External quicksort

Abstract

An external sorting algorithm based on quicksort is presented. The file to be sorted is kept on a disk and only those blocks are fetched into the main memory which are currently needed. At each time, a block is kept in the main memory, if the expected space-time cost of holding it until its next use is smaller than the expected space-time cost of removing it and fetching it again. The efficiency of the algorithm is tested through simulation experiments and the results are compared to those achieved with mergesort in a corresponding environment. The total execution time and the main memory space-time integral are used for measuring the performance. When equal block sizes are used, external quicksort results in a much smaller average space requirement than mergesort. On the other hand, mergesort is somewhat faster than external quicksort. The main memory space-time integral of quicksort is always considerably smaller than that of mergesort. External quicksort is less sensitive to the block size and to the file size. With faster disks, the performance of external quicksort improves faster than that of mergesort. The relative difference of the algorithms is independent of the file size. The external quicksort is also analytically compared to some previous external versions of quicksort. It is shown to be satisfied with less space and fewer block fetches than the others.