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#1124 in Concurrency

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42,138 downloads per month
Used in 62 crates (7 directly)


4.5K SLoC

Note: This is an unstable fork made for use in rustc

Rayon-core represents the "core, stable" APIs of Rayon: join, scope, and so forth, as well as the ability to create custom thread-pools with ThreadPool.

Maybe worth mentioning: users are not necessarily intended to directly access rayon-core; all its APIs are mirror in the rayon crate. To that end, the examples in the docs use rayon::join and so forth rather than rayon_core::join.

rayon-core aims to never, or almost never, have a breaking change to its API, because each revision of rayon-core also houses the global thread-pool (and hence if you have two simultaneous versions of rayon-core, you have two thread-pools).

Please see Rayon Docs for details about using Rayon.

Rayon-core currently requires rustc 1.59.0 or greater.


Rayon-core houses the core stable APIs of Rayon.

These APIs have been mirrored in the Rayon crate and it is recommended to use these from there.

join is used to take two closures and potentially run them in parallel.

  • It will run in parallel if task B gets stolen before task A can finish.
  • It will run sequentially if task A finishes before task B is stolen and can continue on task B.

scope creates a scope in which you can run any number of parallel tasks. These tasks can spawn nested tasks and scopes, but given the nature of work stealing, the order of execution can not be guaranteed. The scope will exist until all tasks spawned within the scope have been completed.

spawn add a task into the 'static' or 'global' scope, or a local scope created by the scope() function.

ThreadPool can be used to create your own thread pools (using ThreadPoolBuilder) or to customize the global one. Tasks spawned within the pool (using install(), join(), etc.) will be added to a deque, where it becomes available for work stealing from other threads in the local threadpool.

Global fallback when threading is unsupported

Rayon uses std APIs for threading, but some targets have incomplete implementations that always return Unsupported errors. The WebAssembly wasm32-unknown-unknown and wasm32-wasi targets are notable examples of this. Rather than panicking on the unsupported error when creating the implicit global threadpool, Rayon configures a fallback mode instead.

This fallback mode mostly functions as if it were using a single-threaded "pool", like setting RAYON_NUM_THREADS=1. For example, join will execute its two closures sequentially, since there is no other thread to share the work. However, since the pool is not running independent of the main thread, non-blocking calls like spawn may not execute at all, unless a lower- priority call like broadcast gives them an opening. The fallback mode does not try to emulate anything like thread preemption or async task switching, but yield_now or yield_local can also volunteer execution time.

Explicit ThreadPoolBuilder methods always report their error without any fallback.

Restricting multiple versions

In order to ensure proper coordination between threadpools, and especially to make sure there's only one global threadpool, rayon-core is actively restricted from building multiple versions of itself into a single target. You may see a build error like this in violation:

error: native library `rayon-core` is being linked to by more
than one package, and can only be linked to by one package

While we strive to keep rayon-core semver-compatible, it's still possible to arrive at this situation if different crates have overly restrictive tilde or inequality requirements for rayon-core. The conflicting requirements will need to be resolved before the build will succeed.


~11K SLoC