Look Disk Scheduling

Look Disk Scheduling

Look disk scheduling is another type of disk scheduling algorithm. Look scheduling is an enhanced version of SCAN disk scheduling. Look disk scheduling is the same as SCAN disk scheduling, but in this scheduling, instead of going till the last track, we go till the last request and then change the direction.

Advantages of Look Disk Scheduling

The advantages of look disk scheduling are:

1. In Look disk scheduling, there is no starvation.
2. Look disk scheduling offers low variance in waiting time and response time.
3. Look disk scheduling offers better performance as compared to the SCAN disk scheduling.
4. In look disk scheduling, there is no requirement of disk head to move till the end to the disk when we do not have any request to be serviced.

 Disadvantages of Look Disk Scheduling

The disadvantages of Look disk scheduling are:

1. In look disk scheduling, there is more overhead to find the end request.
2. Look disk scheduling is not used in case of more load.

Example of Look Disk Scheduling

Consider a disk contains 200 tracks (0-100). The request queue includes track number 82, 170, 43, 140, 24, 16, 190, respectively. The current position of the read/write head is 50. The direction is towards the larger value. Calculate the total number of cylinders moved by head using look disk scheduling.

Solution:

As mentioned in the following example, disk contains 200 tracks, so we take a track line between 0 to 199.

The current position of the read/write head is 50, So we start at 50, then we move the read/write head. (the disk head is moved towards the larger value as mentioned in the given example). When all the requests are addressed, then we calculate the total number of cylinders moved by the head.

                                                               Figure: Look Disk Scheduling

 Total number of cylinders moved by the head = (190-50) + (190-16)

                                                                             = 314

C-Look Disk Scheduling

C-look means circular-look. It takes the advantages of both the disk scheduling C-SCAN, and Look disk scheduling. In C-look scheduling, the disk arm moves and service each request till the head reaches its highest request, and after that, the disk arm jumps to the lowest cylinder without servicing any request, and the disk arm moves further and service those requests which are remaining.

Advantages of C-Look Disk Scheduling

The advantages of C-look disk scheduling are:

1. There is no starvation in C-look disk scheduling.
2. The performance of the C-Look scheduling is better than Look disk scheduling.
3. C-look disk scheduling offers low variance in waiting time and response time.

Disadvantages of C-Look Disk Scheduling

The disadvantages of C-Look disk scheduling are:

1. In C-Look disk scheduling there may be more overhead to determine the end request.
2. There is more overhead in calculations.

Example of C-Look Disk Scheduling

Consider a disk containing 200 tracks (0-100). The request queue contains the track number 93, 176, 42, 148, 27, 14,183 respectively. The current position of the R/W head is 55. The direction is towards the larger value. Calculate the total number of cylinders moved by head using look disk scheduling.

Solution:

As mentioned in the following example, disk contains 200 tracks, so we take a track line between 0 to 199.

The current position of the read/write head is 55, So we start at 55, then we move the read/write head (the disk head is moved towards the larger value as mentioned in the given example). When all the requests are addressed, then we calculate the total number of cylinders moved by the head.

                                                                   Figure: C-Look Disk Scheduling  

Total number of cylinders moved by the head = (180-55) + (180-14) + (42-14)

                                                                    = 125 + 166 + 28

                                                                             = 319