 In the other direction, using "k" > two tapes (and "O"("k") items in memory), we can reduce the number of tape operations in "O"(log "k") times by using a k/2-way merge.A more sophisticated merge sort that optimizes tape (and disk) drive usage is the polyphase merge sort.On modern computers, locality of reference can be of paramount importance in software optimization, because multilevel memory hierarchies are used.