After this, head, bus, and controller performance characteristics were explained. . . . The simulation models the job list as a simple array and the actual disk as a resource. Jobs are created at the appropriate rate by an independent process, and they are added to the job list in a certain order to be serviced later. Another independent process takes jobs off the list according to the scheduling policy, computes the time the job will spend at the disk, and uses the disk for that amount of time. These statistics were collected for 20 independent runs executing simultaneously to compute the beginning of steady state behavior and to determine confidence levels for the data.
Several trends are evident in the table and graphs that follow.
First, the average waiting time and disk utilization increase as the arrival rate increases, which is to be expected. This occurs because FCFS does nothing to reduce seek time while the other two are similar in their seek optimization. However, the difference is not very significant. The simulation of a disk drive shows that as the traffic to the disk increases, the scheduling policy assumes more importance in reducing disk access time. The comparisons of average service times shows that the improvement is due to a reduction of head movements.