OCaml Multicore - October 2021

Welcome to the October 2021 Multicore OCaml monthly report! The previous updates along with this update have been compiled by me, @ctk21, @kayceesrk and @shakthimaan.

As @octachron announced last month, the core team has committed to an OCaml 5.0 release next year with multicore and the effects runtime. This month has seen tremendous activity in our multicore trees to prepare an upstream-friendly version, with a number of changes made to make the code ready for ocaml/ocaml and reduce the size of the diff. Recall that we have been feeding in multicore-related changes steadily since way back in OCaml 4.09, and so we are now down to the really big pieces. Therefore the mainline OCaml trunk code is now being continuously being merged into our 5.00 staging branch, and test coverage has increased accordingly.

In the standard library, we continue to work and improve on thread safety by default. Since effect handlers are confirmed to go into 5.0 as well, they now have their own module in the stdlib as well. The multicore library ecosystem is also evolving with the changes to support OCaml 5.00, and in particular, Domainslib has had significant updates and improvements as more usecases build up. The integration of the Sandmark performance harness with current-bench is also actively being worked upon.

We would like to acknowledge the following people for their contribution:

  • Török Edwin was able to reproduce the bug in Task.pool management Domainslib#43, and has also provided a PR to fix the same.
  • Sid Kshatriya has created PR#83 for Eio to use the Effect Handlers module.

Our focus in November is going to continue to be on relentlessly making a 5.0 staging tree, and we are preparing for a series of working groups with the core OCaml teams (taking up an entire week) to conduct preliminary code review on the full patchset. Stay tuned for how that has gone by the start of December!

As always, the Multicore OCaml updates are listed first, which contain the upstream efforts, merges with trunk, updates to test cases, bug fixes, and documentation improvements. This is followed by the ecosystem updates on Domainslib, Tezos, and Eio. The Sandmark and current-bench tasks are finally listed for your reference.

Multicore OCaml




  • ocaml-multicore/ocaml-multicore#656 Core testsuite workflow

    A draft PR to implement a workflow to run the Core's testsuite once a day.

  • ocaml-multicore/ocaml-multicore#720 Improve ephemerons compatibility with testsuite

    The PR imports upstream fixes to make ephemerons work with infix objects, and provides a fix for weaktest.ml.

  • ocaml-multicore/ocaml-multicore#722 Testsuite: Re-enable signals_alloc testcase

    The signals_alloc testcase has been enabled, and the PR also attempts to ensure the bytecode interpreter polls for signals.

  • ocaml-multicore/ocaml-multicore#723 beat.ml failure on GitHub Action MacOS runners

    An investigation on the beat.ml test failure in the testsuite for the CI execution runs.


  • ocaml-multicore/ocaml-multicore#669 Set thread names for domains

    A patch that implements thread naming for Multicore OCaml. It provides an interface to name Domains and Threads differently.

  • ocaml-multicore/ocaml-multicore#698 Return free pools to the OS

    The pool_release is in the shared_heap and does not return memory to the OS. An ongoing discussion on how much memory to hold, and to reclaim with space overhead setting.

  • ocaml-multicore/ocaml-multicore#703 Possible loop in caml_enter_blocking_section when no domain can handle a blocked signal

    A scenario that can be triggered when a domain that blocks a specific set of signals exists where no other domain can process the signal, and can be caused by a loop in caml_enter_blocking_section.

  • ocaml-multicore/ocaml-multicore#725 Blocked signal infinite loop fix

    A monotonic recorded_signals_counter has been introduced to fix the possible loop in caml_enter_blocking_section when no domain can handle a blocked signal.

  • ocaml-multicore/ocaml-multicore#726 Marshalling of concurrently-modified objects is unsafe

    The marshalling of objects being mutated on a different domain must be handled correctly and should be safe. It should not cause a segmentation fault or crash.



  • ocaml-multicore/ocaml-multicore#662 Disable changes check on 5.0

    The .github/workflows/hygiene.yml has been updated to disable check on 5.00 to avoid noise on the change entries.

  • ocaml-multicore/ocaml-multicore#676 Fix 5.00 install

    The caml/byte_domain_state.tbl has been removed, and README.adoc has been renamed to README.stock.adoc in order to build cleanly with OCaml 5.00 branch.

  • ocaml-multicore/ocaml-multicore#675 Align Bytes.unsafe_of_string / Bytes.unsafe_to_string to OCaml trunk

    The Pbytes_to_string / Pbytes_of_string use in bytecomp/bytegen.ml are now aligned with upstream OCaml.

  • ocaml-multicore/ocaml-multicore#677 Remove debugging nop

    The debugging nop primitive is not required for upstreaming and has been cleaned up. The PR also fixes check-typo whitespace in emit.mlp to match that trunk.

  • ocaml-multicore/ocaml-multicore#679 Remove caml_read_field

    The use of caml_read_field has been removed as the existing Field provides all the necessary information making it closer to upstream OCaml.

  • ocaml-multicore/ocaml-multicore#681 Revert to ocaml/trunk version of otherlibs/unix

    unixsupport.c, cstringv.c and files in otherlibs/unix have been updated to be similar to ocaml/ocaml.

  • ocaml-multicore/ocaml-multicore#684 Remove historical for_handler and Reperform_noloc in lambda/matching

    The for_handler function and Reperform_noloc in lambda/matching.ml{,i} are not required to be upstreamed and hence have been removed.

  • ocaml-multicore/ocaml-multicore#685 Remove Init_field from interp.c

    The interp.c file has been updated to be closer to ocaml/ocaml. The check-typo errors have been fixed, and the Init_field macro has been cleaned up.

  • ocaml-multicore/ocaml-multicore#704 Remove Sync.poll and nanoseconds from Domain

    The Domain module has been updated to include only the changes required for upstreaming. Domain.Sync.poll, and Domain.nanosecond have been removed. Domain.Sync.cpu_relax has been renamed to Domain.cpu_relax. platform.h has been updated with fixes for check-typo.

  • ocaml-multicore/ocaml-multicore#706 Revert otherlibs/win32unix to ocaml/trunk

    The otherlibs/win32unix/* files have been updated to be closer to ocaml/ocaml.

  • ocaml-multicore/ocaml-multicore#708 Remove maybe stats

    The caml_maybe_print_stats primitive to output statistics and the s option to OCAMLRUNPARAM have now been removed.

  • ocaml-multicore/ocaml-multicore#724 Runtime: Remove unused fields from io.h

    Remove revealed and old_revealed from runtime/caml/io.h as they have also been removed from ocaml/ocaml.

  • ocaml-multicore/ocaml#2 Update trunk to the latest upstream trunk

    The PR is an attempt to help with the OCaml 5.0 difference output. With the changes, you can successfully do make && make tests. The summary of the results is provided below:

    2918 tests passed
     40 tests skipped
      0 tests failed
    105 tests not started (parent test skipped or failed)
      0 unexpected errors
    3063 tests considered
  • ocaml-multicore/ocaml#3 Latest 5.00 Commits

    The recent commits from trunk have now been merged to the ocaml-multicore 5.00 branch.

  • ocaml-multicore/ocaml-multicore#718 Deprecate Sync and timer_ticks from Domain

    The patch synchronizes the changes to 4.12.0+domains+effects with the mainline 5.00 branch.

Thread Safe

  • ocaml-multicore/ocaml-multicore#632 Str module multi domain safety

    The PR#635 makes lib-str domain safe to work concurrently with Multicore OCaml.

  • ocaml-multicore/ocaml-multicore#672 Codefrag thread safety

    The PR introduces a lock-free skiplist to make codefrag thread safe. The code fragments cannot be freed as soon as they are removed, but, they are added to a list and cleaned up during a later stop-the-world pause.



  • ocaml-multicore/ocaml-multicore#654 Enable effects tests

    The effect handler tests have now been re-added since the syntax support has been added to Multicore OCaml.

  • ocaml-multicore/ocaml-multicore#658 Enable last dynlink test

    The lib-dynlink-private test has now been enabled to run in the CI.

  • ocaml-multicore/ocaml-multicore#659 Reimport the threadsigmask test and remove systhread-todo test directory

    The lib-systhreads-todo test on signal handling and tick thread missing from systhreads has been reactivated in the the CI.

  • ocaml-multicore/ocaml-multicore#660 Fixups and housekeeping for testsuite/disabled file

    The check-typo problems for 80 character line, and unnecessary test/promotion in testsuite/disabled have been fixed.

  • ocaml-multicore/ocaml-multicore#661 Testsuite: Re-enable pr9971

    The pr9971 test has been re-enabled to run in the CI.

  • ocaml-multicore/ocaml-multicore#688 Better signal handling in systhreads

    Improvements to the signal handling in systhreads that fixes the threadsigmask testcase failure in the CI.

  • ocaml-multicore/ocaml-multicore#712 Otherlibs: Unix.kill should check for pending signals

    The unix_kill test case has been re-enabled to ensure that Unix.kill checks for pending signals on return.


  • ocaml-multicore/ocaml-multicore#672 Check-typo fixes for major_gc, so the changes in #672 don't get clobbered

    A patch that fixes check-typo issues in runtime/major_gc.c.

  • ocaml-multicore/ocaml-multicore#696 Stdlib: Fix typos in effectHandlers.mli

    A few typos in stdlib/effectHandlers.mli have been fixed.

  • ocaml-multicore/ocaml-multicore#697 Remove dead code and clear up comments in the minor gc

    A non-functional change that clears up the comments in the minor and major GC files.

  • ocaml-multicore/ocaml-multicore#699 Cleanup fiber implementation and add documentation

    The unused code in amd64.S has been removed and formatting has been fixed. The addition of 24 bytes at the top of the stack for an external call is no longer needed and has been removed.

  • ocaml-multicore/ocaml-multicore#713 Clarify documentation of Lazy wrt. RacyLazy and Undefined exceptions.

    The documentation in stdlib/lazy.mli has been updated to clarify on the behaviour of try_force and thread safety.

  • ocaml-multicore/ocaml-multicore#717 Tighten code comments in minor_gc.c

    The PR explains promotion of ephemeron keys to avoid introducing a barrier, and uses /* ... */ style comments.

  • ocaml-multicore#docs Docs

    A documentation repository for OCaml 5.00 that contains the design and proposed upstreaming plan.

Effect Handlers



  • ocaml-multicore/domainslib#43 Possible bug in Task.pool management

    Török Edwin has reproduced the segmentation fault using 4.12.0+domains with domainslib 0.3.1 on AMD Ryzen 3900X CPU, and has also provided a draft PR with a fix!

  • ocaml-multicore/domainslib#46 Provide a way to iterate over all the pools

    A requirement to be able to iterate over all the pools created in domainslib. A use case is to tear down all the pools. A weak hash set can be used to store a weak pointer to the pools.

  • ocaml-multicore/domainslib#47 Task.await deadlock (task finished but await never returns)

    A query on nesting Task.await inside Task.async, and Task.async inside Task.async. A sample code snippet, stack trace and platform information have also been provided to reproduce a deadlock scenario.

  • ocaml-multicore/domainslib#48 Move ws_deque to lockfree

    A request to move the work-stealing deque in domainslib to ocaml-multicore/lockfree, and make domainslib depend on this new lockfree implementation.

  • ocaml-multicore/domainslib#49 Should we expose multi-channel from the library?

    A query on whether Multicore OCaml users will find Non-FIFO multi-channel implementation useful. Domainslib already provides FIFO channels.

  • ocaml-multicore/domainslib#50 Multi_channel: Allow more than one instance per program with different configurations

    A draft PR contributed by Török Edwin in lib/multi_channel.ml and lib/task.ml to remove use of a global key with a per-channel key.

  • ocaml-multicore/domainslib#51 Utilise effect handlers

    The tasks are now created using effect handlers, and a new test_deadlock.ml tests the same. The change will work only with 4.12+domains and 5.00. The performance results from the Turing machine (Intel Xeon Gold 5120 CPU @ 2.20 GHz, 28 isolated cores) is shown below:


  • ocaml-multicore/tezos#8 ci.Dockerfile throws warning

    The ci.Dockerfile on Ubuntu 20.10 throws C99 warnings on _Atomic with GCC 10.3.0.

  • ocaml-multicore/tezos#10 Fix make build-deps, fix NixOS support

    conf-perl is no longer required upstream and has been removed from the tezos-opam-repository. The patch also fixes make build-deps/build-dev-deps.

  • ocaml-multicore/ocaml-uring#39 Test failures on NixOS

    The ocaml-uring master branch is showing test failures with dune runtest on NixOS.

  • ocaml-multicore/eio#85 Any plans on supporting js_of_ocaml?

    A query by Konstantin A. Olkhovskiy (Lupus) on whether EIO can compile to JavaScript backend, assuming that js_of_ocaml gets support for effects.

  • ocaml-multicore/domainslib#45 Add named pools

    An optional argument is now added to name a pool during setup. This name can be used to retrieve the pool later.

  • ocaml-multicore/domainslib#52 Use a random number as the cache prefix to disable cache in CI

    The cache-prefix now uses a random number in .github/workflows/main.yml to disable cache in the CI.

  • ocaml-multicore/domainslib#53 Make domainslib build/run with OCaml 5.00 after PR#704

    The CI has been updated to now build and run with OCaml 5.00 branch.

  • ocaml-multicore/domainslib#54 Use last 4.12+domains+effects hash as the cache-key

    The cache-key now uses the last commit hash from OCaml Multicore in order to invalidate the cache in the CI.

  • ocaml-multicore/tezos-opam-repository#3 Add domainslib

    The domainslib.0.3.1 version has now been included in the Tezos OPAM repository as a package.

  • ocaml-multicore/tezos-opam-repository#5 Upstream updates

    The Tezos OPAM repository has been updated with upstream changes using PR#1 and PR#5.

  • ocaml-multicore/retro-httpaf-bench#17 Improve graphs

    Markers have now been added to the graphs generated from the Jupyter notebook to easily distinguish the colour lines.


  • ocaml-multicore/multicore-opam#59 Fix batteries after ocaml-multicore/ocaml-multicore#514

    The batteries.3.3.0+multicore opam file for batteries-included has been updated with the correct src URL.

  • ocaml-multicore/eio#82 Migrate to 4.12.0+domains effects implementation (syntax-free effects version)

    The PR updates eio to support the effects implementation for OCaml 5.0 release.

  • ocaml-multicore/eio#83 Effect handlers have their own module now

    Sid Kshatriya has contributed a patch to rename Obj.Effect_handlers to EffectHandlers since effect handlers have their own module in Stdlib.

  • ocaml-multicore/core

    Jane Street's standard library overlay core has now been added to ocaml-multicore GitHub project repositories.




  • ocaml-bench/sandmark#248 Coq fails to build

    A new Coq tarball, coq-multicore-2021-09-24, builds with Multicore OCaml 4.12.0+domains, but, stdio.v0.14.0 fails to build cleanly with 4.14.0+trunk because of a dune issue that has been reported.

  • ocaml-bench/sandmark#260 Add 5.00 branch for sequential run. Fix notebook.

    A new 5.00 OCaml variant branch has been added to Sandmark to track sequential benchmark runs in the CI.


  • ocaml-bench/sandmark#256 Remove old variants

    The older variants, 4.05.*, 4.06.*, 4.07.*, 4.08.*, 4.10.0.* have now been removed from Sandmark.

  • ocaml-bench/sandmark#258
    Document Makefile variables in README

    The README now contains documentation on the various Makefile variables that are used during building and execution of the benchmarks in Sandmark.



  • ocurrent/current-bench#117 Read stderr from the Docker container

    We would like to see any build failures from the benchmark execution inside the Docker container for debugging purposes.

  • ocurrent/current-bench#146 Replicate ocaml-bench-server setup

    The TAG and OCaml variants need to be abstracted from the Sandmark Makefile to current-bench in order to be able to run the benchmarks for different compiler versions and developer branches.


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

    Custom Dockerfiles are now supported and hence you can pull in any opam image in the Dockerfile.

  • ocurrent/current-bench#119 OCAML_BENCH_DOCKER_CPU does not support range of CPUs for parallel execution

    The Docker CPU setting now uses a string representation instead of an integer to specify the list of CPUs for parallel execution.

  • ocurrent/current-bench#151 Docker versus native performance

    The Docker with current-bench, and native sequential and parallel benchmark runs do not show significant difference with hyper-threading disabled. The Gödel server configuration and the Sandmark-nightly notebook graph results are provided below for reference:

    • CPU: Intel(R) Xeon® Gold 5120 CPU @ 2.20 GHz
    • OS: Ubuntu 20.04.3 LTS (Focal Fossa)
    • Sandmark 2.0-beta branch: https://github.com/ocaml-bench/sandmark/tree/2.0-beta
    • Disabled hyper-threading:
      $ cat /sys/devices/system/cpu/smt/active
    • Memory (62 GB), disk (1.8 TB).
    • OCaml variant: 4.12.0+domains
    • Sandmark-nightly notebooks: https://github.com/ocaml-bench/sandmark-nightly/tree/main/notebooks
    • Average of five iterations for each benchmark
      • run_in_ci for sequential
      • macro_bench for parallel

    Time Current-bench-151-Time|690x236 Normalised Current-bench-151-Time-Normalised|690x315 Top heap words Current-bench-151-Top-heap-words|690x236 Normalised Current-bench-151-Top-heap-words-Normalised|690x323

    MaxRSS (KB) Current-bench-151-MaxRSS|690x237 Normalised Current-bench-151-MaxRSS-Normalised|690x325

    Major Collections Current-bench-151-Major-collections|690x236 Normalised Current-bench-151-Minor-collections-Normalised|690x320

    Parallel Benchmarks Current-bench-151-Parallel-benchmarks-I|486x500 Current-bench-151-Parallel-benchmarks-II|237x500

(see the PR full for the full set of graphs, including major words and time taken)

Our special thanks to all the OCaml users, developers and contributors in the community for their valuable time and continued support to the project. Stay safe!


  • AMD: Advanced Micro Devices
  • CI: Continuous Integration
  • CPU: Central Processing Unit
  • DLS: Domain Local Storage
  • FIFO: First In, First Out
  • GB: Gigabyte
  • GC: Garbage Collector
  • GCC: GNU Compiler Collection
  • IO: Input/Output
  • OPAM: OCaml Package Manager
  • OS: Operating System
  • PR: Pull Request
  • TB: Terabyte
  • URL: Uniform Resource Locator