Compare and contrast any three disk arm scheduling algorithms.
1 Answer

Comparison of Three Disk Arm Scheduling Algorithms

  • A Process makes the I/O requests to the operating system to access the disk.

  • Disk Arm Scheduling Algorithm manages those requests and decides the order of the disk access given to the requests.

  • There are various different Disk Arm Scheduling Algorithm are used to do this such as follows:

    • First Come-First Serve (FCFS)
    • Shortest Seek Time First (SSTF)
    • Elevator Algorithm (SCAN)
    • Circular SCAN (C-SCAN)
    • LOOK (Elevator)
    • C-LOOK

Let's differentiate Three disk arm scheduling algorithms like FCFS, SSTF, and SCAN.

First Come-First Serve Shortest Seek Time First Elevator Algorithm
It services the IO requests in the order in which they arrive. Selects the disk I/O request which requires the least disk arm movement from its current position regardless of the direction. The disk arm moves into a particular direction till the end, satisfying all the requests coming in its path, and then it turns back and moves in the reverse direction satisfying requests coming in its path.
There is no starvation in this algorithm, every request is serviced. It may cause starvation for some requests. Starvation is still possible in this scheduling.
The scheme does not optimize the seek time. It reduces the total seek time as compared to FCFS. It offers an improved variance of seek time as compared to SSTF.
This is acceptable when the load on a disk is light and in small systems where efficiency is not important. It is a reasonable solution for batch processing systems but not applicable for interactive systems. Under a light load, SCAN policy is best.
May not provide the best possible service in terms of good waiting & response time. High variance is present in response & waiting time. Low variance occurs in waiting & response time.
It has extremely low throughput due to lengthy seeks. It has higher throughput than FCFS. It has higher throughput than FCFS and SSTF.
Disadvantage of FCFS is the short processes which are at the back of the queue have to wait for the long process at the front to finish Disadvantage of SSTF is there is an overhead of finding out the closest request. Frequent switching of the Head’s direction slows the algorithm. Disadvantage of SCAN is long waiting time occurs for the cylinders which are just visited by the head. In SCAN the head moves till the end of the disk despite the absence of requests to be serviced.
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