Program for SSTF Disk Scheduling Algorithm

Given an array of disk track numbers and initial head position, our task is to find the total number of seek operations done to access all the requested tracks if Shortest Seek Time First (SSTF) is a disk scheduling algorithm is used.

The basic idea is the tracks that are closer to the current disk head position should be serviced first in order to minimize the seek operations is basically known as Shortest Seek Time First (SSTF).

Advantages of Shortest Seek Time First (SSTF)

• Better performance than the FCFS scheduling algorithm.
• It provides better throughput.
• This algorithm is used in Batch Processing systems where throughput is more important.
• It has a less average response and waiting time.

Disadvantages of Shortest Seek Time First (SSTF)

• Starvation is possible for some requests as it favours easy-to-reach requests and ignores the far-away processes.
• There is a lack of predictability because of the high variance of response time.
• Switching direction slows things down.

Algorithm

Step 1: Let the Request array represents an array storing indexes of tracks that have been requested. ‘head’ is the position of the disk head.

Step 2: Find the positive distance of all tracks in the request array from the head.

Step 3: Find a track from the requested array which has not been accessed/serviced yet and has a minimum distance from the head.

Step 4: Increment the total seek count with this distance.

Step 5: Currently serviced track position now becomes the new head position.

Step 6: Go to step 2 until all tracks in the request array have not been serviced. 

Example:

Request sequence = {176, 79, 34, 60, 92, 11, 41, 114} 
Initial head position = 50 

The following chart shows the sequence in which requested tracks are serviced using SSTF.

Therefore, the total seek count is calculated as: 

SSTF (Shortest Seek Time First)

= (50-41)+(41-34)+(34-11)+(60-11)+(79-60)+(92-79)+(114-92)+(176-114)
= 204
which can also be directly calculated as: (50-11) + (176-11)

Implementation

The implementation of SSTF is given below. Note that we have made a node class having 2 members. ‘distance’ is used to store the distance between the head and the track position. ‘accessed’ is a boolean variable that tells whether the track has been accessed/serviced before by the disk head or not. 

C++

// C++ program for implementation of
// SSTF disk scheduling
#include <bits/stdc++.h>
using namespace std;

// Calculates difference of each
// track number with the head position
void calculatedifference(int request[], int head,
                         int diff[][2], int n)
{
    for(int i = 0; i < n; i++)
    {
        diff[i][0] = abs(head - request[i]);
    }
}

// Find unaccessed track which is
// at minimum distance from head
int findMIN(int diff[][2], int n)
{
    int index = -1;
    int minimum = 1e9;

    for(int i = 0; i < n; i++)
    {
        if (!diff[i][1] && minimum > diff[i][0])
        {
            minimum = diff[i][0];
            index = i;
        }
    }
    return index;
}

void shortestSeekTimeFirst(int request[],
                           int head, int n)
{
    if (n == 0)
    {
        return;
    }

    // Create array of objects of class node
    int diff[n][2] = { { 0, 0 } };

    // Count total number of seek operation
    int seekcount = 0;

    // Stores sequence in which disk access is done
    int seeksequence[n + 1] = {0};

    for(int i = 0; i < n; i++)
    {
        seeksequence[i] = head;
        calculatedifference(request, head, diff, n);
        int index = findMIN(diff, n);
        diff[index][1] = 1;

        // Increase the total count
        seekcount += diff[index][0];

        // Accessed track is now new head
        head = request[index];
    }
    seeksequence[n] = head;

    cout << "Total number of seek operations = "
         << seekcount << endl;
    cout << "Seek sequence is : " << "\n";

    // Print the sequence
    for(int i = 0; i <= n; i++)
    {
        cout << seeksequence[i] << "\n";
    }
}

// Driver code
int main()
{
    int n = 8;
    int proc[n] = { 176, 79, 34, 60, 92, 11, 41, 114 };

    shortestSeekTimeFirst(proc, 50, n);

    return 0;
}

// This code is contributed by manish19je0495

Java

// Java program for implementation of
// SSTF disk scheduling
class node {

    // represent difference between
    // head position and track number
    int distance = 0;

    // true if track has been accessed
    boolean accessed = false;
}

public class SSTF {

    // Calculates difference of each
    // track number with the head position
    public static void calculateDifference(int queue[],
                                        int head, node diff[])

    {
        for (int i = 0; i < diff.length; i++)
            diff[i].distance = Math.abs(queue[i] - head);
    }

    // find unaccessed track
    // which is at minimum distance from head
    public static int findMin(node diff[])
    {
        int index = -1, minimum = Integer.MAX_VALUE;

        for (int i = 0; i < diff.length; i++) {
            if (!diff[i].accessed && minimum > diff[i].distance) {

                minimum = diff[i].distance;
                index = i;
            }
        }
        return index;
    }

    public static void shortestSeekTimeFirst(int request[],
                                                     int head)

    {
        if (request.length == 0)
            return;

        // create array of objects of class node
        node diff[] = new node[request.length];

        // initialize array
        for (int i = 0; i < diff.length; i++)

            diff[i] = new node();

        // count total number of seek operation
        int seek_count = 0;

        // stores sequence in which disk access is done
        int[] seek_sequence = new int[request.length + 1];

        for (int i = 0; i < request.length; i++) {

            seek_sequence[i] = head;
            calculateDifference(request, head, diff);

            int index = findMin(diff);

            diff[index].accessed = true;

            // increase the total count
            seek_count += diff[index].distance;

            // accessed track is now new head
            head = request[index];
        }

        // for last accessed track
        seek_sequence[seek_sequence.length - 1] = head;

        System.out.println("Total number of seek operations = "
                                                     + seek_count);

        System.out.println("Seek Sequence is");

        // print the sequence
        for (int i = 0; i < seek_sequence.length; i++)
            System.out.println(seek_sequence[i]);
    }

    public static void main(String[] args)
    {
        // request array
        int arr[] = { 176, 79, 34, 60, 92, 11, 41, 114 };
        shortestSeekTimeFirst(arr, 50);
    }
}

Python3

# Python3 program for implementation of
# SSTF disk scheduling

# Calculates difference of each
# track number with the head position
def calculateDifference(queue, head, diff):
    for i in range(len(diff)):
        diff[i][0] = abs(queue[i] - head)

# find unaccessed track which is
# at minimum distance from head
def findMin(diff):

    index = -1
    minimum = 999999999

    for i in range(len(diff)):
        if (not diff[i][1] and
                minimum > diff[i][0]):
            minimum = diff[i][0]
            index = i
    return index

def shortestSeekTimeFirst(request, head):
        if (len(request) == 0):
            return

        l = len(request)
        diff = [0] * l

        # initialize array
        for i in range(l):
            diff[i] = [0, 0]

        # count total number of seek operation
        seek_count = 0

        # stores sequence in which disk
        # access is done
        seek_sequence = [0] * (l + 1)

        for i in range(l):
            seek_sequence[i] = head
            calculateDifference(request, head, diff)
            index = findMin(diff)

            diff[index][1] = True

            # increase the total count
            seek_count += diff[index][0]

            # accessed track is now new head
            head = request[index]

        # for last accessed track
        seek_sequence[len(seek_sequence) - 1] = head

        print("Total number of seek operations =",
                                       seek_count)

        print("Seek Sequence is")

        # print the sequence
        for i in range(l + 1):
            print(seek_sequence[i])

# Driver code
if __name__ =="__main__":

    # request array
    proc = [176, 79, 34, 60,
            92, 11, 41, 114]
    shortestSeekTimeFirst(proc, 50)

# This code is contributed by
# Shubham Singh(SHUBHAMSINGH10)

C#

// C# program for implementation of
// SSTF disk scheduling
using System;

public class node
{

    // represent difference between
    // head position and track number
    public int distance = 0;

    // true if track has been accessed
    public Boolean accessed = false;
}

public class SSTF
{

    // Calculates difference of each
    // track number with the head position
    public static void calculateDifference(int []queue,
                                        int head, node []diff)

    {
        for (int i = 0; i < diff.Length; i++)
            diff[i].distance = Math.Abs(queue[i] - head);
    }

    // find unaccessed track
    // which is at minimum distance from head
    public static int findMin(node []diff)
    {
        int index = -1, minimum = int.MaxValue;

        for (int i = 0; i < diff.Length; i++)
        {
            if (!diff[i].accessed && minimum > diff[i].distance)
            {

                minimum = diff[i].distance;
                index = i;
            }
        }
        return index;
    }

    public static void shortestSeekTimeFirst(int []request,
                                                    int head)
    {
        if (request.Length == 0)
            return;

        // create array of objects of class node
        node []diff = new node[request.Length];

        // initialize array
        for (int i = 0; i < diff.Length; i++)

            diff[i] = new node();

        // count total number of seek operation
        int seek_count = 0;

        // stores sequence in which disk access is done
        int[] seek_sequence = new int[request.Length + 1];

        for (int i = 0; i < request.Length; i++)
        {

            seek_sequence[i] = head;
            calculateDifference(request, head, diff);

            int index = findMin(diff);

            diff[index].accessed = true;

            // increase the total count
            seek_count += diff[index].distance;

            // accessed track is now new head
            head = request[index];
        }

        // for last accessed track
        seek_sequence[seek_sequence.Length - 1] = head;

        Console.WriteLine("Total number of seek operations = "
                                                    + seek_count);

        Console.WriteLine("Seek Sequence is");

        // print the sequence
        for (int i = 0; i < seek_sequence.Length; i++)
            Console.WriteLine(seek_sequence[i]);
    }

    // Driver code
    public static void Main(String[] args)
    {
        // request array
        int []arr = { 176, 79, 34, 60, 92, 11, 41, 114 };
        shortestSeekTimeFirst(arr, 50);
    }
}

// This code contributed by Rajput-Ji

Javascript

// Javascript Program for implementation of
// SSTF disk scheduling
function calculatedifference(request, head, diff, n) {
    for (let i = 0; i < n; i++) {
        diff[i][0] = Math.abs(head - request[i]);
    }
}

// Find unaccessed track which is
// at minimum distance from head
function findMIN(diff, n) {
    let index = -1;
    let minimum = 1e9;

    for (let i = 0; i < n; i++) {
        if (!diff[i][1] && minimum > diff[i][0]) {
            minimum = diff[i][0];
            index = i;
        }
    }
    return index;
}

function shortestSeekTimeFirst(request, head, n) {
    if (n == 0) {
        return;
    }

    // Create array of objects of class node
    let diff = new Array(n);
    for (let i = 0; i < n; i++) {
        diff[i] = new Array(2);
    }

    // Count total number of seek operation
    let seekcount = 0;

    // Stores sequence in which disk access is done
    let seeksequence = new Array(n + 1);
    seeksequence[0] = head;

    for (let i = 0; i < n; i++) {
        calculatedifference(request, head, diff, n);
        let index = findMIN(diff, n);
        diff[index][1] = 1;

        // Increase the total count
        seekcount += diff[index][0];

        // Accessed track is now new head
        head = request[index];
        seeksequence[i + 1] = head;
    }

    console.log("Total number of seek operations = " + seekcount);
    console.log("Seek sequence is : ");

    // Print the sequence
    for (let i = 0; i <= n; i++) {
        console.log(seeksequence[i]);
    }
}

// Driver code
let n = 8;
let proc = [176, 79, 34, 60, 92, 11, 41, 114];

shortestSeekTimeFirst(proc, 50, n);

//  This code is contributed by ishankhandelwals.

Output

Total number of seek operations = 204
Seek Sequence: 50, 41, 34, 11, 60, 79, 92, 114, 176 

Time Complexity: O(N^2)

Auxiliary Space: O(N) 

FAQs on SSTF Disk Scheduling Algorithm

1. Will Starvation occur for some requests in SSTF?

Yes, when a continuos stream of requests are coming then it will lead to starvation.

2. Whether SSTF is suitable for minimizing disk arm movement?

Yes, SSTF is helpful in minimizing disk arm movement and it also helps in reducing seek time.

3. In case of two requests having the same seek time, which request SSTF will follow?

If multiple requests have the same seek time, SSTF can choose any of them, depending on the implementation.