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Program for SSTF disk scheduling algorithm

• Difficulty Level : Medium
• Last Updated : 25 Jan, 2023

Prerequisite – Disk scheduling algorithms
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.

Shortest Seek Time First (SSTF) –
Basic idea is the tracks which are closer to current disk head position should be serviced first in order to minimise the seek operations.

Advantages of Shortest Seek Time First (SSTF) â€“

1. Better performance than FCFS scheduling algorithm.
2. It provides better throughput.
3. This algorithm is used in Batch Processing system where throughput is more important.
4. It has less average response and waiting time.

Disadvantages of Shortest Seek Time First (SSTF) â€“

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

Algorithm –

1. Let Request array represents an array storing indexes of tracks that have been requested. ‘head’ is the position of disk head.
2. Find the positive distance of all tracks in the request array from head.
3. Find a track from requested array which has not been accessed/serviced yet and has minimum distance from head.
4. Increment the total seek count with this distance.
5. Currently serviced track position now becomes the new head position.
6. Go to step 2 until all tracks in request array have not been serviced.

Example –
Request sequence = {176, 79, 34, 60, 92, 11, 41, 114}

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

Therefore, total seek count is calculated as:

= (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 –
Implementation of SSTF is given below. Note that we have made a node class having 2 members. â€˜distanceâ€™ is used to store the distance between head and the track position. â€˜accessedâ€™ is a boolean variable which tells whether the track has been accessed/serviced before by disk head or not.

C++

 // C++ program for implementation of// SSTF disk scheduling#include using namespace std; // Calculates difference of each // track number with the head positionvoid 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 headint 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 codeint 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

Python3

 # Python3 program for implementation of# SSTF disk scheduling # Calculates difference of each# track number with the head positiondef 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 headdef 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 codeif __name__ =="__main__":         # request array    proc = [176, 79, 34, 60,            92, 11, 41, 114]    shortestSeekTimeFirst(proc, 50)     # This code is contributed by# Shubham Singh(SHUBHAMSINGH10)

Javascript

 // Javascript Program for implementation of// SSTF disk schedulingfunction 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 headfunction 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 codelet 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 is
50
41
34
11
60
79
92
114
176

Time Complexity: O(N^2)
Auxiliary Space: O(N)

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