parking_lot

Documentation (synchronization primitives)

Documentation (core parking lot API)

Documentation (type-safe lock API)

This library provides implementations of Mutex, RwLock, Condvar and
Once that are smaller, faster and more flexible than those in the Rust
standard library, as well as a ReentrantMutex type which supports recursive
locking. It also exposes a low-level API for creating your own efficient
synchronization primitives.

When tested on x86_64 Linux, parking_lot::Mutex was found to be 1.5x
faster than std::sync::Mutex when uncontended, and up to 5x faster when
contended from multiple threads. The numbers for RwLock vary depending on
the number of reader and writer threads, but are almost always faster than
the standard library RwLock, and even up to 50x faster in some cases.

Features

The primitives provided by this library have several advantages over those
in the Rust standard library:

1. Mutex and Once only require 1 byte of storage space, while Condvar
   and RwLock only require 1 word of storage space. On the other hand the
   standard library primitives require a dynamically allocated Box to hold
   OS-specific synchronization primitives. The small size of Mutex in
   particular encourages the use of fine-grained locks to increase
   parallelism.
2. Since they consist of just a single atomic variable, have constant
   initializers and don't need destructors, these primitives can be used as
   static global variables. The standard library primitives require
   dynamic initialization and thus need to be lazily initialized with
   lazy_static!.
3. Uncontended lock acquisition and release is done through fast inline
   paths which only require a single atomic operation.
4. Microcontention (a contended lock with a short critical section) is
   efficiently handled by spinning a few times while trying to acquire a
   lock.
5. The locks are adaptive and will suspend a thread after a few failed spin
   attempts. This makes the locks suitable for both long and short critical
   sections.
6. Condvar, RwLock and Once work on Windows XP, unlike the standard
   library versions of those types.
7. RwLock takes advantage of hardware lock elision on processors that
   support it, which can lead to huge performance wins with many readers.
8. RwLock uses a task-fair locking policy, which avoids reader and writer
   starvation, whereas the standard library version makes no guarantees.
9. Condvar is guaranteed not to produce spurious wakeups. A thread will
   only be woken up if it timed out or it was woken up by a notification.
10. Condvar::notify_all will only wake up a single thread and requeue the
    rest to wait on the associated Mutex. This avoids a thundering herd
    problem where all threads try to acquire the lock at the same time.
11. RwLock supports atomically downgrading a write lock into a read lock.
12. Mutex and RwLock allow raw unlocking without a RAII guard object.
13. Mutex<()> and RwLock<()> allow raw locking without a RAII guard
    object.
14. Mutex and RwLock support eventual fairness
    which allows them to be fair on average without sacrificing performance.
15. A ReentrantMutex type which supports recursive locking.
16. An experimental deadlock detector that works for Mutex,
    RwLock and ReentrantMutex. This feature is disabled by default and
    can be enabled via the deadlock_detection feature.
17. RwLock supports atomically upgrading an "upgradable" read lock into a
    write lock.
18. Optional support for serde. Enable via the
    feature serde. NOTE! this support is for Mutex, ReentrantMutex,
    and RwLock only; Condvar and Once are not currently supported.

The parking lot

To keep these primitives small, all thread queuing and suspending
functionality is offloaded to the parking lot. The idea behind this is
based on the Webkit WTF::ParkingLot
class, which essentially consists of a hash table mapping of lock addresses
to queues of parked (sleeping) threads. The Webkit parking lot was itself
inspired by Linux futexes,
but it is more powerful since it allows invoking callbacks while holding a queue
lock.

Nightly vs stable

There are a few restrictions when using this library on stable Rust:

• You will have to use the const_* functions (e.g. const_mutex(val)) to
  statically initialize the locking primitives. Using e.g. Mutex::new(val)
  does not work on stable Rust yet.
• RwLock will not be able to take advantage of hardware lock elision for
  readers, which improves performance when there are multiple readers.

To enable nightly-only functionality, you need to enable the nightly feature
in Cargo (see below).

Usage

Add this to your Cargo.toml:

[dependencies]
parking_lot = "0.10"

To enable nightly-only features, add this to your Cargo.toml instead:

[dependencies]
parking_lot = { version = "0.10", features = ["nightly"] }

The experimental deadlock detector can be enabled with the
deadlock_detection Cargo feature.

The core parking lot API is provided by the parking_lot_core crate. It is
separate from the synchronization primitives in the parking_lot crate so that
changes to the core API do not cause breaking changes for users of parking_lot.

Minimum Rust version

The current minimum required Rust version is 1.36. Any change to this is
considered a breaking change and will require a major version bump.

License

Licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any
additional terms or conditions.