Rayon Hash

The rayon-hash crate duplicates the standard HashMap and HashSet, adding native support for Rayon parallel iterators.

Rayon does provide iterators for these standard types already, but since it can't access internal fields, it has to collect to an intermediate vector to be split into parallel jobs. With the custom types in rayon-hash, we can instead read the raw hash table directly, for much better performance.

test rayon_set_sum_parallel ... bench:   1,077,602 ns/iter (+/- 50,610)
test rayon_set_sum_serial   ... bench:   6,363,125 ns/iter (+/- 101,513)
test std_set_sum_parallel   ... bench:   8,519,683 ns/iter (+/- 219,785)
test std_set_sum_serial     ... bench:   6,295,263 ns/iter (+/- 98,600)

This crate currently requires rustc 1.21.0 or greater.

Known limitations

Some compromises have been made to let this work on stable Rust, compared to the standard types that may use unstable features. There are examples included which demonstrate the differences.

• examples/may_dangle.rs: Since we don't use the unstable #[may_dangle] attributes, the type parameters of HashMap<K, V> and HashSet<T> must strictly outlive the container itself.

• examples/nonzero.rs: We use duplicated Unique and Shared pointer types, containing a duplicate NonZero that's not marked #[lang = "non_zero"]. This means the compiler won't apply the intended enum optimizations, so wrappers like Option<HashMap<_,_>> end up larger to store the enum discriminant separately.

Unstable features

Some of the features copied from std would be guarded with #[unstable] attributes, but this isn't available to general crates. Instead, we guard these features with a config flag rayon_hash_unstable. The easiest way to use this is to set the RUSTFLAGS environment variable:

RUSTFLAGS='--cfg rayon_hash_unstable' cargo build

Note that this must not only be done for your crate, but for any crate that depends on your crate. This infectious nature is intentional, as it serves as a reminder that you are outside of the normal semver guarantees. These features also require a nightly Rust compiler.

When such features are stabilized in the standard library, we will remove the rayon_hash_unstable guard here too.

License

Rayon-hash is distributed under the terms of both the MIT license and the Apache License (Version 2.0). See LICENSE-APACHE and LICENSE-MIT for details. Opening a pull requests is assumed to signal agreement with these licensing terms.