We present the design and implementation of a parallel out-of-core sorting algorithm, which is based on Leighton's columnsort algorithm. We show how to relax some of the steps of the original columnsort algorithm to permit a faster out-of-core implementation. Our algorithm requires only 4 passes over the data, and a 3-pass implementation is possible. Although there is a limit on the number of records that can be sorted-as a function of the memory used per processor-this upper limit need not be a severe restriction, and it increases superlinearly with the per-processor memory. To the best of our knowledge, our implementation is the first out-of-core multiprocessor sorting algorithm whose output is in the order assumed by the Parallel Disk Model. We define several measures of sorting efficiency and demonstrate that our implementation's sorting efficiency is competitive with that of NOW-Sort, a sorting algorithm developed to sort large amounts of data quickly on a cluster of workstations.
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