Crate arc_swap [−] [src]
Making Arc
itself atomic
The Arc
uses atomic reference counters, so the object behind it can be safely pointed to by
several threads at once. However, the Arc
itself is quite ordinary ‒ to change its value
(make it point somewhere else), one has to be the sole owner of it (or store it behind a
Mutex
).
On the other hand, there's AtomicPtr
. It can be modified and read from multiple threads,
allowing to pass the value from one thread to another without the use of a Mutex
. The
downside is, tracking when the data can be safely deleted is hard.
This library provides [ArcSwap
] that allows both at once. It can be constructed from ordinary
Arc
, but its value can be loaded and stored atomically, my multiple concurrent threads.
Motivation
For one, the C++ shared_ptr
has this
ability, so it is only fair to
have it too.
For another, it seemed like a really good exercise.
And finally, there are some real use cases for this functionality. For example, when one thread
publishes something (for example configuration) and other threads want to have a peek to the
current one from time to time. There's a global [ArcSwap
], holding the current snapshot and
everyone is free to make a copy and hold onto it for a while. The publisher thread simply
stores a new snapshot every time and the old configuration gets dropped once all the other
threads give up their copies of the pointer.
Performance characteristics
Some benchmarks need to be done. Due to the complexity, it may be possible that using
Mutex<Arc<T>>
might be faster in some cases.
However, this implementation doesn't suffer from contention. Specifically, arbitrary number of readers can access the shared value and won't be blocked. Even when there are many readers and writers at once, they don't block each other. The writers will be somewhat slower when there are active readers at the same time, but won't be stopped indefinitely.
Example
extern crate arc_swap; extern crate crossbeam_utils; use std::sync::Arc; use arc_swap::ArcSwap; use crossbeam_utils::scoped as thread; fn main() { let config = ArcSwap::from(Arc::new(String::default())); thread::scope(|scope| { scope.spawn(|| { let new_conf = Arc::new("New configuration".to_owned()); config.store(new_conf); }); for _ in 0..10 { scope.spawn(|| { loop { let cfg = config.load(); if !cfg.is_empty() { assert_eq!(*cfg, "New configuration"); return; } } }); } }); }
Structs
ArcSwap |
An atomic storage for |