FCFS Disk Scheduling Algorithms

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 First Come First Serve (FCFS) disk scheduling algorithm is used.

First Come First Serve (FCFS) 
FCFS is the simplest disk scheduling algorithm. As the name suggests, this algorithm entertains requests in the order they arrive in the disk queue. The algorithm looks very fair and there is no starvation (all requests are serviced sequentially) but generally, it does not provide the fastest service.

Algorithm: 

1. Let Request array represents an array storing indexes of tracks that have been requested in ascending order of their time of arrival. ‘head’ is the position of disk head.
2. Let us one by one take the tracks in default order and calculate the absolute distance of the track from the head.
3. Increment the total seek count with this distance.
4. Currently serviced track position now becomes the new head position.
5. Go to step 2 until all tracks in request array have not been serviced.

Example:  

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

Output:
Total number of seek operations = 510
Seek Sequence is
176
79
34
60
92
11
41
114

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

Therefore, the total seek count is calculated as: 
 

= (176-50)+(176-79)+(79-34)+(60-34)+(92-60)+(92-11)+(41-11)+(114-41)
= 510

Implementation: 
Implementation of FCFS is given below. Note that distance is used to store absolute distance between head and current track position. 

// C++ program to demonstrate
// FCFS Disk Scheduling algorithm

#include <bits/stdc++.h>
using namespace std;

int size = 8;

void FCFS(int arr[], int head)
{
    int seek_count = 0;
    int distance, cur_track;

    for (int i = 0; i < size; i++) {
        cur_track = arr[i];

        // calculate absolute distance
        distance = abs(cur_track - head);

        // increase the total count
        seek_count += distance;

        // accessed track is now new head
        head = cur_track;
    }

    cout << "Total number of seek operations = "
         << seek_count << endl;

    // Seek sequence would be the same
    // as request array sequence
    cout << "Seek Sequence is" << endl;

    for (int i = 0; i < size; i++) {
        cout << arr[i] << endl;
    }
}

// Driver code
int main()
{

    // request array
    int arr[size] = { 176, 79, 34, 60, 92, 11, 41, 114 };
    int head = 50;

    FCFS(arr, head);

    return 0;
}

// C++ program to demonstrate
// FCFS Disk Scheduling algorithm
#include <stdio.h>
#include <math.h>

int size = 8;

void FCFS(int arr[],int head)
{
    int seek_count = 0;
      int cur_track, distance;
  
    for(int i=0;i<size;i++)
       {
        cur_track = arr[i];
      
          // calculate absolute distance
        distance = fabs(head - cur_track);
      
          // increase the total count
        seek_count += distance;
      
          // accessed track is now new head
        head = cur_track;
    }
  
    printf("Total number of seek operations: %d\n",seek_count);
      
      // Seek sequence would be the same
    // as request array sequence
    printf("Seek Sequence is\n");
  
      for (int i = 0; i < size; i++) {
        printf("%d\n",arr[i]);
    }
}

//Driver code
int main()
{
      // request array
    int arr[8] = { 176, 79, 34, 60, 92, 11, 41, 114 };
      int head = 50;
    
    FCFS(arr,head);
  
    return 0;
}

//This code is contributed by Pratham Kashyap

// Java program to demonstrate
// FCFS Disk Scheduling algorithm
class GFG
{
static int size = 8;

static void FCFS(int arr[], int head)
{
    int seek_count = 0;
    int distance, cur_track;

    for (int i = 0; i < size; i++) 
    {
        cur_track = arr[i];

        // calculate absolute distance
        distance = Math.abs(cur_track - head);

        // increase the total count
        seek_count += distance;

        // accessed track is now new head
        head = cur_track;
    }

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

    // Seek sequence would be the same
    // as request array sequence
    System.out.println("Seek Sequence is");

    for (int i = 0; i < size; i++)
    {
        System.out.println(arr[i]);
    }
}

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

    FCFS(arr, head);
}
}

// This code is contributed by 29AjayKumar

# Python program to demonstrate
# FCFS Disk Scheduling algorithm

size = 8;

def FCFS(arr, head):

    seek_count = 0;
    distance, cur_track = 0, 0;

    for i in range(size):
        cur_track = arr[i];

        # calculate absolute distance
        distance = abs(cur_track - head);

        # increase the total count
        seek_count += distance;

        # accessed track is now new head
        head = cur_track;
    
    print("Total number of seek operations = ", 
                                   seek_count);

    # Seek sequence would be the same
    # as request array sequence
    print("Seek Sequence is");

    for i in range(size):
        print(arr[i]);
    
# Driver code
if __name__ == '__main__':

    # request array
    arr = [ 176, 79, 34, 60,
            92, 11, 41, 114 ];
    head = 50;

    FCFS(arr, head);

# This code contributed by Rajput-Ji

// C# program to demonstrate
// FCFS Disk Scheduling algorithm
using System;

class GFG
{
static int size = 8;

static void FCFS(int []arr, int head)
{
    int seek_count = 0;
    int distance, cur_track;

    for (int i = 0; i < size; i++) 
    {
        cur_track = arr[i];

        // calculate absolute distance
        distance = Math.Abs(cur_track - head);

        // increase the total count
        seek_count += distance;

        // accessed track is now new head
        head = cur_track;
    }

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

    // Seek sequence would be the same
    // as request array sequence
    Console.WriteLine("Seek Sequence is");

    for (int i = 0; i < size; i++)
    {
        Console.WriteLine(arr[i]);
    }
}

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

    FCFS(arr, head);
}
}

// This code is contributed by PrinciRaj1992 

<script>

// Javascript program to demonstrate
// FCFS Disk Scheduling algorithm
var size = 8;

function FCFS(arr, head)
{
    var seek_count = 0;
    var distance, cur_track;

    for(var i = 0; i < size; i++) 
    {
        cur_track = arr[i];
        
        // Calculate absolute distance
        distance = Math.abs(cur_track - head);

        // Increase the total count
        seek_count += distance;

        // Accessed track is now new head
        head = cur_track;
    }

    document.write("Total number of " + 
                   "seek operations = " + 
                   seek_count);

    // Seek sequence would be the same
    // as request array sequence
    document.write("<br>Seek Sequence is");

    for(var i = 0; i < size; i++)
    {
        document.write("<br>" + arr[i]);
    }
}

// Driver code

// request array
var arr = [ 176, 79, 34, 60, 
            92, 11, 41, 114 ];
var head = 50;

FCFS(arr, head);

// This code is contributed by Amit Katiyar 

</script>

Total number of seek operations = 510
Seek Sequence is
176
79
34
60
92
11
41
114

FCFS Disk Scheduling Algorithm in OS

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