swap-buffer-queue

A buffering MPSC queue.

This library is intended to be a (better, I hope) alternative to traditional MPSC queues in the context of a buffering consumer, by moving the buffering part directly into the queue.

It is especially well suited for IO writing workflow, see buffer implementations.

The crate is no_std – some buffer implementations may require alloc crate.

In addition to the low level Queue implementation, a higher level SynchronizedQueue is provided with both blocking and asynchronous methods. Synchronization feature requires std.

Example

use std::ops::Deref;
use swap_buffer_queue::{buffer::{IntoValueIter, VecBuffer}, Queue};

// Initialize the queue with a capacity
let queue: Queue<VecBuffer<usize>> = Queue::with_capacity(42);
// Enqueue some value
queue.try_enqueue([0]).unwrap();
// Multiple values can be enqueued at the same time
// (optimized compared to multiple enqueuing)
queue.try_enqueue([1, 2]).unwrap();
let mut values = vec![3, 4];
queue
    .try_enqueue(values.drain(..).into_value_iter())
    .unwrap();
// Dequeue a slice to the enqueued values
let slice = queue.try_dequeue().unwrap();
assert_eq!(slice.deref(), &[0, 1, 2, 3, 4]);
// Enqueued values can also be retrieved
assert_eq!(slice.into_iter().collect::<Vec<_>>(), vec![0, 1, 2, 3, 4]);

Buffer implementations

In addition to simple ArrayBuffer and VecBuffer, this crate provides useful write-oriented implementations.

`write`

WriteArrayBuffer and WriteVecBuffer are well suited when there are objects to be serialized with a known-serialization size. Indeed, objects can then be serialized directly on the queue's buffer, avoiding allocation.

use std::io::Write;
use swap_buffer_queue::{Queue, write::{WriteBytesSlice, WriteVecBuffer}};

// Creates a WriteVecBuffer queue with a 2-bytes header
let queue: Queue<WriteVecBuffer<2>> = Queue::with_capacity((1 << 16) - 1);
queue
    .try_enqueue((256, |slice: &mut [u8]| { /* write the slice */ }))
    .unwrap();
queue
    .try_enqueue((42, |slice: &mut [u8]| { /* write the slice */ }))
    .unwrap();
let mut slice = queue.try_dequeue().unwrap();
// Adds a header with the len of the buffer
let len = (slice.len() as u16).to_be_bytes();
slice.header().copy_from_slice(&len);
// Let's pretend we have a writer
let mut writer: Vec<u8> = Default::default();
assert_eq!(writer.write(slice.frame()).unwrap(), 300);

`write_vectored`

WriteVectoredArrayBuffer and WriteVectoredVecBuffer allows buffering a slice of IoSlice, saving the cost of dequeuing io-slices one by one to collect them after. (Internally, two buffers are used, one of the values, and one for the io-slices)

As a convenience, total size of the buffered io-slices can be retrieved.

use std::io::{Write};
use swap_buffer_queue::{Queue, write_vectored::WriteVectoredVecBuffer};

// Creates a WriteVectoredVecBuffer queue
let queue: Queue<WriteVectoredVecBuffer<Vec<u8>>, Vec<u8>> = Queue::with_capacity(100);
queue.try_enqueue([vec![0; 256]]).unwrap();
queue.try_enqueue([vec![42; 42]]).unwrap();
let mut total_size = 0u16.to_be_bytes();
let mut slice = queue.try_dequeue().unwrap();
// Adds a header with the total size of the slices
total_size.copy_from_slice(&(slice.total_size() as u16).to_be_bytes());
let mut frame = slice.frame(.., Some(&total_size), None);
// Let's pretend we have a writer
let mut writer: Vec<u8> = Default::default();
assert_eq!(writer.write_vectored(&mut frame).unwrap(), 300);

How it works

Internally, this queue use 2 buffers: one being used for enqueuing while the other is dequeued.

When Queue::try_enqueue is called, it reserves atomically a slot in the current enqueuing buffer. The value is then inserted in the slot.

When Queue::try_dequeue is called, both buffers are swapped atomically, so dequeued buffer will contain previously enqueued values, and new enqueued ones will go to the other (empty) buffer.

As the two-phase enqueuing cannot be atomic, the queue can be in a transitory state, where slots have been reserved but have not been written yet. In this rare case, dequeuing will fail and have to be retried.

Fairness

SynchronizedQueue implementation is not fair, i.e. it doesn't ensure that the oldest blocked enqueuer will succeed when the capacity becomes available.

However, this issue is quite mitigated by the fact that all the capacity becomes available at once, so all blocked enqueuers may succeed (especially with one-sized values).

For the particular case of potential big variable-sized values, it's still possible to combine the queue with a semaphore, e.g. tokio::sync::Semaphore. Performance will be impacted, but the algorithm is fast enough to afford it.

I'm still thinking about a way to include fairness directly in the algorithm, but it's not an easy thing to do.

Unsafe

This library uses unsafe code, for three reasons:

buffers are wrapped in UnsafeCell to allow mutable for the dequeued buffer;
buffers implementation may use unsafe to allow insertion with shared reference;
Buffer trait require unsafe interface for its invariant, because it's public.

To ensure the safety of the algorithm, it uses:

tests (mostly doctests for now, but it needs to be completed)
benchmarks
MIRI (with tests)

Loom is partially integrated for now, but loom tests are on the TODO list.

Performance

swap-buffer-queue is very performant – it's actually the fastest MPSC queue I know.

Here is the crossbeam benchmark forked

benchmark	crossbeam	swap-buffer-queue
bounded1_mpsc	1.545s	1.763s
bounded1_spsc	1.652s	1.000s
bounded_mpsc	0.362s	0.137s
bounded_seq	0.190s	0.114s
bounded_spsc	0.115s	0.092s

However, a large enough capacity is required to reach maximum performance; otherwise, high contention scenario may be penalized. This is because the algorithm put all the contention on a single atomic integer (instead of two for crossbeam).

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