Multicore OCaml: March 2021

Welcome to the March 2021 Multicore OCaml monthly report! The following update and the previous ones have been compiled by me, @kayceesrk and @shakthimaan. We remain broadly on track to integrate the last of the multicore prerequisites into the next (4.13) release, and to propose domains-only parallelism for OCaml 5.0.

Upstream OCaml 4.13 development

The complex safe points PR (#10039) is continuing to make progress, with more refinement towards reducing the binary size increase that results from the introduction of more polling points. Special thanks to @damiendoligez for leaping in with a PR-to-the-PR to home in on a workable algorithm!

Multicore OCaml trees

If there’s one thing we’re not going to miss, it’s git rebasing. The multicore journey began many moons ago with OCaml 4.02, and then 4.04, 4.06, and the current 4.10. We’re pleased to announce the hopefully-last rebase of the multicore OCaml trees to OCaml 4.12.0 are now available. There is now a simpler naming scheme as well to reflect our upstreaming strategy more closely:

  • OCaml 4.12.0+domains is the domains-only parallelism that will be submitted for OCaml 5.0
  • OCaml 4.12.0+domains+effects is the version with domains parallelism and effects-based concurrency.

You can find opam installation instructions for these over at the multicore-opam repository. There is even an ocaml-lsp-server available, so that your favourite IDE should just work!

Domains-only parallelism trees

The bulk of effort this month has been around the integration and debugging of Domain Local Allocation Buffers (DLABs), and also chasing down corner-case failures from stress testing and opam bulk builds. For details, see the long list of PRs in the next section.

We’re also cleaning up historical vestiges in order to reduce the diff to OCaml trunk, in order to clear the path to a clean diff for generating OCaml 5.0 PRs for upstream integration.

Concurrency and Effects trees

The camera-ready paper for PLDI 2021 on Retrofitting Effect handlers onto OCaml is now available on arXiv. The code described in the paper can be used via the 4.12.0+domains+effects opam switches. Please feel free to keep any comments coming to @kayceesrk and myself.

We’ve also been hacking on the multicore IO stack and just beginning to combine concurrency (via effects) and parallelism (via domains) into Linux io_uring, macOS’ Grand Central Dispatch and Windows iocp. We’ll have more to report on this over the next few months, but early benchmarking numbers on Linux are promising.

CI and Benchmarking

We are continuing to expand the testing for different CI configurations for the project. With respect to Sandmark benchmarking, we are in the process of adding the Irmin benchmark. There is also an end-to-end pipeline of using the OCurrent current-bench framework to give us benchmarking results from PRs that can be compared to previous runs.

As always, we begin with the Multicore OCaml updates, which are then followed by the ongoing and completed tasks for the Sandmark benchmarking project. Finally, the upstream OCaml work is listed for your reference.

Detailed Updates

Multicore OCaml



  • ocaml-multicore/ocaml-multicore#484
    Thread allocation buffers

    The PR provides an implementation for thread local allocation
    buffers or Domain Local Allocation Buffers. Code review and
    testing of the changes is in progress.

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

    This is an extension to the Thread allocation buffers PR with
    initialization, lazy resizing of the global minor heap size, and
    rebase to 4.12 branch.


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

    The runtime tests fail when using -O0 instead of -O2 and this
    needs to be investigated further.

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

    The weak-ephe-final tests with a small minor heap (4096 words) cause
    the issue468 test to fail.

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

    A list of requirements to expand the scope and execution of our
    existing CI runs for comprehensive testing.


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

    The ocaml-variants.opam and files have been updated
    to use the Multicore OCaml repository, and to use a local switch
    instead of a global one. The current Multicore OCaml is at the 4.12

  • ocaml-multicore/ocaml-multicore#523
    Systhreads Mutex raises Sys_error

    The error checking for Systhreads Mutex should be inline with trunk,
    instead of the fatal errors reported by Multicore OCaml.

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

    The eventlog implementation has to be ported to the Common Trace
    Format. The log output should be consistent with the
    parallel_minor_gc output, and stress testing need to be performed.



  • ocaml-multicore/ocaml-multicore#490
    Remove getmutablefield from bytecode

    The bytecode compiler and interpreter have been updated by removing
    the getmutablefield opcodes.

  • ocaml-multicore/ocaml-multicore#496
    Replace caml_initialize_field with caml_initialize

    A patch to replace caml_initialize_field, which was earlier used
    with the concurrent minor collector, is now replaced with

  • ocaml-multicore/ocaml-multicore#503
    Re-enable lib-obj and asmcomp/is_static tests

    The lib-obj and asmcomp/is_static tests have been re-enabled and
    the configure settings have been updated for Multicore

  • ocaml-multicore/ocaml-multicore#506
    Replace Op_val with Field

    The use of Op_val (x)[i] has been replaced with Field (x, i) to
    be consistent with trunk implementation.

  • ocaml-multicore/ocaml-multicore#507
    Change interpreter to use naked code pointers

    The changes have been made to identify naked pointers in the
    interpreter stack to be compatible with trunk.

  • ocaml-multicore/ocaml-multicore#516
    Remove caml_root API

    The caml_root variables have been changed to value type and are
    managed as generational global roots. Hence, the caml_root API is
    now removed.


  • ocaml-multicore/ocaml-multicore#511
    Allocate unique root token on the major heap instead of the minor

    The unique root token allocation is now done on the major heap
    allocation that does not raise any exception, and exits cleanly when
    a domain creation fails.

  • ocaml-multicore/ocaml-multicore#513
    Clear the minor heap at the end of a collection in debug runtime

    A debug value is written to every element of the minor heap for
    debugging failures. We now clear the minor heap at the end of a
    minor collection.

  • ocaml-multicore/ocaml-multicore#519
    Make timing test more robust

    The test has been updated to be more resilient for
    testing with DLABs.


  • ocaml-multicore/ocaml-multicore#477
    Move TLS areas to a dedicated memory space

    In order to support Domain Local Allocation Buffer, we now move the
    TLS areas to its own memory alloted space thereby changing the way
    we allocate an individual domain’s TLS.

  • ocaml-multicore/ocaml-multicore#480
    Remove leave_when_done and friends from STW API

    The barriers from caml_try_run_on_all_domains* and stw_request
    are removed by cleaning up the stw_request.leave_when_done

  • ocaml-multicore/ocaml-multicore#481
    Don’t share array amongst domains in gc-roots tests

    Every domain should have its own array, and the parallel global
    roots tests have been updated with this change.

  • ocaml-multicore/ocaml-multicore#494
    Stronger invariants on unix_fork

    We now enforce stronger invariants such that no other domain can run
    alongside domain 0 (caml_domain_alone) for unix_fork.

  • ocaml-multicore/ocaml-multicore#515
    Add memprof stubs to build and stdlib

    The required memprof functions have been added to build stdlib,
    and also to build memprof for the runtime.

Lazy Updates


  • ocaml-multicore/ocaml-multicore#487
    systhreads: set gc_regs_buckets and friends to NULL at thread startup

    Pointers have been initialized to NULL in systhreads/st_stubs.c
    which solves the segmentation

    observed when running the Layers benchmark.

  • ocaml-multicore/ocaml-multicore#491
    Reinitialize child locks after fork

    The runtime needs to operate correctly after a fork, and this
    patch fixes it with proper resetting of domain lock.

  • ocaml-multicore/ocaml-multicore#495
    Fix problems with finaliser orphaning

    A fix for how we merge finalization tables for orphaned finaliser
    work. A test case has also been added to the PR.

  • ocaml-multicore/ocaml-multicore#499
    Fix backtrace unwind

    The unwinding of stacks over callbacks was not happening correctly
    and the discrepancy in caml_next_frame_descriptior is now resolved.

  • ocaml-multicore/ocaml-multicore#509
    Fix for bad setup of Continuation_already_taken exception in bytecode

    A patch to fix the Continuation_already_taken exception which was
    not set up as needed in the bytecode execution.

  • ocaml-multicore/ocaml-multicore#510
    Update a testcase in

    A change in to log the correct output as
    compared to 4.12 branch in ocaml/ocaml.


  • ocaml-multicore/multicore-opam#46
    Multicore compatible ocaml-migrate-parsetree.2.1.0

    The ocaml-migrate-parsetree package uses the effect syntax and now
    builds with Multicore OCaml parallel_minor_gc branch.

  • ocaml-multicore/multicore-opam#47
    Multicore compatible ppxlib

    The effect syntax has been added to ppxlib and is also now
    compatible with Multicore OCaml.

  • ocaml-multicore/multicore-opam#49
    4.12 Multicore configs

    Added configurations to install 4.12.0+domains+effects and
    4.12.0+domains OCaml variants.

  • ocaml-multicore/ocaml-multicore#473
    Building on musl requires dynamically linked execinfo

    The opam files to allow installation on musl-based environments for
    Multicore OCaml have been added to the repository.

  • ocaml-multicore/ocaml-multicore#482
    Check for -lexecinfo in order to build on musl/alpine

    A configure script has been added which checks for -lexecinfo in
    order to support building Multicore OCaml on musl/alpine.


  • ocaml-multicore/ocaml-multicore#502
    Update README to introduce 4.12+domains+effects and 4.12+domains

    We have updated the README file with the current list of active
    branches, and the names of the historic variants.

  • ocaml-multicore/ocaml-multicore#520
    Clarify comment on RacyLazy

    A documentation update in stdlib/lazy.mli that clarifies when
    RacyLazy and Undefined are raised.


  • ocaml-multicore/ocaml-multicore#486
    Sync no-effects-syntax to parallel_minor_gc branch

    The ocaml-multicore:no-effects-syntax branch is now up to date
    with the parallel_minor_gc branch changes.

  • ocaml-multicore/ocaml-multicore#489
    Remove promote_to

    The promote_to function was used in the concurrent minor GC. It is
    not required any more and hence has been removed.

  • ocaml-multicore/ocaml-multicore#500
    Replace caml_modify_field with caml_modify

    The caml_modify_field is no longer necessary and has been replaced
    with caml_modify.



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

    The inclusion of Irmin benchmark with updates to all its
    dependency requirements.

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

    A review of the graph500seq implementation has been
    done, and code changes have been proposed. The macro_bench tag
    will be retained for the graph500 benchmarks.

  • ocaml-bench/sandmark#212
    Increasing the major heap allocation on some benchmarks

    A work in progress to add more longer running benchmarks that
    involve major heap allocation. Some of the parameters have been
    updated with higher values, and more loops have been added as well.

  • We now have integrated the build of Sandmark 2.0 with
    current-bench for
    CI. The results of the benchmark runs are now pushed to a PostgreSQL
    database as shown below:

    docker=# select * from benchmarks;
    -[ RECORD 1 ]--+-------------------------------------------------------
    run_at         | 2021-03-26 11:21:20.64
    repo_id        | local/local
    commit         | 55c6fb6416548737b715d6d8fde6c0f690526e42
    branch         | 2.0.0-alpha+001
    pull_number    | 
    benchmark_name | 
    test_name      | coq.BasicSyntax.v
    metrics        | {"maxrss_kB": 678096, "time_secs": 101.99969387054443}
    duration       | 00:37:52.776357
    -[ RECORD 2 ]--+-------------------------------------------------------
    run_at         | 2021-03-26 11:21:20.64
    repo_id        | local/local
    commit         | 55c6fb6416548737b715d6d8fde6c0f690526e42
    branch         | 2.0.0-alpha+001
    pull_number    | 
    benchmark_name | 
    test_name      | thread_ring_lwt_mvar.20_000
    metrics        | {"maxrss_kB": 8096, "time_secs": 2.6146790981292725}
    duration       | 00:37:52.776357

    We will continue to work on adding more workflows and features to
    current-bench to support Sandmark builds.


  • ocaml-bench/sandmark#202
    Added bench clean target in the Makefile

    A benchclean target to remove the generated benchmarks and its
    results while still retaining the _opam folder has been added to
    the Makefile.

  • ocaml-bench/sandmark#203
    Implement ITER support

    The use of ITER variable is now supported in Sandmark, and you can
    run multiple iterations of the benchmarks. For example, with
    ITER=2, a couple of summary .bench files are created with the
    benchmark results as shown below:

    $ TAG='"run_in_ci"' make run_config_filtered.json
    $ ITER=2 RUN_CONFIG_JSON=run_config_filtered.json make ocaml-versions/4.10.0+multicore.bench 
    $ ls _results/
    4.10.0+multicore_1.orun.summary.bench  4.10.0+multicore_2.orun.summary.bench
  • ocaml-bench/sandmark#208
    Fix params for simple-tests/capi

    A minor fix in run_config.json to correctly pass the arguments to
    the simple-tests/capi benchmark execution. You can verify the same
    using the following commands:

    $ TAG='"lt_1s"' make run_config_filtered.json
    $ RUN_CONFIG_JSON=run_config_filtered.json make ocaml-versions/4.10.0+multicore.bench
  • ocaml-bench/sandmark#210
    Don’t share array in global roots parallel benchmarks

    A patch to not share array in global roots implementation for
    parallel benchmarks.

  • ocaml-bench/sandmark#213
    Resolve dependencies for 4.12.1+trunk, 4.12.0+domains and 4.12.0+domains+effects

    The dependencies/packages have now been updated to be able to
    build 4.12.1+trunk, 4.12.0+domains and 4.12.0+domains+effects
    branches with Sandmark.



  • ocaml/ocaml#10039

    The review of the Safepoints PR is in progress. Special thanks to
    Damien Doligez for his code
    on safepoints
    and inserting polls. There is still work to be done on

Many thanks to all the OCaml users, developers and contributors in the
community for their support to the project. Stay safe!


  • API: Application Programming Interface
  • CI: Continuous Integration
  • CTF: Common Trace Format
  • DLAB: Domain Local Allocation Buffer
  • GC: Garbage Collector
  • OPAM: OCaml Package Manager
  • PR: Pull Request
  • STW: Stop The World
  • TLS: Thread Local Storage

Sorry if this has been answered elsewhere – am I reading this correctly that effects aren’t intended for OCaml 5? If not, are there plans to merge effect handlers into mainline OCaml at some later stage, or is it going to be kept separate?

Thanks for the thorough monthly reports!

1 Like

I believe effects are slated for 5.1 release. According to @sadiq.

1 Like

Just to be clear, effects are not slated for any official release of OCaml yet. The focus is on making sure that OCaml 5.0 is a really solid release with domains-only parallelism, and then to look into subsequent changes required for concurrency. There are some considerations about how precisely effects should work when they materialise in a mainline OCaml release:

  • semantics: how will the introduction of effect handlers complicate (or improve!) reasoning about OCaml programs? @fpottier and Paulo Emílio de Vilhena have a paper on “separation logic for effect handlers” at POPL 2021 that is a good starting point to read from.
  • performance: on the other side, we want to make sure that effect handlers do not introduce a runtime overhead to sequential OCaml code. Our paper on “retrofitting effect handlers onto OCaml” addresses this, with a particular restriction on making sure handlers are one-shot only (a position conveniently reinforced by @fpottier’s paper above, as it also makes program reasoning much easier).
  • type safety: our current implementation of effects is untyped, which doesn’t fit that well into the spirit of ML. There is active research ongoing into developing a theory of typed effects lead by @lpw25, and you can see a tech talk about this.
  • portability: OCaml is an extremely portable language, we want to make sure we can maintain clean compilation to existing targets (such as js_of_ocaml) and new ones (such as webassembly). @kayceesrk is active in the wasm working groups on this topic, and we’re using MirageOS (which has compiler forks for esp32 embedded systems) to experiment with overheads using unikernels.
  • ergonomics: and last but not least, the hardest one to answer: once we develop a workable model for typed effects in OCaml, does it work at scale on large codebases and also for writing incremental code that can be locally reasoned about? The only answer to this is to build prototypes of a significant size, which is something we’re working on at OCaml Labs as part of efforts like our multicore IO stack. I don’t talk about this much at the moment as we regularly thrown prototypes away, but we’re converging on some stability in recent months.

To risk an analogy, imagine that we’re flying from Cambridge to Fiji for a nice post-pandemic vacation. To get to our end goal, we have to fly via a few intermediate cities (say, London and Sydney). We need to be very careful not to screw up our landing into Sydney because we’re so excited about eventually getting to Fiji – if we do that, there is no glorious holiday for us! And you know what? Sydney’s not so bad a place to hang out and adjust to jetlag while we carefully plan the flight path to Fiji.

We could really use the community’s help with OCaml 5.0, as it’s going to be a major release. Rest assured that we’re thinking (and working hard) on how concurrency should also show up in OCaml, but it won’t happen until we get OCaml 5.0 out of the door. Core OCaml development is all about careful, iterative progress to avoid the risk of losing the properties that make OCaml so reliable for all the downstream projects that depend on us.

Having said all that, I can’t wait to get to get to Fiji either. It’s going to be an awesome vacation when we finally arrive.


Apologies if the question is misguided. I started re-learning OCaml and want to try multicore. I am having a problem with installing the LSP server. Here’s my step-by-step:

opam update
opam switch create 4.12.0+domains+effects --packages=ocaml-variants.4.12.0+domains+effects --repositories=multicore=git+,default
eval $(opam env)
opam switch set 4.12.0+domains+effects
opam install dune domainslib

Then finally:

opam install dune merlin.3.4.1 dot-merlin-reader.3.4.1 ocaml-lsp-server                                                                                                                                                    ─╯
[NOTE] Package dune is already installed (current version is 2.8.5).
The following dependencies couldn't be met:
  - merlin → ocaml < 4.12
      base of this switch (use `--unlock-base' to force)

No solution found, exiting

OK then, let’s try with --unlock-base:

opam install --unlock-base dune merlin.3.4.1 dot-merlin-reader.3.4.1 ocaml-lsp-server                                                                                                                                      ─╯
[NOTE] Package dune is already installed (current version is 2.8.5).
The following actions will be performed:
  ↘ downgrade ocaml-variants      4.12.0+domains+effects to 4.11.2+spacetime [required by ocaml]
  ⊘ remove    ocaml-option-nnp    1
  ↘ downgrade ocaml-config        2 to 1                                     [required by ocaml]
  ↘ downgrade ocaml               4.12.0 to 4.11.2                           [required by merlin, dot-merlin-reader, dune, ocaml-lsp-server]
  ↻ recompile ocamlfind           1.9.1                                      [uses ocaml]
  ↻ recompile dune                2.8.5
  ∗ install   stdlib-shims        0.3.0                                      [required by ocaml-lsp-server]
  ∗ install   result              1.5                                        [required by merlin, dot-merlin-reader, ocaml-lsp-server]
  ∗ install   easy-format         1.3.2                                      [required by yojson]
  ∗ install   dune-build-info     2.8.5                                      [required by ocaml-lsp-server]
  ∗ install   csexp               1.5.1                                      [required by merlin, dot-merlin-reader, ocaml-lsp-server]
  ∗ install   cppo                1.6.7                                      [required by yojson]
  ∗ install   biniou              1.2.1                                      [required by yojson]
  ∗ install   yojson              1.7.0                                      [required by merlin, dot-merlin-reader, ocaml-lsp-server]
  ∗ install   ppx_yojson_conv_lib v0.14.0                                    [required by ocaml-lsp-server]
  ∗ install   merlin              3.4.1
  ∗ install   dot-merlin-reader   3.4.1
  ∗ install   ocaml-lsp-server    1.4.1
===== ∗ 12   ↻ 2   ↘ 3   ⊘ 1 =====
Do you want to continue? [Y/n]

At this point I cancel because I find this worrying:

  ↘ downgrade ocaml-variants      4.12.0+domains+effects to 4.11.2+spacetime [required by ocaml]

Any recommendations? I only have 4.12 as the previous default switch in opam, if that’s helpful.

This is likely because the current tip of multicore is maybe at 4.11.2? I am just looking for confirmation that the output of the command is normal and is what I am supposed to be seeing.

EDIT: I went ahead and ran it anyway but it made no difference because compiling merlin.3.4.1 and dot-merlin-reader.3.4.1 failed (macOS).

Thanks for your fantastic work. I’ll try again after the next update! :slight_smile:

1 Like

You don’t need to install merlin with ocaml-lsp-server. the latter vendors the former.

1 Like

Oh. Thank you, didn’t realize.

Thanks for mentioning about the error @dimitarvp and thanks @Ulugbek for clarifying about ocaml-lsp-server. The LSP installation instructions in multicore-opam were meant for 4.10.0+multicore. They’ve been updated to reflect the 4.12 branch and should work now with 4.12.0+domains and 4.12.0+domains+effects. Some changes are necessary to get merlin and ocaml-lsp working with 4.10 multicore variants, it’s not the case with 4.12, they just work out of the box now. Apologies for the confusion!

Feel free to post here or open an issue in the issue tracker if you run into any other errors.


I did all the steps from scratch (after deleting the opam switch beforehand) and this time everything went smooth and I was able to compile and run a hobby project of mine on 4.12+multicore. Thank you!

Having in mind the rather specific way of installing 4.12.+multicore+effects, can I expect an opam update / opam upgrade to renew that installation in the future? Dumb question I know, but I am absolutely clueless of the OCaml’s ecosystem still.

Surely not a dumb question! :slight_smile:
Multicore’s compatibility with the wider ecosystem has been steadily improving. I’d expect opam update && opam upgrade to work just fine on a 4.12+domains+effects switch.

An exception could be multicore specific packages, whose upstream updates may not work with multicore. In which case I’d suggest pinning the multicore version of those packages.