Time-Space Optimal Parallel Merging and Sorting

  • Authors:
  • Xiaojun Guan;Michael A. Langston

  • Affiliations:
  • Washington State Univ., Pullman;Univ. of Tennessee, Knoxville, TN

  • Venue:
  • IEEE Transactions on Computers
  • Year:
  • 1991

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Abstract

The authors present a parallel merging algorithm that, on an exclusive-read exclusive-write (EREW) parallel random-access machine (PRAM) with k processors merges two sorted lists of total length n in O(n/k+log n) time and constant extra space per processor, and hence is time-space optimal for any value of kor=n/(log n). The authors also describe how this gives rise to a stable version of the parallel merging algorithm that is similarly time-space optimal on an EREW PRAM. The authors observe that this technique for achieving stability incurs two penalties: a slightly more complicated algorithm and somewhat larger constants of proportionality. These two parallel merges naturally lead to time-space optimal parallel sorting algorithms. Extensions to sorting and open topics for future research are discussed.