Operating Systems Day 07

Seek time: the time required to position the read/write head on the proper track from the time the I/O request is issued.

First-come, first-served (FCFS): the simplest scheduling algorithm for direct access storage devices that satisfies track requests in the order in which they are received.

Shortest Seek Time First (SSTF): a scheduling strategy for direct access storage devices that’s used to optimize seek time. The track requests are ordered so the one closest to the currently active track is satisfied first and the ones farthest away are made to wait.

SCAN is a seek strategy in which it uses a directional bit to indicate whether the arm is moving toward the center of the disk or away from it. In this technique, the algorithm moves the arm methodically from the outer to the inner track, servicing every request in its path. When it reaches the innermost track, it reverses direction and moves toward the outer tracks, again servicing every request in its path. In addition, SCAN strategy moves to extreme tracks even when there is no need.

LOOK is a seek strategy which is the most common variation of SCAN. In this strategy, the arm does not necessarily go all the way to either edge unless there are requests, hence eliminating the possibility of indefinite postponement.

Direct Memory Access (DMA) technique that allows a control unit to access main memory directly and transfer data without the intervention of the CPU. This technique is used for high-speed devices such as disks. Without DMA, the CPU is responsible for the physical movement of data between main memory and the device, a time-consuming task that results in significant overhead and decreased CPU utilization.


1. FCFS effective with light loads

• Service time unacceptably long under high loads

2. SSTF effective with moderate loads

• Localization problem under heavy loads

3. SCAN effective with light to moderate loads

• Eliminates indefinite postponement

4. C(Cicular)-SCAN effective with moderate to heavy loads

• Very small service time variances

References

1. McHoes, A., & Flynn, I.M. (2008). Understanding operating systems (5th ed.). CENGAGE Learning. 
2. Melbourne Institute of Technology (Semester 1, 2012). BN104 Operating Systems Lecture Notes.