In this experiment a 4 MB file was sorted on a particular computer. A simple mergesort that did not build initial sorted runs took 451 seconds. A 2-way mergesort that used initial runs of 128 KB took only 160 seconds. A multiway mergesort that used the same initial runs took only 103 seconds. Clearly, using initial sorted runs dramatically speeds up the sorting.
References See the references below for more complete information and more advanced methods.
A Practical Introduction to Data Structures and Algorithm Analysis . Clifford A. Shaffer. See chapter 9. William Ford, William Topp. See pages 830 and following.