OCaml Multicore - 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 layers.ml 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 configure.ac 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 branch.

  • 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 caml_initialize.

  • 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 NO_NAKED_POINTERS.

  • 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 timing.ml 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 implementation.

  • 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 fault 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 principality-and-gadts.ml

    A change in principality-and-gadts.ml 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 layers.ml as a benchmark to Sandmark

    The inclusion of Irmin layers.ml 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 kernel1.ml 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 Safepoints

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

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