MPLLang / mpl

The MaPLe compiler for efficient and scalable parallel functional programming

MaPLe (MPL)

MaPLe (MPL) is a functional language for provably efficient and safe multicore parallelism, developed at Carnegie Mellon University.

Features:

• Support for the full Standard ML programming language, extended with task-parallel and data-parallel primitives.
• Whole-program compilation based on MLton, with aggressive optimizations to achieve performance competitive with languages such as C/C++.
• Efficient memory representations, including:
  • Untagged and unboxed native integers and floating-point numbers.
  • Flattened tuples and records.
  • Native arrays with contiguous unboxed elements.
• Simple and fast foreign-function calls into C, based on MLtonFFI.
• Support for both regular and irregular fine-grained parallelism, with provably efficient automatic parallelism management [7] to control the overheads of task creation.
• Provably efficient parallel garbage collection based on hierarchical memory management and disentanglement [1,2,3,4,5,6].
• Support for large core counts and large memory sizes. MPL scales to hundreds of cores, and can efficiently handle heap sizes of as much as 1TB or more.

MPL is being actively developed. If you are interested in contributing to the project, PRs are welcome!

If you are you interested in using MPL, consider checking out the tutorial. You might also be interested in exploring mpllib (a library for MPL) and the Parallel ML benchmark suite.

References

[7] Automatic Parallelism Management. Sam Westrick, Matthew Fluet, Mike Rainey, and Umut A. Acar. POPL 2024.

[6] Efficient Parallel Functional Programming with Effects. Jatin Arora, Sam Westrick, and Umut A. Acar. PLDI 2023.

[5] Entanglement Detection with Near-Zero Cost. Sam Westrick, Jatin Arora, and Umut A. Acar. ICFP 2022.

[4] Provably Space-Efficient Parallel Functional Programming. Jatin Arora, Sam Westrick, and Umut A. Acar. POPL 2021.

[3] Disentanglement in Nested-Parallel Programs. Sam Westrick, Rohan Yadav, Matthew Fluet, and Umut A. Acar. POPL 2020.

[2] Hierarchical Memory Management for Mutable State. Adrien Guatto, Sam Westrick, Ram Raghunathan, Umut Acar, and Matthew Fluet. PPoPP 2018.

[1] Hierarchical Memory Management for Parallel Programs. Ram Raghunathan, Stefan K. Muller, Umut A. Acar, and Guy Blelloch. ICFP 2016.

Try It Out

If you want to quickly try out using MPL, you can download the Docker image and run one of the examples.

$ docker pull shwestrick/mpl
$ docker run -it shwestrick/mpl /bin/bash
...# examples/bin/primes @mpl procs 4 --

To write and compile your own code, we recommend mounting a local directory inside the container. For example, here's how you can use MPL to compile and run your own main.mlb in the current directory. (To mount some other directory, replace $(pwd -P) with a different path.)

$ ls
main.mlb
$ docker run -it -v $(pwd -P):/root/mycode shwestrick/mpl /bin/bash
...# cd /root/mycode
...# mpl main.mlb
...# ./main @mpl procs 4 --

Benchmark Suite

The Parallel ML benchmark suite provides many examples of sophisticated parallel algorithms and applications in MPL, as well as cross-language performance comparisons with C++, Go, Java, and multicore OCaml.

Libraries

We recommend using the smlpkg package manager. MPL supports the full SML language, so existing libraries for SML can be used.

In addition, here are a few libraries that make use of MPL for parallelism:

• github.com/MPLLang/mpllib: implements a variety of data structures (sequences, sets, dictionaries, graphs, matrices, meshes, images, etc.) and parallel algorithms (map, reduce, scan, filter, sorting, search, tokenization, graph processing, computational geometry, etc.). Also includes basic utilies (e.g. parsing command-line arguments) and benchmarking infrastructure.
• github.com/shwestrick/sml-audio: a library for audio processing with I/O support for .wav files.

Parallel and Concurrent Extensions

MPL extends SML with a number of primitives for parallelism and concurrency. Take a look at examples/ to see these primitives in action.

The ForkJoin Structure

val par: (unit -> 'a) * (unit -> 'b) -> 'a * 'b
val parfor: int -> (int * int) -> (int -> unit) -> unit
val alloc: int -> 'a array

The par primitive takes two functions to execute in parallel and returns their results.

The parfor primitive is a "parallel for loop". It takes a grain-size g, a range (i, j), and a function f, and executes f(k) in parallel for each i <= k < j. The grain-size g is for manual granularity control: parfor splits the input range into approximately (j-i)/g subranges, each of size at most g, and each subrange is processed sequentially. The grain-size must be at least 1, in which case the loop is "fully parallel".

The alloc primitive takes a length and returns a fresh, uninitialized array of that size. Warning: To guarantee no errors, the programmer must be careful to initialize the array before reading from it. alloc is intended to be used as a low-level primitive in the efficient implementation of high-performance libraries. It is integrated with the scheduler and memory management system to perform allocation in parallel and be safe-for-GC.

The MLton.Parallel Structure

val compareAndSwap: 'a ref -> ('a * 'a) -> 'a
val arrayCompareAndSwap: ('a array * int) -> ('a * 'a) -> 'a

compareAndSwap r (x, y) performs an atomic CAS which attempts to atomically swap the contents of r from x to y, returning the original value stored in r before the CAS. Polymorphic equality is determined in the same way as MLton.eq, which is a standard equality check for simple types (char, int, word, etc.) and a pointer equality check for other types (array, string, tuples, datatypes, etc.). The semantics are a bit murky.

arrayCompareAndSwap (a, i) (x, y) behaves the same as compareAndSwap but on arrays instead of references. This performs a CAS at index i of array a, and does not read or write at any other locations of the array.

Using MPL

MPL uses .mlb files (ML Basis) to describe source files for compilation. A typical .mlb file for MPL is shown below. The first three lines of this file respectively load:

• The SML Basis Library
• The ForkJoin structure, as described above
• The MLton structure, which includes the MPL extension MLton.Parallel as described above, as well as various MLton-specific features. Not all MLton features are supported (see "Unsupported MLton Features" below).

(* libraries *)
$(SML_LIB)/basis/basis.mlb
$(SML_LIB)/basis/fork-join.mlb
$(SML_LIB)/basis/mlton.mlb

(* your source files... *)
A.sml
B.sml

Compiling a Program

The command to compile a .mlb is as follows. By default, MPL produces an executable with the same base name as the source file, i.e. this would create an executable named foo:

$ mpl [compile-time options...] foo.mlb

MPL has a number of compile-time options derived from MLton, which are documented here. Note that MPL only supports C codegen and does not support profiling.

Some useful compile-time options are

• -output <NAME> Give a specific name to the produced executable.
• -default-type int64 -default-type word64 Use 64-bit integers and words by default.
• -debug true -debug-runtime true -keep g For debugging, keeps the generated C files and uses the debug version of the runtime (with assertions enabled). The resulting executable is somewhat peruse-able with tools like gdb.

For example:

$ mpl -default-type -int64 -output foo sources.mlb

Running a Program

MPL executables can take options at the command line that control the run-time system. The syntax is

$ <program> [@mpl [run-time options...] --] [program args...]

The runtime arguments must begin with @mpl and end with --, and these are not visible to the program via CommandLine.arguments.

Some useful run-time options are

• procs <N> Use N worker threads to run the program.
• set-affinity Pin worker threads to processors. Can be used in combination with affinity-base <B> and affinity-stride <S> to pin thread i to processor number B + S*i.
• block-size <X> Set the heap block size to X bytes. This can be written with suffixes K, M, and G, e.g. 64K is 64 kilobytes. The block-size must be a multiple of the system page size (typically 4K). By default it is set to one page.

For example, the following runs a program foo with a single command-line argument bar using 4 pinned processors.

$ foo @mpl procs 4 set-affinity -- bar

Bugs and Known Issues

Basis Library

The basis library is inherited from (sequential) SML. It has not yet been thoroughly scrubbed, and some functions may not be safe for parallelism (#41).

Garbage Collection

• (#115) The GC is currently disabled at the "top level" (outside any calls to ForkJoin.par). For highly parallel programs, this has generally not been a problem so far, but it can cause a memory explosion for programs that are mostly (or entirely) sequential.

Unsupported MLton Features

Many MLton-specific features are unsupported, including (but not limited to):

• share
• shareAll
• size
• Finalizable
• Profile
• Signal
• Thread (partially supported but not documented)
• Cont (partially supported but not documented)
• Weak
• World

Build and Install (from source)

Requirements

MPL has only been tested on Linux with x86-64. The following software is required.

• GCC
• GMP (GNU Multiple Precision arithmetic library)
• GNU Make, GNU Bash
• binutils (ar, ranlib, strip, ...)
• miscellaneous Unix utilities (diff, find, grep, gzip, patch, sed, tar, xargs, ...)
• Standard ML compiler and tools:
  • Recommended: MLton (mlton, mllex, and mlyacc). Pre-built binary packages for MLton can be installed via an OS package manager or (for select platforms) obtained from http://mlton.org.
  • Supported but not recommended: SML/NJ (sml, ml-lex, ml-yacc).
• (If using mpl-switch): Python 3, and git.

Installation with mpl-switch

The mpl-switch utility makes it easy to install multiple versions of MPL on the same system and switch between them. After setting up mpl-switch, you can install MPL as follows:

$ mpl-switch install v0.5
$ mpl-switch select v0.5

You can use any commit hash or tag name from the MPL repo to pick a particular version of MPL. Installed versions are stored in ~/.mpl/; this folder is safe to delete at any moment, as it can always be regenerated. To see what versions of MPL are currently installed, do:

$ mpl-switch list

Manual Instructions

Alternatively, you can manually build mpl by cloning this repo and then performing the following.

Build the executable. This produces an executable at build/bin/mpl:

$ make

Put it where you want it. After building, MPL can then be installed to /usr/local:

$ make install

or to a custom directory with the PREFIX option:

$ make PREFIX=/opt/mpl install