OCaml Multicore - 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 ephetest6.ml fails when more number of domains are spawned, and also deadlocks at times.

  • ocaml-multicore/ocaml-multicore#547 Investigate weaktest.ml failure

    The weaktest.ml 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 configure.ac 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 test_finaliser_gc.ml 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 weaklifetime.ml 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 domain_dls.ml and ephetest_par.ml 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 Dockerfile.

  • 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 4.12+domains+effects.

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

    The README.md 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 eh_frame_check.py 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:

current-bench Sandmark-2.0 frontend |312x499

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 kernel1.ml implementation based on the code review suggestions is currently being worked upon.

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

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


  • 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:

current-bench frontend fix 0 baseline

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

    The Multicore OCaml uses ocaml/opam:ubuntu-20.10-ocaml-4.10 image while the pipeline/lib/pipeline.ml 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 HACKING.md 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 HACKING.md 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 layers.ml as a benchmark to Sandmark

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



  • ocaml/ocaml#10039 Safepoints

    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