WebAssembly Threads ready to try in Chrome 70

WebAssembly thread support has shipped in Chrome 70 under an origin-trial.

Alex Danilo

WebAssembly (Wasm) enables compilation of code written in C++ and other languages to run on the web. One very useful feature of native applications is the ability to use threads - a primitive for parallel computation. Most C and C++ developers would be familiar with pthreads which is a standardized API for thread management in an application.

The WebAssembly Community Group has been working on bringing threads to the web to enable real multi-threaded applications. As part of this effort, V8 has implemented necessary support for threads in the WebAssembly engine, available through an Origin Trial. Origin Trials allow developers to experiment with new web features before they are fully standardized. This allows us to gather real-world feedback from intrepid developers, which is critical to validate and improve new features.

The Chrome 70 release supports threads for WebAssembly and we encourage interested developers to start using them and give us feedback.

Threads? What about Workers?

Browsers have supported parallelism via Web Workers since 2012 in Chrome 4; in fact it's normal to hear terms like 'on the main thread' etc. However, Web Workers do not share mutable data between them, instead relying on message-passing for communication. In fact, Chrome allocates a new V8 engine for each of them (called isolates). Isolates share neither compiled code nor JavaScript objects, and thus they cannot share mutable data like pthreads.

WebAssembly threads, on the other hand, are threads that can share the same Wasm memory. The underlying storage of the shared memory is accomplished with a SharedArrayBuffer, a JavaScript primitive that allows sharing a single ArrayBuffer's contents concurrently between workers. Each WebAssembly thread runs in a Web Worker, but their shared Wasm memory allows them to work much like they do on native platforms. This means that the applications that use Wasm threads are responsible for managing access to the shared memory as in any traditional threaded application. There are many existing code libraries written in C or C++ that use pthreads, and those can be compiled to Wasm and run in true threaded mode, allowing more cores to work on the same data simultaneously.

A simple example

Here's an example of a simple 'C' program that uses threads.

#include <pthread.h>
#include <stdio.h>

// Calculate Fibonacci numbers shared function
int fibonacci(int iterations) {
    int     val = 1;
    int     last = 0;

    if (iterations == 0) {
        return 0;
    for (int i = 1; i < iterations; i++) {
        int     seq;

        seq = val + last;
        last = val;
        val = seq;
    return val;
// Start function for the background thread
void *bg_func(void *arg) {
    int     *iter = (void *)arg;

    *iter = fibonacci(*iter);
    return arg;
// Foreground thread and main entry point
int main(int argc, char *argv[]) {
    int         fg_val = 54;
    int         bg_val = 42;
    pthread_t   bg_thread;

    // Create the background thread
    if (pthread_create(&bg_thread, NULL, bg_func, &bg_val)) {
        perror("Thread create failed");
        return 1;
    // Calculate on the foreground thread
    fg_val = fibonacci(fg_val);
    // Wait for background thread to finish
    if (pthread_join(bg_thread, NULL)) {
        perror("Thread join failed");
        return 2;
    // Show the result from background and foreground threads
    printf("Fib(42) is %d, Fib(6 * 9) is %d\n", bg_val, fg_val);

    return 0;

That code begins with the main() function which declares 2 variables fg_val and bg_val. There is also a function called fibonacci(), which will be called by both of the threads in this example. The main() function creates a background thread using pthread_create() whose task is to calculate the fibonacci number sequence value corresponding to the value of the bg_val variable. Meanwhile, the main() function running in the foreground thread calculates it for the fg_val variable. Once the background thread has completed running, the results are printed out.

Compile for thread support

First, you should have the emscripten SDK installed, preferably version 1.38.11 or later. To build our example code with threads enabled for running in the browser, we need to pass a couple of extra flags to the emscripten emcc compiler. Our command line looks like this:

emcc -O2 -s USE_PTHREADS=1 -s PTHREAD_POOL_SIZE=2 -o test.js test.c

The command line argument '-s USE_PTHREADS=1' turns on threading support for the compiled WebAssembly module and the argument '-s PTHREAD_POOL_SIZE=2' tells the compiler to generate a pool of two (2) threads.

When the program is run, under the hood it will load the WebAssembly module, create a Web Worker for each of the threads in the thread pool, share the module with each of the workers, in this case it's 2, and those will be used whenever a call to pthread_create() is made. Each worker instantiates the Wasm module with the same memory, allowing them to cooperate. V8's newest changes in 7.0 share the compiled native code of Wasm modules that are passed between workers, which allows even very large applications to scale to many workers. Note, it makes sense to make sure the thread pool size is equal to the maximum number of threads your application needs, or thread creation may fail. At the same time, if the thread pool size is too large, you'll be creating unnecessary Web Workers that'll sit around doing nothing but using memory.

How to try it out

The quickest way to test out our WebAssembly module is to turn on the experimental WebAssembly threads support in Chrome 70 onwards. Navigate to the URL about://flags in your browser as shown below:

Chrome flags page

Next, find the experimental WebAssembly threads setting which looks like this:

WebAssembly threads setting

Change the setting to Enabled as shown below, then restart your browser.

WebAssembly threads setting enabled

After the browser has restarted, we can try loading the threaded WebAssembly module with a minimal HTML page, containing just this content:

<!DOCTYPE html>
  <title>Threads test</title>
    <script src="test.js"></script>

To try this page, you'll need to run some form of web server and load it from your browser. That will cause the WebAssembly module to load and run. Opening DevTools shows us the output from the run, and you should see something like the output image below in the console:

Console output from fibonacci program

Our WebAssembly program with threads has executed successfully! We'd encourage you to try out your own threaded application using the steps outlined above.

Testing in the field with an Origin Trial

Trying out threads by turning on experimental flags in the browser is fine for development purposes, but if you'd like to test your application out in the field, you can do so with what's known as an origin trial.

Origin trials let you try out experimental features with your users by obtaining a testing token that's tied to your domain. You can then deploy your app and expect it to work in a browser that can support the feature you're testing (in this case Chrome 70 onwards). To obtain your own token to run an origin trial, use the application form here.

We've hosted our simple example above using an origin trial token, so you can try it out for yourself without needing to build anything.

If you want to see what 4 threads running in parallel can do for ASCII art, then you must take a look at this demo as well!

Give us feedback

WebAssembly threads are an extremely useful new primitive for porting applications to the web. It's now possible to run C and C++ applications and libraries which require pthreads support in the WebAssembly environment.

We're looking for feedback from developers trying out this feature as it'll help inform the standardization process as well as validate its usefulness. The best way to send feedback is to report issues and/or get involved with the standardization process in the WebAssembly Community Group.