LOOK and CLOOK Algorithm

CLOOK Algorithm

CLOOK is comparable to the CSCAN disc scheduling algorithm in the same way that LOOK is comparable to the SCAN algorithm. Despite traveling to the end, the disc arm in CLOOK only goes to the last request to be serviced first in front of the head and then to the opposite end's last request. As a result, it avoids the additional time caused by unneeded traversal to the disk's end.

Algorithm

1. Arrange all the I/O requests in ascending order.
2. The initial direction in which the head is moving is given and it serves in the same direction.
3. The head services all the requests one by one in the direction it is moving.
4. The head continues to move in the same direction until all the requests in this direction have been serviced.
5. While reversing the direction, pending requests will not be served which are left to the Head pointer in the case of where the direction is given towards a large number of values. Instead, the disc arm will move directly toward the first request and then reverses its direction to serve the pending requests by moving forth. 
6. When the direction is given towards the less number of values the procedure would be the same regardless of the direction will change. 

Consider an example,

Suppose the requests to be addressed are 82, 140, 170, 190, 70, 35, and 20. Head pointer starts at 50, and it is also given that the disk arm should move “towards the larger value”. Calculate the seek time.

The seek time can be calculated as,

Seek time can be calculated by counting the head movements. 

OR

By subtracting the last service requests(direction of head pointer) and the head pointer in addition to the first service requests(direction of head pointer)  and last service requests(direction opposite to the head pointer) in addition to the last service requests(direction opposite of head pointer).

The seek time can be calculated as,

Advantages

• It provides a more uniform waiting time compared to LOOK and C-SCAN.
• It is more efficient compared to C-SCAN.

Disadvantages

• More overhead in calculations.
• Look algorithm is not recommended to use in terms of heavy load.
   
• Prims and Kruskal Algorithm

LOOK vs CLOOK 

Also see, Difference between C-LOOK and LOOK Algorithms, Multiprogramming vs Multitasking

Frequently Asked Questions

Which scheduling has the best performance considering all the algorithms like FCFS, SSTF, SCAN, CSCAN, LOOK, and C-LOOK?

CLOOK performance is best in comparison to all the scheduling algorithms. Because of uniform waiting time and response time.

In which algorithm, does the disc arm in the algorithm goes as far as the final request in each direction, then instantly reverse direction without reaching the end of the disk?

Suppose the requests to be addressed are 176, 79, 34, 60, 92, 11, 41, and 114. The Head pointer starts at 50, and it is also given that the disk arm should move “towards the larger value”. Calculate the seek time.

Conclusion

This article discusses how page fault occurs and How Page Fault Handling works in will work in the Operating System. 

Recommended Reading:

• Parallel Operating System
• scheduling algorithms in operating system
• Open Source Operating System
• Kadane's Algorithm
• Disk Management in Operating Systems
• SSTF Disc Scheduling Algortihm
• Design and Analysis of Algorithms
• Free Space Management

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