C-SCAN Disk Scheduling Algorithm

Prerequisite: Disk Scheduling Algorithms and SCAN 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 a C-SCAN disk scheduling algorithm is used.

C-SCAN (Circular Elevator) Disk Scheduling Algorithm

The circular SCAN (C-SCAN) scheduling algorithm is a modified version of the SCAN disk scheduling algorithm that deals with the inefficiency of the SCAN algorithm by servicing the requests more uniformly. Like SCAN (Elevator Algorithm) C-SCAN moves the head from one end servicing all the requests to the other end. However, as soon as the head reaches the other end, it immediately returns to the beginning of the disk without servicing any requests on the return trip (see chart below) and starts servicing again once reaches the beginning. This is also known as the “Circular Elevator Algorithm” as it essentially treats the cylinders as a circular list that wraps around from the final cylinder to the first one.

Advantages of C-SCAN (Circular Elevator) Disk Scheduling Algorithm:

• Works well with moderate to heavy loads.
• It provides better response time and uniform waiting time.

Disadvantages of C-SCAN (Circular Elevator) Disk Scheduling Algorithm:

• May not be fair to service requests for tracks at the extreme end.
• It has more seek movements as compared to the SCAN Algorithm.

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. The head services only in the right direction from 0 to the size of the disk.
3. While moving in the left direction do not service any of the tracks.
4. When we reach the beginning(left end) reverse the direction.
5. While moving in the right direction it services all tracks one by one.
6. While moving in the right direction calculate the absolute distance of the track from the head.
7. Increment the total seek count with this distance.
8. Currently serviced track position now becomes the new head position.
9. Go to step 6 until we reach the right end of the disk.
10. If we reach the right end of the disk reverse the direction and go to step 3 until all tracks in the request array have not been serviced.

Examples:

Input:  
Request sequence = {176, 79, 34, 60, 92, 11, 41, 114}
Initial head position = 50
Direction = right(We are moving from left to right)

Output:
Initial position of head: 50
Total number of seek operations = 389
Seek Sequence is
60
79
92
114
176
199
0
11
34
41

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

Therefore, the total seek count is calculated as: 

= (60-50)+(79-60)+(92-79)
        +(114-92)+(176-114)+(199-176)+(199-0)
        +(11-0)+(34-11)+(41-34)
= 389

Implementation:

The implementation of C-SCAN algorithm is given below.

Note:
The distance variable is used to store the absolute distance between the head and current track position. disk_size is the size of the disk. Vectors left and right store all the request tracks on the left-hand side and the right-hand side of the initial head position respectively.

C++

// C++ program to demonstrate
// C-SCAN Disk Scheduling algorithm
#include <bits/stdc++.h>
using namespace std;

// Code by Vikram Chaurasia

int size = 8;
int disk_size = 200;

void CSCAN(int arr[], int head)
{
    int seek_count = 0;
    int distance, cur_track;
    vector<int> left, right;
    vector<int> seek_sequence;

    // appending end values
    // which has to be visited
    // before reversing the direction
    left.push_back(0);
    right.push_back(disk_size - 1);

    // tracks on the left of the
    // head will be serviced when
    // once the head comes back
    // to the beginning (left end).
    for (int i = 0; i < size; i++) {
        if (arr[i] < head)
            left.push_back(arr[i]);
        if (arr[i] > head)
            right.push_back(arr[i]);
    }

    // sorting left and right vectors
    std::sort(left.begin(), left.end());
    std::sort(right.begin(), right.end());

    // first service the requests
    // on the right side of the
    // head.
    for (int i = 0; i < right.size(); i++) {
        cur_track = right[i];
        // appending current track to seek sequence
        seek_sequence.push_back(cur_track);

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

        // increase the total count
        seek_count += distance;

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

    // once reached the right end
    // jump to the beginning.
    head = 0;

    // adding seek count for head returning from 199 to 0
    seek_count += (disk_size - 1);

    // Now service the requests again
    // which are left.
    for (int i = 0; i < left.size(); i++) {
        cur_track = left[i];

        // appending current track to seek sequence
        seek_sequence.push_back(cur_track);

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

        // increase the total count
        seek_count += distance;

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

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

    cout << "Seek Sequence is" << endl;

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

// Driver code
int main()
{

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

    cout << "Initial position of head: " << head << endl;
    CSCAN(arr, head);

    return 0;
}

Java

// Java program to demonstrate
// C-SCAN Disk Scheduling algorithm
import java.util.*;

class GFG {

    static int size = 8;
    static int disk_size = 200;

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

        Vector<Integer> left = new Vector<Integer>();
        Vector<Integer> right = new Vector<Integer>();
        Vector<Integer> seek_sequence
            = new Vector<Integer>();

        // Appending end values which has
        // to be visited before reversing
        // the direction
        left.add(0);
        right.add(disk_size - 1);

        // Tracks on the left of the
        // head will be serviced when
        // once the head comes back
        // to the beggining (left end).
        for (int i = 0; i < size; i++) {
            if (arr[i] < head)
                left.add(arr[i]);
            if (arr[i] > head)
                right.add(arr[i]);
        }

        // Sorting left and right vectors
        Collections.sort(left);
        Collections.sort(right);

        // First service the requests
        // on the right side of the
        // head.
        for (int i = 0; i < right.size(); i++) {
            cur_track = right.get(i);

            // Appending current track to seek sequence
            seek_sequence.add(cur_track);

            // 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;
        }

        // Once reached the right end
        // jump to the beggining.
        head = 0;

        // adding seek count for head returning from 199 to
        // 0
        seek_count += (disk_size - 1);

        // Now service the requests again
        // which are left.
        for (int i = 0; i < left.size(); i++) {
            cur_track = left.get(i);

            // Appending current track to
            // seek sequence
            seek_sequence.add(cur_track);

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

            // Increase the total count
            seek_count += distance;

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

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

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

        for (int i = 0; i < seek_sequence.size(); i++) {
            System.out.println(seek_sequence.get(i));
        }
    }

    // Driver code
    public static void main(String[] args) throws Exception
    {

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

        System.out.println("Initial position of head: "
                           + head);

        CSCAN(arr, head);
    }
}

// This code is contributed by divyesh072019

Python3

# Python3 program to demonstrate
# C-SCAN Disk Scheduling algorithm
size = 8
disk_size = 200


def CSCAN(arr, head):

    seek_count = 0
    distance = 0
    cur_track = 0
    left = []
    right = []
    seek_sequence = []

    # Appending end values
    # which has to be visited
    # before reversing the direction
    left.append(0)
    right.append(disk_size - 1)

    # Tracks on the left of the
    # head will be serviced when
    # once the head comes back
    # to the beggining (left end).
    for i in range(size):
        if (arr[i] < head):
            left.append(arr[i])
        if (arr[i] > head):
            right.append(arr[i])

    # Sorting left and right vectors
    left.sort()
    right.sort()

    # First service the requests
    # on the right side of the
    # head.
    for i in range(len(right)):
        cur_track = right[i]

        # Appending current track
        # to seek sequence
        seek_sequence.append(cur_track)

        # Calculate absolute distance
        distance = abs(cur_track - head)

        # Increase the total count
        seek_count += distance

        # Accessed track is now new head
        head = cur_track

    # Once reached the right end
    # jump to the beggining.
    head = 0

    # adding seek count for head returning from 199 to 0
    seek_count += (disk_size - 1)

    # Now service the requests again
    # which are left.
    for i in range(len(left)):
        cur_track = left[i]

        # Appending current track
        # to seek sequence
        seek_sequence.append(cur_track)

        # Calculate absolute distance
        distance = abs(cur_track - head)

        # Increase the total count
        seek_count += distance

        # Accessed track is now the new head
        head = cur_track

    print("Total number of seek operations =",
          seek_count)
    print("Seek Sequence is")
    print(*seek_sequence, sep="\n")

# Driver code


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

print("Initial position of head:", head)

CSCAN(arr, head)

# This code is contributed by rag2127

C#

// C# program to demonstrate
// C-SCAN Disk Scheduling algorithm
using System;
using System.Collections.Generic;

class GFG {

    static int size = 8;
    static int disk_size = 200;

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

        List<int> left = new List<int>();
        List<int> right = new List<int>();
        List<int> seek_sequence = new List<int>();

        // Appending end values which has
        // to be visited before reversing
        // the direction
        left.Add(0);
        right.Add(disk_size - 1);

        // Tracks on the left of the
        // head will be serviced when
        // once the head comes back
        // to the beggining (left end).
        for (int i = 0; i < size; i++) {
            if (arr[i] < head)
                left.Add(arr[i]);
            if (arr[i] > head)
                right.Add(arr[i]);
        }

        // Sorting left and right vectors
        left.Sort();
        right.Sort();

        // First service the requests
        // on the right side of the
        // head.
        for (int i = 0; i < right.Count; i++) {
            cur_track = right[i];

            // Appending current track to seek sequence
            seek_sequence.Add(cur_track);

            // 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;
        }

        // Once reached the right end
        // jump to the beggining.
        head = 0;

        // adding seek count for head returning from 199 to
        // 0
        seek_count += (disk_size - 1);

        // Now service the requests again
        // which are left.
        for (int i = 0; i < left.Count; i++) {
            cur_track = left[i];

            // Appending current track to
            // seek sequence
            seek_sequence.Add(cur_track);

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

            // Increase the total count
            seek_count += distance;

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

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

        Console.WriteLine("Seek Sequence is");

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

    // Driver code
    static void Main()
    {

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

        Console.WriteLine("Initial position of head: "
                          + head);

        CSCAN(arr, head);
    }
}

// This code is contributed by divyeshrabadiya07

Javascript

<script>
    // Javascript program to demonstrate
    // C-SCAN Disk Scheduling algorithm
    let size = 8;
    let disk_size = 200;

    function CSCAN(arr, head)
    {
        let seek_count = 0;
        let distance, cur_track;
        let left = [], right = [];
        let seek_sequence = [];

        // appending end values
        // which has to be visited
        // before reversing the direction
        left.push(0);
        right.push(disk_size - 1);

        // tracks on the left of the
        // head will be serviced when
        // once the head comes back
        // to the beggining (left end).
        for (let i = 0; i < size; i++) {
            if (arr[i] < head)
                left.push(arr[i]);
            if (arr[i] > head)
                right.push(arr[i]);
        }

        // sorting left and right vectors
        left.sort(function(a, b){return a - b});
        right.sort(function(a, b){return a - b});

        // first service the requests
        // on the right side of the
        // head.
        for (let i = 0; i < right.length; i++)
        {
            cur_track = right[i];

            // appending current track to seek sequence
            seek_sequence.push(cur_track);

            // 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;
        }

        // once reached the right end
        // jump to the beggining.
        head = 0;

        // adding seek count for head returning from 199 to 0
        seek_count += (disk_size - 1);

        // Now service the requests again
        // which are left.
        for (let i = 0; i < left.length; i++) {
            cur_track = left[i];

            // appending current track to seek sequence
            seek_sequence.push(cur_track);

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

            // increase the total count
            seek_count += distance;

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

        document.write("Total number of seek operations = " + seek_count + "</br>");
        document.write("Seek Sequence is" + "</br>");
        for (let i = 0; i < seek_sequence.length; i++)
        {
            document.write(seek_sequence[i] + "</br>");
        }
    }

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

    document.write("Initial position of head: " + head + "</br>");
    CSCAN(arr, head);

    // This code is contributed by mukesh07.
</script>

Output

Initial position of head: 50
Total number of seek operations = 389
Seek Sequence is
60
79
92
114
176
199
0
11
34
41