CPU Scheduling MCQ Quiz - Objective Question with Answer for CPU Scheduling - Download Free PDF

The correct option is (3)

Concept:-

Microcomputer:- Microcomputer was formerly a commonly used term for personal computers, whose CPU is contained on a single integrated semiconductor chip. Thus, a microcomputer uses a single microprocessor for its CPU, which performs all logic and arithmetic operations.

Key Points

• Microcomputers have been used for applications including data and word processing, electronic spreadsheets, professional presentation and graphic programs, communications, and DBMS.
• Modern desktop computers, video game consoles, laptops, tablet PCs, and many types of handheld devices, including mobile phones, and pocket calculators, may all be examples of microcomputers.
• A microchip (also called a chip, a computer chip, an integrated circuit, or IC) is a set of electronic circuits on a small flat piece of silicon.

Additional InformationMainframe computer:- A mainframe computer is a large integrated machine with a lot of memory, a lot of storage capacity, and many high-end processors. The mainframe is critical to commercial databases, transaction servers, and applications that require high resiliency, security and agility.

Minicomputers:- Minicomputers are small computers that were introduced in 1960 and are used for various business enterprises and scientific applications work. A minicomputer may also be called a mid-range computer.

Supercomputer:- Supercomputer, any of a class of extremely powerful computers. Such computers have been used primarily for scientific and engineering work requiring exceedingly high-speed computations.

Concept:

Scan Algorithm

It is also called as Elevator Algorithm. In this algorithm, the disk arm moves into a particular direction till the end, satisfying all the requests coming in its path,and then it turns backand moves in the reverse direction satisfying requests coming in its path.

Advantages 

1. This algorithm is simple and easy to understand.
2. SCAN algorithm have no starvation.
3. This algorithm is better than FCFS Scheduling algorithm .

Disadvantages 

1. More complex algorithm to implement.
2. This algorithm is not fair because it cause long waiting time for the cylinders just visited by the head.

Hence Option 2 is correct

The correct answer is Option 1) Branch and bound.

Key Points

• Branch and Bound is an algorithm design technique used to solve optimization problems like the Travelling Salesman Problem, Knapsack Problem, etc. It is not a CPU scheduling algorithm.
• CPU scheduling algorithms determine the order in which processes are executed by the processor, especially in a multitasking environment.
• Some commonly used CPU scheduling algorithms include:
  • First Come, First Served (FCFS): Executes processes in the order they arrive.
  • Shortest Job Next (SJN): Picks the job with the shortest execution time next.
  • Round Robin (RR): Assigns a fixed time slice to each process in a cyclic order.

Additional Information

• CPU Scheduling is essential for managing process execution in multi-tasking systems, improving CPU utilization, and minimizing waiting and response time.
• Other advanced CPU scheduling algorithms include:
  • Priority Scheduling
  • Multilevel Queue Scheduling
  • Multilevel Feedback Queue Scheduling

Conclusion: Branch and Bound is not used in CPU scheduling; it belongs to the category of problem-solving algorithms in operations research and computer science.

The correct answer is Option 3) 520.

Key Points

• First Come First Served (FCFS) Disk Scheduling Algorithm serves the requests in the order they arrive.
• Given:
  • Initial head position = 40
  • Request queue = (90, 150, 50, 140, 30, 20)
• We calculate the total distance moved by the disk head step-by-step:

Additional Information

• FCFS Characteristics:
  • Simple to implement
  • Processes requests in the order of arrival
  • Does not optimize seek time
• Other disk scheduling algorithms:
  • SSTF (Shortest Seek Time First): Chooses the request closest to current head position
  • SCAN: Moves head in one direction, servicing requests in order until end is reached, then reverses
  • LOOK: Like SCAN, but stops at last request instead of going to the end of disk

Concept:

In the priority scheduling algorithm, a priority is associated with each process, and CPU is allocated to the process with the highest priority. 

Explanation:

Priority scheduling can be either preemptive or non-preemptive. A major problem with the priority scheduling algorithm is indefinite blocking or starvation. This algorithm can leave some low priority processes waiting indefinitely. 

The solution to the problem of starvation is aging. Aging is a technique of gradually increasing the priority of processes that wait in the system for a long time. 

Example: if priorities range from 100 to 0(high), we could increase the priority of a waiting process by 1 every 15 minutes. Due to this, even a process with 100 priority would have the highest priority.

 Correct Explanation for FCFS and RR2 Scheduling

🔹 Given:

🔸 FCFS (First Come First Serve)

• P1 arrives at time 0 → executed first → finishes at time 5
• P2 arrives at time 1 → executed next → finishes at time 5 + 7 = 12
• P3 arrives at time 3 → executed last → finishes at time 12 + 4 = 16

✅ FCFS Completion Order: P1, P2, P3

🔸 RR2 (Round Robin with Time Quantum = 2)

• Time 0–2: P1 executes → remaining = 3
• Time 2–4: P2 executes → remaining = 5
• Time 4–6: P3 executes → remaining = 2
• Time 6–8: P1 executes → remaining = 1
• Time 8–10: P2 executes → remaining = 3
• Time 10–12: P3 executes → completes ✅
• Time 12–13: P1 executes → completes ✅
• Time 13–15: P2 executes → remaining = 1
• Time 15–16: P2 executes → completes ✅

✅ RR2 Completion Order: P3, P1, P2

📌 Summary

✔️ Final Answer: FCFS: P1, P2, P3    RR2: P3, P1, P2

• The round-robin (RR) scheduling algorithm is designed especially for timesharing systems.
• It is similar to FCFS scheduling, but preemption is added to enable the system to switch between processes.
• A small unit of time, called a time quantum or time slice, is defined.
• The ready queue is treated as a circular queue. The CPU scheduler goes around the ready queue, allocating the CPU to each process for a time interval of up to 1-time​ quantum.

The correct answer is "option 3".

EXPLANATION:

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".

Pre-emptive is the most suitable scheduling scheme for the real-time operating system 

Key Points

• 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.

Additional Information

• The round-robin (RR) scheduling algorithm is designed especially for timesharing systems.
• It is similar to FCFS scheduling, but preemption is added to enable the system to switch between processes.
• A small unit of time, called a time quantum or time slice, is defined.
• The ready queue is treated as a circular queue. The CPU scheduler goes around the ready queue, allocating the CPU to each process for a time interval of up to 1-time​ quantum.

Concept:

In the priority scheduling algorithm, a priority is associated with each process, and CPU is allocated to the process with the highest priority. 

Explanation:

Priority scheduling can be either preemptive or non-preemptive. A major problem with the priority scheduling algorithm is indefinite blocking or starvation. This algorithm can leave some low priority processes waiting indefinitely. 

The solution to the problem of starvation is aging. Aging is a technique of gradually increasing the priority of processes that wait in the system for a long time. 

Example: if priorities range from 100 to 0(high), we could increase the priority of a waiting process by 1 every 15 minutes. Due to this, even a process with 100 priority would have the highest priority.

The number of processes that are completed by a system per time unit, is called throughput.

Explanation:-

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.

Key Points

• The rate at which an amount of work or data can be processed is called throughput. It is a measure of the comparative effectiveness of a process or an operation.
• Throughput is the amount of packet per second or MBS per second that a device can process.
• The amount of material or items passing through a system or process
• Throughput is a measure of how many units of information a system can process in a given amount of time.

Additional InformationProcess output:- The outputs are the data and materials flowing out of the transformation process. The inputs represent the flow of data and materials into the process from the outside.

Turnaround time:- It is the amount of time taken to complete a process or fulfill a request. In another word, the total time taken between the submission of a program for execution and the return of the complete output to the customer.

Response time:- The response time is the total elapsed time from when a request is made to the time it is completed.

Concept:

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.

Explanation

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

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. 

Concepts:

Shortest remaining time first (SRTF), is a scheduling method that is a pre-emptive version of shortest job first (SJF).

In this scheduling algorithm, the process with the smallest amount of time remaining until completion is selected to execute.

Explanation:

At t = 0:

Only Process P₁ is available and hence P₁ is executed for 1 ms.

At t = 1:

Process P₁ and Process P₂ are available, between them P₂ has least burst time. Therefore, P₁ gets pre-empted and hence P₂ is executed for 1 ms.

At t = 2:

All three process are available. Since P₂ is having least burst time it executes till its completion followed by Process P₁ and process P₃ respectively

Gantt chart:

Preemptive SJF scheduling is equal to SRTF

The correct answer is option 3.

Key Points

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.