The Expected Difference between the Sltf and Mtpt Drum Scheduling Disciplines

This report is a sequel to an earlier report [Fuller, 19711 that develops a minimal-total-processing-time (MTPT) drum scheduling algorithm. A quantative comparison between MTPT schedules and shortest-latency-timefirst (SLTF) schedules, commonly acknowledged as good schedules for drumlike storage units, is presented here. The analysis develops an analogy to random walks and proves several asymptotic properties of collections of records on drums. These properties are specialized to the MTPI' and SLTF algorithms and it is shown that for sufficiently large sets of records, the expected processing time of a SLTF schedule is longer than a MTPT schedule by the expected record length. The results of a simulation study are also presented to show the difference in MTPT and SLTF schedules for small sets of records and for situations not covered in the analytic discussion.