Time-space optimal parallel merging and sorting

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 k⩽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