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OCaml Multicore - May 2020

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.

    perf Random

  • 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:

    Sequentials select comparison

    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