A constant-size lock-free and almost wait-free ring buffer

Upsides

• Very fast, push and pop should be approx. O(1) even during heavy concurrency
• Only 4 words of memory overhead
• no memory allocations after initial creation

Downsides

• Fixed size, growing/shrinking is not supported
• No method for blocking poll
• Only efficient on 64bit architectures, due to
• maximum capacity of 65536 entries
• capacity is rounded up to the next power of 2

Implementation details

This implementation uses a 64 Bit atomic to store the state

 +64------+56------+48--------------+32------+24------+16-------------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 buzffer.
• 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 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.

Dependencies

This package only depends on atomic64

Usage

AtomicRingBuffer is on crates.io and on docs.rs

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

[dependencies]
atomicring = "0.1.0"

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());