Multicore OCaml: May 2020 update

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Multicore OCaml: May 2020

Welcome to the May 2020 update from the Multicore OCaml team! As with previous updates, many thanks to @shakthimaan and @kayceesrk for help assembling this month’s roundup.

A major milestone in May 2020 has been the completion of rebasing of Multicore OCaml all the way from 4.06 to 4.10! The Parallel Minor GC variant that performs stop-the-world parallel minor collection is the default branch for the compiler, which means that compatibility with C bindings is now much simpler than with the older minor GC design.

I’ve received many questions asking if this means that multicore OCaml will “just work” with the opam ecosystem now. Not quite yet: we estimate that we are now one PR away from this working, which requires that the existing Threads module is backported to multicore OCaml to support the older (non-parallel-in-the-runtime but concurrent) uses of threading that existing OCaml supports. This effort was begun a year ago by @jhw in #240 and now rebased and being reviewed by @engil in #342. Once that is merged and tested by us on a bunch of packages and bulk builds, we should be good to start using Multicore OCaml with opam. Stay tuned for more on that next month!

The ongoing and completed tasks for the Multicore OCaml are listed first, which are then followed by improvements to the Sandmark benchmarking project. Finally, the status of the contributions to upstream OCaml are mentioned for your reference. This month has also seen a meeting of the core OCaml runtime developers to assign post-rebasing tasks (such as also porting statmemprof, how to handle non-x86 architectures, Windows support, etc) to ensure a more complete view of the upstreaming tasks ahead. The task list is long, but steadily decreasing in length.

As to how to contribute currently, there is an incredibly exciting seam of work that has now started on the appropriate programming abstractions to support parallel algorithms in OCaml. See this thread for more on that, and also on the Domainslib repository for more low-level examples of traditional parallel algorithms. In a month or so, we expect that the multicore switch will also be more suitable for use with opam, but don’t let that stop you from porting your favourite parallel benchmark to Domainslib today.

Multicore OCaml

Ongoing

• ocaml-multicore/ocaml-multicore#339
  Proposal for domain-local storage

  A new proposal for implementing a domain-local storage in Multicore
  OCaml has been created.

• ocaml-multicore/domainslib#8
  Task library slowdown if the number of domains is greater than 8

  This is an ongoing investigation on why there is a slowdown with
  domainslib version 0.2 for the Game of Life benchmark when the
  number of domains is greater than eight.

• ocaml-multicore/ocaml-multicore#340
  Fix Atomic.exchange in concurrent_minor_gc

  An implementation is provided for Atomic.exchange using
  Atomic.get and Atomic.compare_and_set to obtain the correct
  semantics to handle assertion failure in interp.c.

• ocaml-multicore/ocaml-multicore#338
  Introduce Lazy.try_force and Lazy.try_force_val

  The Lazy.try_force and Lazy.try_force_val functions are
  implemented for concurrent lazy abstractions to handle the RacyLazy
  exception.

• ocaml-multicore/ocaml-multicore#333
  Random module functions slowdown on multiple cores

  There is an observed slowdown for the Random module on multiple
  cores, and the issue is being analysed in detail.

  

• ocaml-multicore/ocaml-multicore#343
  Fix extcall noalloc DWARF

  The patch provides a fix for the emitted DWARF information for
  extcall noalloc. This PR is currently under review.

Completed

• ocaml-multicore/ocaml-multicore#337
  Update opam file to 4.10.0+multicore

  The rebasing of Multicore OCaml to 4.11 branch
  (parallel_minor_gc_4_11) point is now complete! The
  opam
  file for 4.10.0+multicore has been made the default in the
  multicore-opam repository.

• ocaml-multicore/ocaml-multicore#335
  Add byte_domain_state.tbl to install files

  A patch to install byte_domain_state.tbl and caml/*.h files has
  now been included in the runtime/Makefile which is required for
  parallel_minor_gc_4_10 branch.

• The Multicore OCaml major GC implementation verification using the
  SPIN model checker is available at the following GitHub repository
  ocaml-multicore/multicore-ocaml-verify.

Benchmarking

Ongoing

• ocaml-bench/sandmark#115
  Task API Port: LU-Decomposition, Floyd Warshall, Mandelbrot, N-body

  Porting of the following programs - LU-Decomposition, Floyd
  Warshall, Mandelbrot and N-body to use the Task API.

• ocaml-bench/sandmark#37
  Make benchmark wrapper user configurable

  The ability to dynamically specify the input commands and their
  respective arguments to the benchmark scripts is currently being
  evaluated.

• ocaml-bench/sandmark#106
  Promote dune > 2.0

  Sandmark works with dune 1.11.4 and we need to support dune greater
  than 2.0 moving forward. The upgrade path with the necessary package
  builds is being tested.

Completed

• ocaml-bench/sandmark#109
  Added sequential-interactive.ipynb

  An interactive notebook to run and analyse sequential benchmarks has
  been included. Given an artifacts directory with the benchmark
  files, the notebook prompts you in the GUI to select different
  commit and compiler variants for analysis. A sample screenshot of
  the UI is shown below:

  

  The PR adds error handling, user input validation and the project
  README has also been updated.

• ocaml-bench/sandmark#111
  Add parallel initialisation and parallel copy to LU decomposition benchmark

  The parallel initialisation is now added to LU decomposition
  numerical benchmark
  (benchmarks/multicore-numerical/LU_decomposition_multicore.ml).

• ocaml-bench/sandmark#113
  Use --format=columns with pip3 list in Makefile

  A fix for the “DEPRECATION: The default format will switch to
  columns in the future” warning when using pip3 list has now been
  added to the Makefile with the use of the --format=columns option.

• ocaml-bench/sandmark#116
  Use sudo for parallel benchmark builds

  The Makefile has been updated with the right combination of sudo
  and OPAM environment variables so that we can now run parallel
  benchmarks in Sandmark. The sudo command is required exclusively for
  using the chrt command. We can now perform nightly builds for both
  serial and parallel benchmarks!

• ocaml-bench/sandmark#118
  Refactored README and added JupyterHub info

  The Sandmark README file has now been updated to include information
  on configuration, usage of JupyterHub, benchmarking and a quick
  start guide!

OCaml

Ongoing

• ocaml/ocaml#9541
  Add manual page for the instrumented runtime

  A draft manual for the instrumented runtime eventlog tracing has
  been created. Please feel free to review the document and share your
  valuable feedback.

• ocaml/dune#3500
  Support building executables against OCaml 4.11 instrumented runtime

  OCaml 4.11.0 has built-in support for the instrumented runtime, and
  it will be useful to have dune generate instrumented targets.

Completed

• ocaml/ocaml#9082
  Eventlog tracing system

  The Eventlog tracing proposal for the OCaml runtime that uses the
  Binary Trace Format (CTF) is now merged with upstream OCaml
  (4.11.0).

• ocaml/ocaml#9534
  [RFC] Dynamic check for naked pointers

  An RFC for adding the ability to dynamically identify naked pointers
  in the 4.10.0 compiler.

• ocaml/ocaml#9573
  Reimplement Unix.create_process and related functions without Unix.fork

  The use of process creation functions in the Unix module is not
  suitable for Multicore OCaml, for both behaviour and efficiency. The
  patch provides an implementation that uses posix_spawn.

• ocaml/ocaml#9564
  Add a macro for out-of-heap block header

  This PR adds a macro definition to construct a out-of-heap block
  header in runtime/caml/mlvalues.h. The objective is to use the
  header for out of heap objects.

As always, we would like to thank all the OCaml developers and users for their continued support and contribution to the project. Stay safe out there.

Acronyms

• API: Application Programming Interface
• CTF: Common Trace Format
• DWARF: Debugging With Attributed Record Formats
• GC: Garbage Collector
• GUI: Graphical User Interface
• LU: Lower-Upper
• OPAM: OCaml Package Manager
• PIP: Pip Installs Python
• PR: Pull Request
• RFC: Request for Comments
• UI: User Interface