Difference between LOOK and C-LOOK Disk scheduling algorithms

1. LOOK disk scheduling algorithm :
Look Algorithm is actually an improves version of SCAN Algorithm. In this algorithm, the head starts from first request at one side of disk and moves towards the other end by serving all requests in between. After reaching the last request of one end, the head reverse its direction and returns to first request, servicing all requests in between. Unlike SCAN, in this the head instead of going till last track, it goes till last request and then direction is changed.

Example –
Consider a disk with 200 tracks (0-199) and the disk queue having I/O requests in the following order as follows: 98, 183, 40, 122, 10, 124, 65. The current head position of the Read/Write head is 53 and will move in Right direction . Calculate the total number of track movements of Read/Write head using LOOK algorithm.

Total head movements,

= (65-53)+(98-65)+(122-98)
           +(124-122)+(183-124)+(183-40)+(40-10)
= 303 



2. C-LOOK disk scheduling algorithm :
C-LOOK is the modified version of both LOOK and SCAN algorithms. In this algorithm, the head starts from first request in one direction and moves towards the last request at other end, serving all request in between. After reaching last request in one end, the head jumps in other direction and move towards the remaining requests and then satisfies them in same direction as before. Unlike LOOK, it satisfies requests only in one direction.



Example –
Consider a disk with 200 tracks (0-199) and the disk queue having I/O requests in the following order as follows: 98, 183, 40, 122, 10, 124, 65. The current head position of the Read/Write head is 53 and will move in Right direction . Calculate the total number of track movements of Read/Write head using C-LOOK algorithm.

Total head movements,

= (65-53)+(98-65)+(122-98)
               +(124-122)+(183-124)+(183-10)+(40-10)
= 333 



Difference between LOOK and C-LOOK disk scheduling algorithm :

LOOK DISK SCHEDULING ALGORITHM C-LOOK SCHEDULING ALGORITHM
1. In LOOK, the head can serve the requests in both the directions. In C-LOOK algorithm, head can serves the requests only in one direction.
2. It lags in performance as compared to C-LOOK. C-LOOK algorithm has the best performance in all disk scheduling algorithms.
3. In above example of LOOK algorithm, the head moves from 53, serves all requests in right direction till it reaches the last request in one end. Then it reverses the direction and serves the remaining requests in left direction. In above example of C-LOOK algorithm, the head moves from 53, serves all requests in right direction till it reaches the last request in one end. Then it jumps to the remaining requests and serve them in right direction only.
4. Here handling of request is not so good as compared to C-LOOK algorithm. C-LOOK algorithm can handle requests more effectively than LOOK.
5. LOOK has higher throughput and provides low variance response time. C-LOOK provides uniform waiting time and response time.

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