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LOOK Disk Scheduling Algorithm

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The LOOK Disk Scheduling Algorithm is the advanced version of the SCAN (elevator) disk scheduling algorithm which gives slightly better seek time than any other algorithm in the hierarchy (FCFS->SRTF->SCAN->C-SCAN->LOOK). It is used to reduce the amount of time it takes to access data on a hard disk drive by minimizing the seek time between read/write operations. The LOOK algorithm operates by scanning the disk in a specific direction, but instead of going all the way to the end of the disk before reversing direction like the SCAN algorithm, it reverses direction as soon as it reaches the last request in the current direction.

The LOOK algorithm services request similarly to the SCAN Algorithm meanwhile it also “looks” ahead as if there are more tracks that are needed to be serviced in the same direction. The main reason behind the better performance of the LOOK algorithm in comparison to SCAN is that in this algorithm the head is not allowed to move till the end of the disk.

Steps Involved in the LOOK Algorithm

  • Determine the initial direction of disk head movement.
  • Sort the pending disk requests in the order in which they will be serviced.
  • Scan the disk in the chosen direction, servicing requests as they are encountered.
  • When the last request in the current direction has been serviced, reverse the direction and continue scanning until all requests have been serviced.

Advantages of the LOOK Disk Scheduling Algorithm

  • It can provide better performance than the FCFS (first-come, first-served) and SSTF (shortest-seek-time-first) algorithms because it reduces the number of head movements required to access data on the disk.
  • It is relatively simple to implement and does not require a large amount of memory or processing power.
  • It is efficient in terms of disk usage because it scans only the areas of the disk where data is located.

Disadvantages of the LOOK Disk Scheduling Algorithm

  • It may not be optimal in situations where there are large amounts of data to be read or written in one direction, as it could lead to a large number of requests being queued up in the opposite direction.
  • It may not be suitable for real-time systems where fast response times are critical, as it does not prioritize requests based on their urgency or importance.
  • It may lead to starvation of requests that are located far away from the current position of the disk head.
  • Given an array of disk track numbers and initial head position, our task is to find the total number of seek operations to access all the requested tracks if the LOOK disk scheduling algorithm is used. Also, write a program to find the seek sequence using the LOOK disk scheduling algorithm.

Algorithm for LOOK Disk Scheduling

Step 1: Let the 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 the disk head.

Step 2: The initial direction in which the head is moving is given and it services in the same direction.

Step 3: The head services all the requests one by one in the direction head is moving.

Step 4: The head continues to move in the same direction until all the requests in this direction are finished.

Step 5: While moving in this direction calculate the absolute distance of the track from the head.

Step 6: Increment the total seek count with this distance.

Step 7: Currently serviced track position now becomes the new head position.

Step 8: Go to step 5 until we reach at last request in this direction.

Step 9: If we reach where no requests are needed to be serviced in this direction reverse the direction and go to step 3 until all tracks in the request array have not been serviced.

Example 

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 = 291
Seek Sequence: 60, 79, 92, 114, 176, 41, 34, 11

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

LOOK Disk Scheduling Algorithm

LOOK Disk Scheduling Algorithm

Therefore, the total seek count is calculated as

Total Seek Time = (60-50) + (79-60) + (92-79) + (114-92) + (176-114) + (176-41) + (41-34) + (34-11)

= 291

Implementation of LOOK Disk Scheduling Algorithm

Implementation of the LOOK Algorithm is given below.

Note: The distance variable is used to store the absolute distance between the head and the 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
// LOOK Disk Scheduling algorithm
int size = 8;
#include <bits/stdc++.h>
using namespace std;
  
// Code by Vikram Chaurasia
  
int disk_size = 200;
  
void LOOK(int arr[], int head, string direction)
{
    int seek_count = 0;
    int distance, cur_track;
    vector<int> left, right;
    vector<int> seek_sequence;
  
    // appending values which are
    // currently at left and right
    // direction from the head.
    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
    // for servicing tracks in the
    // correct sequence.
    std::sort(left.begin(), left.end());
    std::sort(right.begin(), right.end());
  
    // run the while loop two times.
    // one by one scanning right
    // and left side of the head
    int run = 2;
    while (run--) {
        if (direction == "left") {
            for (int i = left.size() - 1; i >= 0; 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;
            }
            // reversing the direction
            direction = "right";
        }
        else if (direction == "right") {
            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;
            }
            // reversing the direction
            direction = "left";
        }
    }
  
    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;
    string direction = "right";
  
    cout << "Initial position of head: "
         << head << endl;
  
    LOOK(arr, head, direction);
  
    return 0;
}


Java




// Java program to demonstrate
// LOOK Disk Scheduling algorithm
import java.util.*;
  
class GFG{
      
static int size = 8
static int disk_size = 200;
  
public static void LOOK(int arr[], int head,
                        String direction) 
    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 values which are 
    // currently at left and right 
    // direction from the head. 
    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 
    // for servicing tracks in the 
    // correct sequence. 
    Collections.sort(left);  
    Collections.sort(right);  
      
    // Run the while loop two times. 
    // one by one scanning right 
    // and left side of the head 
    int run = 2
    while (run-- > 0)
    
        if (direction == "left"
        
            for(int i = left.size() - 1
                    i >= 0; 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; 
            
              
            // Reversing the direction 
            direction = "right"
        
        else if (direction == "right")
        
            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; 
            
              
            // Reversing the direction 
            direction = "left"
        
    
      
    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
    String direction = "right"
    
    System.out.println("Initial position of head: "
                        head); 
    
    LOOK(arr, head, direction); 
}
}
  
// This code is contributed by divyesh072019


Python3




# Python3 program to demonstrate
# LOOK Disk Scheduling algorithm
size = 8
disk_size = 200
  
def LOOK(arr, head, direction):
      
    seek_count = 0
    distance = 0
    cur_track = 0
  
    left = []
    right = []
   
    seek_sequence = []
  
    # Appending values which are
    # currently at left and right
    # direction from the head.
    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
    # for servicing tracks in the
    # correct sequence.
    left.sort()
    right.sort()
  
    # Run the while loop two times.
    # one by one scanning right
    # and left side of the head
    run = 2
    while (run):
        if (direction == "left"):
            for i in range(len(left) - 1, -1, -1):
                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
  
            # Reversing the direction
            direction = "right"
              
        elif (direction == "right"):
            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
  
            # Reversing the direction
            direction = "left"
              
        run -= 1
  
    print("Total number of seek operations ="
          seek_count)
    print("Seek Sequence is")
  
    for i in range(len(seek_sequence)):
        print(seek_sequence[i])
  
# Driver code
  
# Request array
arr = [ 176, 79, 34, 60, 92, 11, 41, 114 ]
head = 50
  
direction = "right"
  
print("Initial position of head:", head)
  
LOOK(arr, head, direction)
  
# This code is contributed by rag2127


C#




// C# program to demonstrate
// LOOK Disk Scheduling algorithm
using System;
using System.Collections.Generic;
  
class GFG{
      
static int size = 8; 
  
static void LOOK(int[] arr, int head, 
                 string direction) 
    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 values which are 
    // currently at left and right 
    // direction from the head. 
    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 
    // for servicing tracks in the 
    // correct sequence. 
    left.Sort();  
    right.Sort();  
     
    // Run the while loop two times. 
    // one by one scanning right 
    // and left side of the head 
    int run = 2; 
    while (run-- > 0)
    
        if (direction == "left"
        
            for(int i = left.Count - 1; i >= 0; 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; 
            
               
            // Reversing the direction 
            direction = "right"
        
        else if (direction == "right")
        
            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; 
            
               
            // Reversing the direction 
            direction = "left"
        
    
       
    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; 
    string direction = "right"
     
    Console.WriteLine("Initial position of head: "
                       head); 
     
    LOOK(arr, head, direction); 
}
}
  
// This code is contributed by divyeshrabadiya07


Javascript




<script>
    // Javascript program to demonstrate
    // LOOK Disk Scheduling algorithm
      
    let size = 8;
   
    function LOOK(arr, head, direction)
    {
        let seek_count = 0;
        let distance, cur_track;
  
        let left = [];
        let right = [];
        let seek_sequence = [];
  
        // Appending values which are
        // currently at left and right
        // direction from the head.
        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
        // for servicing tracks in the
        // correct sequence.
        left.sort(function(a, b){return a - b});
        right.sort(function(a, b){return a - b});
  
        // Run the while loop two times.
        // one by one scanning right
        // and left side of the head
        let run = 2;
        while (run-- > 0)
        {
            if (direction == "left")
            {
                for(let i = left.length - 1; i >= 0; 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;
                }
  
                // Reversing the direction
                direction = "right";
            }
            else if (direction == "right")
            {
                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;
                }
  
                // Reversing the direction
                direction = "left";
            }
        }
  
        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;
    let direction = "right";
      
    document.write("Initial position of head: " + head + "</br>");
      
    LOOK(arr, head, direction);
      
</script>


Output

Initial position of head: 50
Total number of seek operations = 291
Seek Sequence: 60, 79, 92, 114, 176, 41, 34, 11

FAQs On LOOK Disk Scheduling Algorithm

Q.1: Is Look Disk Scheduling Algorithms Suitable for Solid State Drives (SSD)?

Answer:

Most probably, the Look Disk Scheduling Algorithm is suitable for traditional hard drives. Since SSDs don’t have a moving part, therefore Look Disk Scheduling Algorithm is not so suitable for Solid State Drives.

Q.2: Is Look Disk Scheduling Algorithms fair to all Requests?

Answer:

Look Disk Scheduling Algorithms are not fair to all requests. It mostly prefers to those requests that come in its path, where it is currently scanning.



Last Updated : 06 Sep, 2023
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