AtomicRingBuffer

A constant-size almost lock-free concurrent ring buffer for 64bit platforms

Upsides

• fast, try_push and pop are O(1)
• scales well even during heavy concurrency
• only 5 words of memory overhead
• no memory allocations after initial creation

Downsides

• growing/shrinking is not supported
• no blocking poll support
• only efficient on 64bit architectures (uses a Mutex on non-64bit architectures)
• maximum capacity of 65535 entries
• capacity is rounded up to the next power of 2

This queue should perform similar to 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 a 64 Bit atomic to store the entire state

 +63----56+55----48+47------------32+31----24+23----16+15-------------0+
 | w_done | w_pend |  write_index   | r_done | r_pend |   read_index   |
 +--------+--------+----------------+--------+--------+----------------+

• write_index/read_index (16bit): 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.

Dependencies

This package has no dependencies

Usage

To use AtomicRingBuffer, add this to your Cargo.toml:

[dependencies]
atomicring = "0.2.0"

And something like this to your code

// create an AtomicRingBuffer with capacity of 1024 elements 
let ring = ::atomicring::AtomicRingBuffer::new(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 and LICENSE-APACHE for details.