Test: Disk Scheduling Algorithm - Computer Science Engineering (CSE) MCQ

The given data,

"towards the larger number
" means it works like a scan algorithm.
Let's say there are seven requests: 16, 24, 43, 82, 140, 170, and 190.
The read/write arm is at 50.

The seek time is calculated as:
=(199-50)+(199-16)
=332

Hence the correct answer is 332.

Algorithm - CSCAN
Requests - 5, 40, 75, 195, 145, 203, 106, 122
Current Cylinder - (208 - 57 + 1) = 151

• Here, we start from the current cylinder, that is, 151 because as the question states, we are starting from the 57th cylinder from the largest cylinder. So, as per the chart, 57th cylinder from the right, that is, 151.
• The direction is from largest cylinder to innermost cylinder, that is, right to left.
• Now R/W head will move from 151 to 150, 150 to 149 and so on till it reaches 145. It reaches there and access the data. Total seek time till now can be calculated by: (151 - 145) * 1 = 6 msec. Similarly, R/W head will move from 145 to 122, 122 to 106 and so on till 0.
• Now when the R/W head reaches 0, it will jump to 208 and then continue moving in the same direction. Now, this jumping will take 9 msec. After that, it will move as is mentioned in (step - 3).

Calculation - 
{[(151 - 145) + (145 - 122) + (122 - 106) + (106 - 75) + (75 - 40) + (40 - 5) + (5 - 0)] * 1} + {(208 - 0) * 9} + {[(208 - 203) + (203 - 195)] * 1} = 203

Option 1: 
The elevator algorithm is also known as the SCAN algorithm. 

In this algorithm, the disk moves in a particular direction servicing the requests coming in the way till the end & reverse the direction & service all the requests.

Option 2: 
In Shortest Seek Time First(SSTF), the algorithm selects disk I/O which requires the least disk arm movement from the current position

Option 3: 
In C-scan i.e. Circular Elevator, the disk moves in a particular direction servicing the requests coming in the way till the end & reverse the direction & again goes to the start point & starts servicing the requests once reach the start point. 

Option 4:
In First Come First Serve(FCFS), the disk service the request that comes first.
Hence, the correct answer is "option 3".

Shortest Seek Time First (SSTF)

SSTF =(22-20+22-10+10-6+6-2+38-2+40-38)
          =2+12+4+4+36+2
          =60

It takes 2 milliseconds to move from one cylinder to adjacent one = 60x2  =120 milliseconds
∴ Hence the correct answer is 120 milliseconds.

Stack algorithm:
A page replacement algorithm is said to satisfy the inclusion property, or it is called as stack algorithm if the set of pages in a k frame memory is always a subset of the pages in a k+1 frame memory.

Elevator algorithm:
It is a disk scheduling algorithm, also known as scan algorithm in which head, starts from one end of disk and moves towards other end by servicing each request in between and reach other end.

Priority scheduling algorithm:
It is a CPU scheduling algorithm in which each process is assigned a priority and processes according to their priority.

Havender’s Algorithm:
It is related with deadlock. In this, all the resources required by process will be requested at once. 

Total head movements in SSTF= (86−80)+(86−72)+(134−72)+(134−16)=200
Power dissipated in moving 100 cylinder = 20mW
Power dissipated by 200 movements = 0.2∗200=40mW = P
Power dissipated in reversing head direction once= 15mW
Number of times head changes its direction= 3
Power dissipated in reversing head direction = 3∗15= 45mW = Q
​​​​​​​Total Power Consumption is P + Q = 85mW

Data:
Seek time = t = 6 ms per cylinder
Head = 20

Concept:
Since, it is a first-come-first-serve scheduling requests will be served in the given sequence:
10, 22, 20, 2, 40, 6 and 38

Formula:
Total Seek time = total head movement × t

Calculation:
Total head movement = (20 - 10) + (22 - 10) + (22 - 2) + (40 - 2) + (40 - 6) + (38 - 6) =
= 10 + 12 + 20 + 38 + 34 + 32 = 146
Total Seek time = 146 x 6 = 876 ms.

Magnetic Disks:

Magnetic Disks provide the bulk of secondary storage for modern computer systems. Conceptually, disks are relatively simple. Each disk platter has a flat circular shape, like a CD. The two surfaces of a platter are covered with magnetic material. We store information by recording it magnetically on the platters.

A read-write head "flies" just above each surface of every platter. The heads are attached to a disk arm that moves all the heads as a unit. The surface of a platter is logically divided into circular tracks which are subdivided into sectors.

The given data,
The process track request are= 98, 183, 37, 122, 14, 124, 14, 124, 65, 67 
The initial seek = 53
C-Look algorithm:

Total number of seeks= ( 183-53 )+(183-14) +(37-14)
Total number of seeks= 322
Hence the correct answer is 322.

Given disk queue requests 93, 173, 132, 20, 35, 150, 100, 175, 55

Total head movements = 10 + 20 + 15 + 73 + 7 + 32 + 18 + 23 + 2 
Total head movements = 200 cylinders
Hence the correct answer is option 2 and option 3.