Test: CPU Scheduling - Computer Science Engineering (CSE) MCQ

Semaphores: It is an integer variable which is used by various processes in mutually exclusive manner to achieve synchronisation. There are two types of semaphores -

• Binary Semaphore
• Counting Semaphore

There are two operations which are atomic in nature -

• Block: it suspends the process for which it is invoked
• wakeup(P): it resumes the execution of a blocked process P.
  • If there are two or more processes present in the suspended list, and there are no other processes present to wake these up, then these processes are said to be in deadlock.
  • If there are two or more processes present in the suspended list, then the selection of the process will be done on the basis of FCFS (First Come First Serve) for the wakeup()

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".

In Shortest Seek Time First (SSTF) scheduling, after servicing every I/O request, the disk head finds the nearest block to be serviced and moves to that particular block.
For Example, after servicing block 53, the nearest block request is at 65 and moves to block 65. Similarly all the above requests are serviced as shown in the figure below

Total number of head movements = (67-53) + (67-14) + (183-14) = 236

• Preemptive Scheduling is suitable for Real-Time Programming, as it will allow a real-time process to preempt a process currently running in the Kernel.
• A preemptive Kernel may be more responsive since there is less risk that a Kernel-mode process will run for arbitrarily long periods before relinquishing the processor to the waiting process.
• Real-time systems require that results be produced within a specified deadline. One of the key features of a Real-time system is its ability to respond to real-time processes as soon as the process requires CPU.
• As a result, the scheduler for the real-time system must support a priority-based algorithm with preemption.

Shortest job first scheduling:
In this job scheduling algorithm, the process having the shortest burst time or runtime will be executed first. It can be preemptive or non - preemptive. It reduces the waiting time for other processes. It results in maximum throughput.

Round-robin scheduling:
In this, each process has a time quantum associated with it. Each process gets an equal share of time to execute. It is a preemptive scheduling algorithm.

FCFS:
FCFS is a first come first serve scheduling algorithm. The process which comes first in the queue will be executed first. Processes are executed in the order of their arrival. It is the simplest scheduling algorithm.

Priority scheduling:
It is a non-preemptive scheduling algorithm. Each process is assigned a priority and they are executed in the order of their priority. Process having the highest priority will be executed first and so on. Priority is decided on the basis of memory requirements, time, and resources.
Therefore Shortest job first scheduling (SJF) will result in the highest throughput.

As we shall see when we consider the various scheduling algorithms, a ready queue can be implemented as a FIFO queue, a priority queue, a tree, or simply an unordered linked list. Conceptually, however, all the processes in the ready queue are lined up waiting for a chance to run on the CPU. The records in the queues are generally process control blocks (PCBs) of the processes

Another component involved in the CPU-scheduling function is the dispatcher. This function involves the following

• Switching context
• Switching to user mode
• Jumping to the proper location in the user program to restart that program

Concept:
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.

Concept:
C-Look algorithm:
An enhanced variant of the LOOK Algorithm is the Circular-LOOK Algorithm. The head goes across the disc from the first request at one end to the last request at the other, serving all the requests in between. The Head turns around when it gets to the final request on the other end.
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.

Throughput:- It is the number of units that can be produced by a production process within a certain period of time. For example, if 800 units can be produced during an eight-hour shift, then the production process generates a throughput of 100 units per hour.
Throughput = total good units produced / time.