Maintaining data structure semantics of concurrent queues such as first-in first-out (FIFO) ordering requires expensive synchronization mechanisms which limit scalability. However, deviating from the original semantics of a given data structure may allow for a higher degree of scalability and yet be tolerated by many concurrent applications. We introduce the notion of a k-FIFO queue which may be out of FIFO order up to a constant k (called semantical deviation). Implementations of k-FIFO queues may be distributed and therefore be accessed unsynchrontzed while still being starvation-free. We show that k-FIFO queues whose implementations are based on state-of-the-art FIFO queues, which typically do not scale under high contention, provide scalability. Moreover, probabilistic versions of k-FIFO queues improve scalability further but only bound semantical deviation with high probability.
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