Enhancing Concurrent Data Structures with Concurrent Iteration Operations: Consistency and Algorithms

Report, 2013

Concurrent data structures provide the means to multi-threaded applications to share data. Data structures come with a set of predefined operations, specified by the semantics of the data structure. In the literature and in several contemporary commonly used programming environments, the notion of iterations has been introduced for collection data structures, as a bulk operation enhancing the native set of their operations. Iterations in several of these contexts are treated as sequential in nature and may freeze the data structure while operating or provide a variety of consistency guarantees when running concurrently with the native operations of the data structures. In this work we study iterations in concurrent data structures with respect to their coexistence with the native operations of the latter and the guarantees that they provide under concurrency. Besides linearizability, we propose a set of consistency specifications for such bulk operations, including also concurrency-aware properties by building on Lamport's systematic definitions for registers. By using queues, fixed-domain sets and composite registers as case-studies of underlying objects, we show a set of constructions of iteration operations, satisfying these properties. Besides the trade-off between consistency and throughput, we demonstrate the trade-off between the overhead of the bulk operation and possible support (helping) by the native operations of the data structure. We show a set of algorithms that demonstrate these and study the implications on the efficiency of the implementations.