Rayon Hash

This crate is now deprecated, because the new implementation in std also exists as the hashbrown crate with its own "rayon" feature.

The rayon-hash crate duplicates the former 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.

Benchmarks using rustc 1.36.0-nightly (e938c2b9a 2019-04-23), before the hashbrown implementation had merged into std:

test hashbrown_set_sum_parallel  ... bench:     617,405 ns/iter (+/- 58,565)
test hashbrown_set_sum_serial    ... bench:   2,655,882 ns/iter (+/- 15,104)
test rayon_hash_set_sum_parallel ... bench:   1,368,058 ns/iter (+/- 75,984)
test rayon_hash_set_sum_serial   ... bench:   7,558,175 ns/iter (+/- 190,545)
test std_hash_set_sum_parallel   ... bench:   6,869,490 ns/iter (+/- 47,897)
test std_hash_set_sum_serial     ... bench:   7,591,704 ns/iter (+/- 154,438)

This crate currently requires rustc 1.31.0 or greater.

Known limitations

Some compromises may be made to let this work on stable Rust, compared to the standard types that may use unstable features. There is an example included which demonstrates one difference.

• 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.

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 may 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.