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OCaml Multicore - June 2020

Welcome to the June 2020 Multicore OCaml report! As with previous updates, many thanks to @shakthimaan and @kayceesrk for collating the updates for the month of June 2020. This is an incremental update; new readers may find it helpful to flick through the previous posts first.

This month has seen a tremendous surge of activity on the upstream OCaml project to prepare for multicore integration, as @xavierleroy and the core team have driven a number of initiatives to prepare the OCaml project for the full multicore featureset. To reflect this, from next month we will have a status page on the ocaml-multicore wiki with the current status of both our multicore branch and the upstream OCaml project itself.

Why not from this month? Well, there's good news and bad news. Last month, I observed that we are a PR away from most of the opam ecosystem working with the multicore branch. The good news is that we are still a single PR away from it working, but it's a different one :-) The retrofitting of the Threads library has brought up some design complexities, and so rather than putting in a "bandaid" fix, we are integrating a comprehensive solution that will work with system threads, domains and (eventually) fibres. That work has taken some time to get right, and I hope to be able to update you all on an opam-friendly OCaml 4.10.0+multicore in a few weeks.

Aside from this, there have been a number of other improvements going into the multicore branches: mingw Windows support, callstack improvements, fixing the Unix module and so on. The full list is in the detailed report later in this update.

Sandmark benchmarks

A major milestone in this month has been the upgrade to the latest dune.2.6.0 to build Multicore OCaml 4.10.0 for the Sandmark benchmarking project. A number of new OPAM packages have been added, and the existing packages have been upgraded to their latest versions. The Multicore OCaml code base has seen continuous performance improvements and enhancements which can be observed from the various PRs mentioned in the report.

We would like to thank:

  • @xavierleroy for working on a number of multicore-prequisite PRs to make stock OCaml ready for Multicore OCaml.
  • @camlspotter has reviewed and accepted the camlimages changes and made a release of camlimages.5.0.3 required for Sandmark.
  • @dinosaure for updating the decompress test benchmarks for Sandmark to build and run with dune.2.6.0 for Multicore OCaml 4.10.0.

A chapter on Parallel Programming in Multicore OCaml with topics on task pool, channels section, profiling with code examples is being written. We shall provide an early draft version of the document to the community for your valuable feedback.

Papers

Our "Retrofitting Parallism onto OCaml" paper has been officially accepted at ICFP 2020 which will be held virtually between August 23-28, 2020. A preprint of the paper was made available earlier, and will be updated in a few days with the camera-ready version for ICFP. Please do feel free to send on comments and queries even after the paper is published, of course.

Excitingly, another multicore-related paper on Cosmo: A Concurrent Separation Logic for Multicore OCaml will also be presented at the same conference.

The Multicore OCaml updates are first listed in our report, which are followed by improvements to the Sandmark benchmarking project. Finally, the changes made to upstream OCaml which include both the ongoing and completed tasks are mentioned for your reference.

Multicore OCaml

Ongoing

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

    An RFC proposal to implement a domain-local storage in Multicore OCaml. Kindly review the idea and share your feedback!

  • ocaml-multicore/ocaml-multicore#342 Implementing the threads library with Domains

    An effort to rebase @jhwoodyatt's implementation of the Thread library for Domains.

  • ocaml-multicore/ocaml-multicore#357 Implementation of systhreads with pthreads

    Exploring the possibilty of implementing systhreads with pthreads, while still maintaining compatibility with the existing solution.

  • ocaml/dune#3548 Dune fails to pick up secondary compiler

    The ocaml-secondary-compiler fails to install with dune.2.6.0. This is required as Multicore OCaml cannot build the latest dune without systhreads support.

Completed

  • ocaml-multicore/multicore-opam#22 Update dune to 2.6.0

    The dune version in the Multicore OPAM repository is now updated to use the latest 2.6.0.

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

    An implementation of Lazy.try_force and Lazy.try_force_val functions to implement concurrent lazy abstractions.

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

    A patch that introduces Atomic.exchange through Atomic.get that provides the appropriate read barrier for correct exchange semantics for caml_atomic_exchange in memory.c.

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

    The DWARF information emitted for extcall noalloc had broken backtraces and this PR fixes the same.

  • ocaml-multicore/ocaml-multicore#345 Absolute exception stack

    The representation of the exception stack is changed from relative addressing to absolute addressing and the results are promising. The Sandmark serial benchmark results after the change is illustrated in the following graph:

Absolute addressing improvements |690x218

  • ocaml-multicore/ocaml-multicore#347 Turn on -Werror by default

    Adds a --enable-warn-error option to configure to treat C compiler warnings as errors.

  • ocaml-multicore/ocaml-multicore#353 Poll for interrupts in cpu_relax without locking

    Use Caml_check_gc_interrupt first to poll for interrupts without locking, and then proceeding to handle the interrupt with the lock.

  • ocaml-multicore/ocaml-multicore#354 Add Caml_state_field to domain_state.h

    The Caml_state_field macro definition in domain_state.h is required for base-v0.14.0 to build for Multicore OCaml 4.10.0 with dune.2.6.0.

  • ocaml-multicore/ocaml-multicore#355 One more location to poll for interrupts without lock

    Another use of Caml_check_gc_interrupt first to poll for interrupts without lock, similar to ocaml-multicore/ocaml-multicore#353.

  • ocaml-multicore/ocaml-multicore#356 Backup threads for domain

    Introduces backup threads to perform GC and handle service interrupts when the domain is blocked in the kernel.

  • ocaml-multicore/ocaml-multicore#358 Fix up bad CFI information in amd64.S

    Add missing CFI_ADJUST directives in runtime/amd64.S for caml_call_poll and caml_allocN.

  • ocaml-multicore/ocaml-multicore#359 Inline caml_domain_alone

    The PR makes caml_domain_alone an inline function to improve performance for caml_atomic_cas_field and other atomics in memory.c.

  • ocaml-multicore/ocaml-multicore#360 Parallel minor GC inline mask rework

    The inline mask rework for the promotion path to the parallel_minor_gc branch gives a 3-5% performance improvement for test_decompress sandmark benchmark, and a decrease in the executed instructions for all other benchmarks.

  • ocaml-multicore/ocaml-multicore#361 Mark stack push work credit

    The PR improves the Multicore mark work accounting to be in line with stock OCaml.

  • ocaml-multicore/ocaml-multicore#362 Iloadmut does not clobber rax and rdx when we do not have a read barrier

    A code clean-up to free the registers rax and rdx for OCaml code when Iloadmut is used.

Benchmarking

Ongoing

  • ocaml-bench/sandmark#8 Ability to run compiler variants in Sandmark

    A feature to specify configure options when building compiler variants such as flambda is useful for development and testing. This feature is being worked upon.

  • ocaml-bench/sandmark#107 Add Coq benchmarks

    We are continuing to add more benchmarks to Sandmark for Multicore OCaml and investigating adding the Coq benchmarks to our repertoire!

  • ocaml-bench/sandmark#124 User configurable paramwrapper added to Makefile

    A PARAMWRAPPER environment variable can be passed as an argument by specifying the --cpu-list to be used for parallel benchmark runs.

  • ocaml-bench/sandmark#131 Update decompress benchmarks

    Thanks to @dinosaure for updating the decompress benchmarks in order to run them with dune.2.6.0 for Multicore OCaml 4.10.0.

  • ocaml-bench/sandmark#132 Update dependency packages to use dune.2.6.0 and Multicore OCaml 4.10.0

    Sandmark has been running with dune.1.11.4, and we need to move to the latest dune.2.6.0 for using Multicore OCaml 4.10.0 and beyond, as mentioned in Promote dune to > 2.0. The PR updates over 30 dependency packages and successfully builds both serial and parallel benchmarks!

Completed

  • camlspotter/camlimages#1 Use dune-configurator instead of configurator for camlimages

    A new release of camlimages.5.0.3 was made by @camlspotter after accepting the changes to camlimages.opam in order to build with dune.2.6.0.

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

    The changes to use the Domainslib.Task API for the listed benchmarks have been merged.

  • ocaml-bench/sandmark#121 Mention sudo access for run_all_parallel.sh script

    The README.md file has been updated with the necessary sudo configuration steps to execute the run_all_parallel.sh script for nightly builds.

  • ocaml-bench/sandmark#125 Add cubicle benchmarks

    The German PFS and Szymanski's mutual exclusion algorithm cubicle benchmarks have been included in Sandmark.

  • ocaml-bench/sandmark#126 Update ocaml-versions README to reflect 4.10.0+multicore

    The README has now been updated to reflect the latest 4.10.0 Multicore OCaml compiler and its variants.

  • ocaml-bench/sandmark#129 Add target to run parallel benchmarks in the CI

    The .drone.yml file used by the CI has been updated to run both the serial and parallel benchmarks.

  • ocaml-bench/sandmark#130 Add missing dependencies in multicore-numerical

    The domainslib library has been added to the dune file for the multicore-numerical benchmark.

OCaml

Ongoing

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

    The instrumented runtime has been merged to OCaml 4.11.0. A manual for the same has been created and is under review.

Completed

  • ocaml/ocaml#9619 A self-describing representation for function closures

    The PR provides a way to record the position of the environment for each entry point for function closures.

  • ocaml/ocaml#9649 Marshaling for the new closure representation

    The output_value marshaler has been updated to use the new closure representation. There is no change required for the input_value unmarshaler.

  • ocaml/ocaml#9655 Introduce type Obj.raw_data and functions Obj.raw_field, Obj.set_raw_field to manipulate out-of-heap pointers

    The PR introduces a type Obj.bits, and functions Obj.field_bits and Obj.set_field_bits to read and write bit representation of block fields to support the no-naked-pointer operation.

  • ocaml/ocaml#9678 Reimplement Obj.reachable_word using a hash table to detect sharing

    The caml_obj_reachable_words now uses a hash table instead of modifying the mark bits of block headers to detect sharing. This is required for compatibility with Multicore OCaml.

  • ocaml/ocaml#9680 Naked pointers and the bytecode interpreter

    The bytecode interpreter implementation is updated to support the no-naked-pointers mode operation as required by Multicore OCaml.

  • ocaml/ocaml#9682 Signal handling in native code without the page table

    The patch uses the code fragment table instead of a page table lookup for signal handlers to know whether the signal came from ocamlopt-generated code.

  • ocaml/ocaml#9683 globroots.c: adapt to no-naked-pointers mode

    The patch considers out-of-heap pointers as major-heap pointers in no-naked-pointers mode for global roots management.

  • ocaml/ocaml#9689 Generic hashing for the new closure representation

    The hashing functions have been updated to use the latest closure representation from ocaml/ocaml#9619 for the no-naked-pointers mode.

  • ocaml/ocaml#9698 The end of the page table is near

    The PR eliminates some of the use of the page tables in the runtime system when built with no-naked-pointers mode.

Our thanks to all the OCaml developers and users in the community for their continued support and contribution to the project. Stay safe!

Acronyms

  • API: Application Programming Interface
  • CFI: Call Frame Information
  • CI: Continuous Integration
  • DWARF: Debugging With Attributed Record Formats
  • GC: Garbage Collector
  • ICFP: International Conference on Functional Programming
  • OPAM: OCaml Package Manager
  • PR: Pull Request
  • RFC: Request for Comments