Efficient Low-Contention Parallel Algorithms

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The queue-read, queue-write ( qrqw ) parallel random access machine ( pram ) model permits concurrent reading and writing to shared memory locations, but at a cost proportional to the number of readers/writers to any one memory location in a given step. The qrqw pram model reflects the contention properties of most commercially available parallel machines more accurately than either the well-studied crcw pram or erew pram models, and can be efficiently emulated with only logarithmic slowdown on hypercube-type noncombining networks. This paper describes fast, low-contention, work-optimal, randomized qrqw pram algorithms for the fundamental problems of load balancing, multiple compaction, generating a random permutation, parallel hashing, and distributive sorting. These logarithmic or sublogarithmic time algorithms considerably improve upon the best known erew pram algorithms for these problems, while avoiding the high-contention steps typical of crcw pram algorithms. An illustrative experiment demonstrates the performance advantage of a new qrqw random permutation algorithm when compared with the popular erew algorithm. Finally, this paper presents new randomized algorithms for integer sorting and general sorting.