State of the Art of Parallel Javascript

Javascript is defined as a single-threaded programming and execution model, but several independent designs and implementations for parallel Javascript exist covering a spectrum of capabilities. This list aims to capture the state of the art for parallel Javascript and enabling technologies like shared-memory.

This is a work in progress, additions and corrections are welcome. Please feel free to submit a PR to this list or send an update to mogill@synsem.com.

Classification System

This list separates parallelism and synchronization into separate concerns to capture implementations ranging from shared memory with no source of parallelism to shared-nothing multiprocessing.

• Types of Parallelism: How processes/threads are created and managed
  • Fork-Join - New threads/processes are created explicitly
  • Bulk Synchronous Parallel - A fixed number of processes are started when the program is executed, all entering the program at main().
  • Loop Level - Iterations of loops are distributed to multiple processes/threads
  • Asynchronous - Non-blocking function calls or continuation passing
  • None - No form of parallelism is provided by the platform
• Types of Synchronization: Categories of how processes enforce order of operations:
  • Messaging - Active Messages/Remote Procedure Calls cause a function to execute using the message payload at the destination
  • Atomic Read-Modify-Write - Fetch-And-Phi memory operations that atomically load from memory, perform an arithmetic or comparison operation, store the result back to memory, and return the original value stored in memory
  • Barrier - Wait for a known number of processes to arrive before any proceeds. Frequently used with Bulk Synchronous Parallelism
  • Mutex - Mutual exclusion locks and semaphores
  • None - No synchronization is provided
• Shared Memory: Directly sharing memory between processes/threads
  • Sharing Model: What is shared between threads/processes
    • Shared Everything - All heap variables are shared unless explicitly made thread-private
    • Explicit Sharing - Declaration of shared variables requires different syntax than private data
    • Shared Nothing - No shared memory capabilities
  • Coherency: Whether or not all processes see the most recent copy of data
  • Atomicity: Whether or not other processes can see a partially completed memory operation

What is Considered "Parallel Javascript" and "Shared Memory"

Because parallelism is not part of the base language, parallel JS might mean anything from two mutually dependent Node.js programs communicating via sockets, to transpiling a downloaded module before evaluating it, to loop-level parallelism with mutex locks.

Accessing shared memory differs from accessing shared remote caches, object stores, or databases in that it is too fast to merit being made asynchronous. Specifically, the overhead for scheduling a callback is greater than the amount of time needed to perform the work. Shared memory also introduces the potential hazards when one process modifies the value of a variable used by another thread.

Javascript projects do not use the terms multithreaded or shared memory consistently, projects that describe themselves with those terms but might better be described as asynchronous (i.e.: the async Node.js package) and/or message passing (i.e.: Node's built-in cluster) are not listed.

Shared memory can be implemented separately from a parallel programming model, indeed the de-facto standard, Shared Array Buffers, exists solely to support parallel programming but does not itself implement any parallelism, that is provided by WebWorkers. Parallel memory models with no parallel execution model are included in this list because they are complementary to other forms of parallelism.

Concurrency vs. Parallelism

This document uses the terms concurrent and parallel according the definitions given by the Go language:

Concurrency is about dealing with lots of things at once. Parallelism is about doing lots of things at once.

-GoBlog Concurrency is Not Parallelism

List of Parallel and Shared-Memory Javascript Projects

Project Web Site	Parallelism Model(s)	Shared Memory Model	Unique Features	Intended Uses	Usage Examples	Set # Procs	First Release	Latest Release	Active	Platform
AliOS	Fork-Join	N/A	Proposal only, no implementation yet, work in progress	Asynchronous processing	N/A	?	N/A	N/A	Yes	V8
Concurrent Javascript	Fork-Join	Shared everything, atomic Read-Modify-Write operations	C-like shared everything memory proposal, no implementation yet, work in progress	?	N/A	?	N/A	N/A	Yes	WebKit
Extended Memory Semantics (EMS.js)	Loop decomposition, Fork-Join, Bulk Synchronous Parallel	Atomic Read-Modify-Write operations on persistent objects in shared memory	Allows any number or kind of processes and devices to share objects. Hardware accelerators supported.	High Performance Computing, ad-hoc analytics on unstructured data.	Parallel web server, word counting, transaction processing	Yes	2013	2017	Yes	V8
Fibers	Fork-Join, Futures	N/A	Implements asynchronous functionality used by other asynchronous libraries	Asynchronous processing	Fibonacci	N/A	2011	2017	Yes	V8
isolated-vm	Fork-Join	Limited sharing within an isolate, Shared arrays	Exposes v8 runtime API to JS, isolates created in JS run on separate threads	Sandboxed execution of user functions in an application	Quicksort	Yes	2017	2018	Yes	V8
JXcore	Fork-Join	N/A	Supported multiple engines (V8, Spider Monkey, Chakra), multi-threading, shared objects	?	Fibonacci, messaging	Yes	2014	2016	No	V8, SM, Chk
mmap-object	N/A	Serially accessed shared object memory	Native C++ implementation based on Boost	?	N/A	N/A	2017	2017	Yes	V8
Napa.js	Fork-Join	Non-coherent shared objects	Microsoft implemented for V8	"...highly iterative services with non-compromised performance in Bing..."	Fibonacci, Pi, Max square sub-matrix	Yes	2017	2017	Yes	V8
Nexus.js	Asynchronous I/O	N/A	Makes possible promise-based multi-threading for WebKit	Node.js-like asynchronous I/O for implementing web servers	Web server	No	2016	2017	Yes	JSC
node-multithread	Fork-Join	Non-coherent shared objects	Map-reduce oriented	?	N/A	No	2017	2017	No	V8
node-shared-cache	N/A	Limited atomicity on shared objects	BSON for internal storage Key-value store	N/A	N/A	N/A	2015	2016	No	V8
node-worker-farm	Fork-Join	N/A	Basic F-J running in a pool of separate Node.js processes	Asynchronous processing	Pi	Yes (poolsz)	2013	2017	Yes	Node.js
Parallel.js	Map-Reduce	N/A	Web-worker based	Map-reduce	N/A	No	2014	2016	No	V8
promise-worker	Fork-Join	N/A	Implements promises as workers	Asynchronous processing	N/A	N/A	2017	2017	Yes	V8
RiverTrail	SIMD, loop-level	Shared arrays and global variables via closures(?)	Browser plug-in, OpenCL support, explicitly SIMD with implicit loop-level parallelism	Array & image processing, other OpenCL amenable algorithms	Image processing, OpenCL acceleration of JS	?	2011	2017	No	SM
Shared Array Buffers	N/A	Atomic Read-Modify-Write operations on a shared integer array	De-facto standard supported by major JS implementations, C-like semantics	Target for Emscripten	N/A	N/A	2016	2017	Yes	V8, SM
WebWorkers	Fork-Join	Messaging, shared array buffers	De-facto standard supported in major browsers	Streaming media	N/A	Implicit	2010	2017	Yes	Chrome, FF
Worker	Fork-Join	Atomic Read-Modify-Write operations on a shared integer array	Native to Node	Asynchronous processing	N/A	Implicit	2018	2018	Yes	Node
Workerpool	Fork-Join	N/A	Small implementation	Asynchronous processing	Background worker	N/A	2014	2017	Yes	V8

• Project Web Site - Name of and link to the project
• Parallelism Model - Type(s) of parallelism the project exposes to applications
• Shared Memory Model - Coherency and communication of shared data
• Unique Features - Notable differentiators
• Intended Uses - Target application(s) of the project
• Examples - Examples provided by implementation
• Set # Procs - Does the API allow the program to specify the number of parallel processes
• First Release - Date of initial public release
• Latest Release - Date of most recent release
• Active - Does the project have ongoing development in the past 6 months
• Engines - Supported Javascript engines