# AtomicRingBuffer / AtomicRingQueue
[](https://travis-ci.org/eun-ice/atomicring)
[](https://github.com/eun-ice/atomicring)
[](https://crates.io/crates/atomicring)
[](https://docs.rs/atomicring)
A constant-size almost lock-free concurrent ring buffer
Upsides
- fast, try_push and try_pop are O(1)
- scales well even during heavy concurrency
- AtomicRingBuffer has only 4 words of memory overhead (AtomicRingQueue has 6 words of overhead)
- blocking pop supported via AtomicRingQueue
- no memory allocations after initial creation
Downsides
- growing/shrinking is not supported
- maximum capacity of (usize >> 16) entries
- capacity is rounded up to the next power of 2
This queue should perform similar to [mpmc](https://github.com/brayniac/mpmc) but with a lower memory overhead.
If memory overhead is not your main concern you should run benchmarks to decide which one to use.
## Implementation details
This implementation uses two atomics to store the read_index/write_index
```Text
Read index atomic
+63------------------------------------------------16+15-----8+7------0+
Write index atomic
+63------------------------------------------------16+15-----8+7------0+
| write_index | w_done | w_pend |
+----------------------------------------------------+--------+--------+
```
- write_index/read_index (16bit on 32bit arch, 48bits on 64bit arch): current read/write position in the ring buffer (head and tail).
- r_pend/w_pend (8bit): number of pending concurrent read/writes
- r_done/w_done (8bit): number of completed read/writes.
For reading r_pend is incremented first, then the content of the ring buffer is read from memory.
After reading is done r_done is incremented. read_index is only incremented if r_done is equal to r_pend.
For writing first w_pend is incremented, then the content of the ring buffer is updated.
After writing w_done is incremented. If w_done is equal to w_pend then both are set to 0 and write_index is incremented.
In rare cases this can result in a race where multiple threads increment r_pend in turn and r_done never quite reaches r_pend.
If r_pend == 255 or w_pend == 255 a spinloop waits it to be <255 to continue. This rarely happens in practice, that's why this is called almost lock-free.
## Structs
This package provides ```AtomicRingBuffer``` without blocking pop support and ```AtomicRingQueue``` with blocking pop support.
## Dependencies
This package depends on [parking_lot](https://github.com/Amanieu/parking_lot) for blocking support in AtomicRingQueue
## Usage
To use AtomicRingBuffer, add this to your `Cargo.toml`:
```toml
[dependencies]
atomicring = "0.5.4"
```
And something like this to your code
```rust
// create an AtomicRingBuffer with capacity of 1024 elements
let ring = ::atomicring::AtomicRingBuffer::with_capacity(900);
// try_pop removes an element of the buffer and returns None if the buffer is empty
assert_eq!(None, ring.try_pop());
// push_overwrite adds an element to the buffer, overwriting the oldest element if the buffer is full:
ring.push_overwrite(1);
assert_eq!(Some(1), ring.try_pop());
assert_eq!(None, ring.try_pop());
```
## License
Licensed under the terms of MIT license and the Apache License (Version 2.0).
See [LICENSE-MIT](LICENSE-MIT) and [LICENSE-APACHE](LICENSE-APACHE) for details.