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152 lines
6.7 KiB
152 lines
6.7 KiB
parking_lot
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============
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[](https://crates.io/crates/parking_lot)
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[Documentation (synchronization primitives)](https://docs.rs/parking_lot/)
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[Documentation (core parking lot API)](https://docs.rs/parking_lot_core/)
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[Documentation (type-safe lock API)](https://docs.rs/lock_api/)
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This library provides implementations of `Mutex`, `RwLock`, `Condvar` and
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`Once` that are smaller, faster and more flexible than those in the Rust
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standard library, as well as a `ReentrantMutex` type which supports recursive
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locking. It also exposes a low-level API for creating your own efficient
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synchronization primitives.
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When tested on x86_64 Linux, `parking_lot::Mutex` was found to be 1.5x
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faster than `std::sync::Mutex` when uncontended, and up to 5x faster when
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contended from multiple threads. The numbers for `RwLock` vary depending on
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the number of reader and writer threads, but are almost always faster than
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the standard library `RwLock`, and even up to 50x faster in some cases.
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## Features
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The primitives provided by this library have several advantages over those
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in the Rust standard library:
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1. `Mutex` and `Once` only require 1 byte of storage space, while `Condvar`
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and `RwLock` only require 1 word of storage space. On the other hand the
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standard library primitives require a dynamically allocated `Box` to hold
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OS-specific synchronization primitives. The small size of `Mutex` in
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particular encourages the use of fine-grained locks to increase
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parallelism.
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2. Since they consist of just a single atomic variable, have constant
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initializers and don't need destructors, these primitives can be used as
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`static` global variables. The standard library primitives require
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dynamic initialization and thus need to be lazily initialized with
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`lazy_static!`.
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3. Uncontended lock acquisition and release is done through fast inline
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paths which only require a single atomic operation.
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4. Microcontention (a contended lock with a short critical section) is
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efficiently handled by spinning a few times while trying to acquire a
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lock.
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5. The locks are adaptive and will suspend a thread after a few failed spin
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attempts. This makes the locks suitable for both long and short critical
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sections.
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6. `Condvar`, `RwLock` and `Once` work on Windows XP, unlike the standard
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library versions of those types.
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7. `RwLock` takes advantage of hardware lock elision on processors that
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support it, which can lead to huge performance wins with many readers.
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8. `RwLock` uses a task-fair locking policy, which avoids reader and writer
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starvation, whereas the standard library version makes no guarantees.
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9. `Condvar` is guaranteed not to produce spurious wakeups. A thread will
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only be woken up if it timed out or it was woken up by a notification.
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10. `Condvar::notify_all` will only wake up a single thread and requeue the
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rest to wait on the associated `Mutex`. This avoids a thundering herd
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problem where all threads try to acquire the lock at the same time.
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11. `RwLock` supports atomically downgrading a write lock into a read lock.
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12. `Mutex` and `RwLock` allow raw unlocking without a RAII guard object.
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13. `Mutex<()>` and `RwLock<()>` allow raw locking without a RAII guard
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object.
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14. `Mutex` and `RwLock` support [eventual fairness](https://trac.webkit.org/changeset/203350)
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which allows them to be fair on average without sacrificing performance.
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15. A `ReentrantMutex` type which supports recursive locking.
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16. An *experimental* deadlock detector that works for `Mutex`,
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`RwLock` and `ReentrantMutex`. This feature is disabled by default and
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can be enabled via the `deadlock_detection` feature.
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17. `RwLock` supports atomically upgrading an "upgradable" read lock into a
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write lock.
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18. Optional support for [serde](https://docs.serde.rs/serde/). Enable via the
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feature `serde`. **NOTE!** this support is for `Mutex`, `ReentrantMutex`,
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and `RwLock` only; `Condvar` and `Once` are not currently supported.
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19. Lock guards can be sent to other threads when the `send_guard` feature is
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enabled.
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## The parking lot
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To keep these primitives small, all thread queuing and suspending
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functionality is offloaded to the *parking lot*. The idea behind this is
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based on the Webkit [`WTF::ParkingLot`](https://webkit.org/blog/6161/locking-in-webkit/)
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class, which essentially consists of a hash table mapping of lock addresses
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to queues of parked (sleeping) threads. The Webkit parking lot was itself
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inspired by Linux [futexes](http://man7.org/linux/man-pages/man2/futex.2.html),
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but it is more powerful since it allows invoking callbacks while holding a queue
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lock.
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## Nightly vs stable
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There are a few restrictions when using this library on stable Rust:
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- You will have to use the `const_*` functions (e.g. `const_mutex(val)`) to
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statically initialize the locking primitives. Using e.g. `Mutex::new(val)`
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does not work on stable Rust yet.
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- `RwLock` will not be able to take advantage of hardware lock elision for
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readers, which improves performance when there are multiple readers.
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- The `wasm32-unknown-unknown` target is only supported on nightly and requires
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`-C target-feature=+atomics` in `RUSTFLAGS`.
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To enable nightly-only functionality, you need to enable the `nightly` feature
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in Cargo (see below).
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## Usage
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Add this to your `Cargo.toml`:
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```toml
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[dependencies]
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parking_lot = "0.11"
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```
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To enable nightly-only features, add this to your `Cargo.toml` instead:
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```toml
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[dependencies]
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parking_lot = { version = "0.11", features = ["nightly"] }
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```
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The experimental deadlock detector can be enabled with the
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`deadlock_detection` Cargo feature.
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To allow sending `MutexGuard`s and `RwLock*Guard`s to other threads, enable the
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`send_guard` option.
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Note that the `deadlock_detection` and `send_guard` features are incompatible
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and cannot be used together.
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The core parking lot API is provided by the `parking_lot_core` crate. It is
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separate from the synchronization primitives in the `parking_lot` crate so that
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changes to the core API do not cause breaking changes for users of `parking_lot`.
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## Minimum Rust version
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The current minimum required Rust version is 1.36. Any change to this is
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considered a breaking change and will require a major version bump.
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## License
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Licensed under either of
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* Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
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* MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
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at your option.
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### Contribution
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Unless you explicitly state otherwise, any contribution intentionally submitted
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for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any
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additional terms or conditions.
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