of the graph is shown below:

The Sandmark 2.0 benchmarking is moving to use the current-bench
tooling. You can now create necessary issues and PRs for the
Multicore OCaml project in the current-bench project using the
multicore label.

  • ocaml-bench/sandmark#209
    Use rule target kronecker.txt and remove from macro_bench

    A rewrite of the graph500seq implementation based on
    the code review suggestions is currently being worked upon.

  • ocaml-bench/sandmark#215
    Remove Gc.promote_to from

    We are updating Sandmark to run with 4.12+domains and
    4.12+domains+effects, and this patch removes Gc.promote_to from the


  • ocurrent/current-bench#87
    Run benchmarks for old commits

    We would like to be able to re-run the benchmarks for older commits
    in a project for analysis and comparison.

  • ocurrent/current-bench#103
    Ability to set scale on UI to start at 0

    The raw results plotted in the graph need to start from [0, y_max+delta] for the y-axis for better comparison. A PR is available for the same, and the fixed output is shown in the following graph:

  • ocurrent/current-bench#105
    Abstract out Docker image name from pipeline/lib/

    The Multicore OCaml uses ocaml/opam:ubuntu-20.10-ocaml-4.10 image
    while the pipeline/lib/ uses ocaml/opam, and it will
    be useful to use an environment variable for the same.

  • ocurrent/current-bench#106
    Use --privileged with Docker run_args for Multicore OCaml

    The Sandmark environment uses bwrap for Multicore OCaml benchmark
    builds, and hence we need to run the Docker container with
    --privileged option. Otherwise, the build exits with an Operation not permitted error.

  • ocurrent/current-bench#107
    Ability to start and run only PostgreSQL and frontend

    For Multicore OCaml, we provision the hardware with different
    configuration settings for various experiments, and using an ETL
    tool to just load the results to the PostgreSQL database and
    visualize the same in the frontend will be useful.

  • ocurrent/current-bench#108
    Support for native builds for bare metals

    In order to avoid any overhead with Docker, we need a way to run the
    Multicore OCaml benchmarks on bare metal machines.



  • ocurrent/current-bench#75
    Fix production deployment; add instructions

    The is now updated with documentation for doing a
    production deployment of current-bench.

  • ocurrent/current-bench#90
    Add some solutions to errors that users might run into

    Based on our testing of current-bench with Sandmark-2.0, we now have
    updated the FAQ in the file.


  • ocurrent/current-bench#96
    Remove hardcoded URL for the frontend

    The frontend URL is now abstracted out from the code, so that we can
    deploy a current-bench instance on any new pristine server.

  • ocaml-bench/sandmark#204
    Adding as a benchmark to Sandmark

    The Irmin benchmark is now added to Sandmark along with
    its dependencies. This is tagged with gt_100s.



  • ocaml/ocaml#10039

    This PR is a work-in-progress. Thanks to Mark Shinwell and Damien
    Doligez and Xavier Leroy for their valuable feedback and code suggestions.

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


  • AMD: Advanced Micro Devices
  • CI: Continuous Integration
  • CTF: Common Trace Format
  • DLAB: Domain Local Allocation Buffer
  • DWARF: Debugging With Attributed Record Formats
  • ETL: Extract Transform Load
  • GC: Garbage Collector
  • OPAM: OCaml Package Manager
  • PR: Pull Request
  • UI: User Interface
  • URL: Uniform Resource Locator
  • ZMQ: ZeroMQ