Until now, one piece missing from WebAssembly standalone engines was the ability to spawn threads. Browsers have had this ability for some time via Web Workers, but standalone engines had no standard way to do this. This post describes the work of several collaborators to bring about wasi-threads, a proposal to add threads to WASI. It will explain the history to this proposal, the work done to get to this point, and how one can experiment with threaded applications in engines like Wasmtime and WAMR. To showcase this, we’ll look at how performing parallel compression with wasi-threads drastically improves performance.


The core WebAssembly specification has a threads proposal to add support for thread primitives that is at phase 3. That proposal adds support for three major features:

  • shared linear memory, via a shared annotation on memory declarations
  • atomic operations, e.g., i32.atomic.rmw.cmpxchg
  • wait and notify

Significantly, the WebAssembly threads proposal did not specify a mechanism for creating new threads. A future vision for how this might work is described in Weakening WebAssembly, but, for practical reasons, the proposal left this “spawn” detail up to implementors. After all, WebAssembly engines in browsers could already spawn new threads using Web Workers (see Emscripten’s pthreads support). But not specifying a spawn mechanism meant that standalone WebAssembly engines (i.e., not in a browser) could avoid the complexity of supporting multi-threaded execution. The key assumption was that not all users of WebAssembly wanted or needed a threaded execution environment.

The lack of a specification did not stop people from investigating, though. In some areas of the WebAssembly world, there is some interesting previous work to mention (a non-exhaustive list):

  • in 2018, Alex Crichton did some in-depth investigation to enable a WebAssembly raytracing example in the browser using Rust’s wasm-bindgen (blog post)
  • in 2020, Ann Yuan and Marat Dukhan published browser results showing that the TensorFlow.js WebAssembly backend was fastest with threads and SIMD (blog post)
  • in 2021, Ingvar Stepanyan and Surma updated Squoosh, a browser-based image manipulation app that relied on multi-threaded C++ and Rust libraries; they built wasm-bindgen-rayon to compile Rust threads to browser threads (blog post)
  • in 2022, Alexandru Ene proposed wasi-threads largely based on the pthreads API

Building on Alexandru’s initial proposal, an informal working group gathered to hash out the details. These discussions pared down the API to a single call, wasi_thread_spawn, with some simple parameters (e.g., see the simpler version). The current specification is experimental and minimal; it is enough to run parallel programs (as we will show below) but, due to its early nature, could change as users point out gaps.


It is hard to deny the performance implications of parallelism. Libraries like pthreads for C and rayon for Rust are used in countless projects. Many algorithms have both parallel and sequential versions, implying that peak performance requires parallelism. And in many cases, for systems with multiple CPU cores available, the best performance is only available by using multiple threads. With wasi-threads, this performance is now available in standalone WebAssembly engines.

Besides performance, another motivation for wasi-threads is to unlock even more of the ecosystem. Until now, many libraries using multi-threading (e.g., for multi-tasking) could not be run in a standalone engine. By enabling threads, wasi-threads expands the pool of libraries available.

But, in some standalone use cases, WebAssembly parallelism is not desired. For example, a blockchain environment billing per WebAssembly instruction may never desire the complications of multiple threads of execution. And some environments already parallelize execution along a different dimension — per incoming HTTP request. In fact, it would be natural for some WebAssembly use cases to remain solidly single-threaded. This motivates the opt-in aspect of wasi-threads — users who need maximum performance from their WebAssembly host should be able to use it and others should be able to ignore it.

It has been difficult to estimate the need for wasi-threads due to lack of tooling, preventing experimentation — but this is changing. Until now, users could not compile pthreads applications to WebAssembly — at least not easily and not for standalone engines (Emscripten has had browser support for some time). With the pre-release of wasi-sdk 20 with threads enabled, one can now compile pthreads programs to the wasm32-wasi-threads target (we’ll show that next). This pre-release pulls together work from many contributors to build a pthreads-enabled wasi-libc. Why is all of this necessary? Because of how WebAssembly features work with LLVM currently, all linked WebAssembly objects need to share the same features (i.e., threads), all the way down to the libc objects. Hopefully threads support can be extended to C++ and Rust soon, but at least this pre-release enables C users to experiment with wasi-threads, providing some idea for the demand for parallelism in the standalone WebAssembly ecosystem.


We can start with a simple example to demonstrate what wasi-threads makes possible. After retrieving the wasi-sdk-20+threads pre-release for your OS, create the following program:

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

#define NUM_THREADS 10

void *thread_entry_point(void *ctx) {
  int id = (int) ctx;
  printf(" in thread %d\n", id);
  return 0;

int main(int argc, char **argv) {
  pthread_t threads[10];
  for (int i = 0; i < NUM_THREADS; i++) {
    int ret = pthread_create(&threads[i], NULL, &thread_entry_point, (void *) i);
    if (ret) {
      printf("failed to spawn thread: %s", strerror(ret));
  for (int i = 0; i < NUM_THREADS; i++) {
    pthread_join(threads[i], NULL);
  return 0;

This can be compiled with the following flags:

$ $WASI_SDK_DIR/bin/clang --sysroot $WASI_SDK_DIR/share/wasi-sysroot \
    --target=wasm32-wasi-threads -pthread \
    -Wl,--import-memory,--export-memory,--max-memory=67108864 \
    threads.c -o threads.wasm

This set of flags can seem daunting so let’s walk through them:

  • --sysroot wasi-sdk/share/wasi-sysroot is necessary when wasi-sdk is used outside its normal installation directory
  • --target=wasm32-wasi-threads tells the compiler we want to generate wasi-threads compatible modules (e.g., with shared memory, atomics support, a thread entry point, a pthreads-enabled wasi-libc)
  • -pthread adds pthreads support, as expected
  • -Wl,--import-memory,--export-memory,--max-memory=67108864 tells the linker to import the memory (this is required by the wasi-threads specification), then export it (this is required by other WASI specifications), and make the memory large enough (without enough memory for their stack and TLS, threads cannot spawn)

Once we have the compiled threads.wasm file, we can run it in a recent version of Wasmtime (or WAMR 1):

$ git clone --recurse-submodules https://github.com/bytecodealliance/wasmtime
$ cargo build --release
$ target/release/wasmtime run --wasm-features=threads --wasi-modules=experimental-wasi-threads threads.wasm
  in thread 4
  in thread 2
  in thread 0
  in thread 5
  in thread 6
  in thread 1
  in thread 3
  in thread 7
  in thread 8
  in thread 9


Now we can spawn threads, but it turns out that some of the Rust safeguards added to Wasmtime impact threaded performance negatively. After investigating and removing sub-optimal copies, performance was back to normal (#5566). The workload I used for this performed parallel compression — the idea is that the work of compressing chunks of a file can be efficiently split into separate threads. (Why use WebAssembly for this? You tell me; I would typically use natively-compiled binaries myself, but for demo purposes gzip is an easily-parallelizable, recognizable workload).

The components compiled together with wasi-sdk for the parallel compression benchmark.

I found a parallel implementation of gzip, called pigz, and added Makefiles to compile it to WebAssembly. This also involved compiling zlib, a dependency, and patching pigz lightly. I collected all of the tools necessary for compiling and running pigz and put them all in one place here. The Makefiles essentially compile the WebAssembly objects using the same flags as shown above, but with the added project-specific complexities of pigz and libz. You can build and run the benchmark yourself with:

$ git clone --recurse-submodules https://github.com/bytecodealliance/wasm-parallel-gzip
$ make
$ make benchmark NUM_THREADS=1
$ make benchmark NUM_THREADS=8

Using eight threads to compress 100MB of random bytes, I see linear speedups when compared to a single thread! With Wasm, ~8x improvement for eight threads is actually higher than the ~6x improvement I see using the natively-compiled pigz binary (i.e., eight threads versus a single thread but using pigz’s default Makefile). More investigation is needed, but note that the benchmarking setup in the sample repository is intentionally rough (e.g., time). I hope in the future to integrate the parallel compression workload more carefully into the Sightglass benchmark suite. I am interested in the results you observe on your system as well; contact me on Zulip to discuss more!


This blog post demonstrates how standalone WebAssembly engines can now use wasi-threads to spawn threads — with the associated performance improvements that brings. But work is not finished: these are early days for wasi-threads and there are still issues to work through. Read through the specification and file issues there or feel free to create a Zulip thread to discuss with the various contributors. Remember that wasi-threads, as a new WASI proposal, is still experimental — do not expect strong stability in the ABI, especially as WASI transitions to the component model.

We are watching and coordinating wasi-threads with the component model transition. wasi-threads, in an attempt to maintain as much compatibility with browser WebAssembly as possible, is designed to match the “instance per thread” model — each new thread is instantiated separately with only shared memory to connect the parallel execution. The component model uses instances to isolate subcomponents. If a subcomponent were to spawn a thread, it could no longer instantiate a single module into a thread (should it instantiate a sub-tree of modules?!). Also, we are working through how to express wasi-threads in WIT. We expect to continue discussing these issues in the future.

In the meantime, we need to understand how wasi-threads will be used. Please experiment with wasi-threads and provide feedback! Note that Wasmtime is not the only standalone WebAssembly engine implementing wasi-threads; you may also be interested in trying out the examples above on WAMR. If you are interested in contributing, please contact one of us; a good start might be to look at the open thread-related issues.

Finally, I want to give a a special thanks to Alexandru Ene, Marcin Kolny, Takashi Yamamoto for all of the joint work on the wasi-threads specification. To Sam Clegg and Dan Gohman, thank you for all of the toolchain help on wasi-libc and wasi-sdk. And to Alex Crichton, thank you for all of the in-depth help making Wasmtime threads-friendly.

  1. To run the threads.wasm example in WAMR:

    $ git clone https://github.com/bytecodealliance/wasm-micro-runtime.git
    $ cd wasm-micro-runtime/product-mini/platforms/linux
    $ ./iwasm threads.wasm