Shortest Seek Time First (SSTF) Disk Scheduling

Aim

To simulate the SSTF disk scheduling algorithm and compute the total head movement, displaying Step, From, To, Movement.

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Theory

Shortest Seek Time First (SSTF) selects the disk request that is closest to the current head position.

• Minimizes immediate seek time

• Improves performance compared to FCFS

• May cause starvation for far-away requests

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Algorithm

1. Read number of disk requests.

2. Read request queue.

3. Read initial head position.

4. Mark all requests as unvisited.

5. From current head:

   • Find request with minimum absolute distance

   • Move head to that request

   • Add movement to total

   • Mark request visited

6. Repeat until all requests are serviced.

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Program: SSTF Disk Scheduling (C)

#include <stdio.h>
#include <stdlib.h>

int main() {
    int n, head, total = 0;

    printf("Enter number of requests: ");
    scanf("%d", &n);

    int req[n], visited[n];

    printf("Enter the requests:\n");
    for(int i = 0; i < n; i++) {
        scanf("%d", &req[i]);
        visited[i] = 0;
    }

    printf("Enter initial head position: ");
    scanf("%d", &head);

    printf("\nStep\tFrom\tTo\tMovement\n");
    printf("--------------------------------\n");

    for(int step = 0; step < n; step++) {

        int min = 100000, index = -1;

        for(int i = 0; i < n; i++) {
            if(!visited[i]) {
                int dist = abs(head - req[i]);
                if(dist < min) {
                    min = dist;
                    index = i;
                }
            }
        }

        printf("%d\t%d\t%d\t%d\n", step+1, head, req[index], min);

        total += min;
        head = req[index];
        visited[index] = 1;
    }

    printf("--------------------------------\n");
    printf("Total head movement = %d\n", total);

    return 0;
}

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Same Example as FCFS

Input

Number of requests = 8
Queue = 98 183 37 122 14 124 65 67
Initial head = 53

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Sample Run (SSTF Order Selection)

Current head = 53

Nearest sequence:

53 → 65 → 67 → 37 → 14 → 98 → 122 → 124 → 183

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Output Table

Step    From    To      Movement
--------------------------------
1       53      65      12
2       65      67      2
3       67      37      30
4       37      14      23
5       14      98      84
6       98      122     24
7       122     124     2
8       124     183     59
--------------------------------
Total head movement = 236

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Observation

Algorithm	Total Head Movement
FCFS	640
SSTF	236

👉 SSTF significantly reduces head movement compared to FCFS.

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Result

The SSTF disk scheduling algorithm was successfully simulated.
For the given example, total head movement = 236 tracks, which is lower than FCFS.