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

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.

Key Points

• Scheduling emails helps in better time management as it allows you to plan and send emails at a specific time, ensuring you don't forget important communications.
• It enables you to reach your audience at the optimal time, increasing the likelihood that your email will be read and responded to.
• By scheduling emails, you can ensure consistent communication even when you are busy or out of office.
• It helps in managing your workflow more efficiently and avoids the need for real-time email handling, thus freeing up time for other tasks.

Additional Information

• Time management
  • It is the process of planning and controlling how much time to spend on specific activities.
  • Good time management enables an individual to complete more tasks in a shorter period, lowers stress, and leads to career success.
  • It involves tools and techniques such as setting goals, prioritizing tasks, and planning.
  • Effective time management helps individuals and organizations achieve their objectives efficiently.
• Increased response time
  • It refers to the period between sending an email and receiving a reply.
  • Strategically scheduling emails can sometimes improve response rates but is not a guaranteed benefit.
• Immediate delivery
  • This refers to the instant transmission of emails upon clicking the send button.
  • Scheduling emails is about delaying the send time to a future point rather than immediate delivery.
• Reduced inbox clutter
  • This involves techniques to minimize the number of emails in your inbox.
  • Scheduling emails does not directly reduce inbox clutter; instead, email organization techniques such as filtering and archiving are more relevant.

Scheduling Algorithm: Non-preemptive Priority Scheduling (Low number = High Priority)

Gantt chart:

0    2    7    8    12    15

Process Table:

Average response time = ​​​​​\(\frac{{12 + 7 + 2 + 8 + 0}}{5} = 5.8\) ms

Answer: The corrected average response time is 5.8 ms.

Note: The Priority column is essential to justify the scheduling order. The correct formula for response time is Response Time = Start Time − Arrival Time.

The correct answer is Highest Response Ratio Next.

Key Points

• Shortest Job First (SJF) scheduling algorithm selects the process with the smallest execution time. This can lead to issues such as starvation, where longer processes keep getting delayed by shorter processes.
• The Highest Response Ratio Next (HRRN) algorithm addresses the starvation problem by considering the waiting time of the processes. It calculates the response ratio for each process using the formula:
  • Response Ratio = (Waiting Time + Burst Time) / Burst Time
• The process with the highest response ratio is selected next for execution. This ensures that even processes with longer burst times get a chance to execute after waiting for a sufficient period.
• By considering both the waiting time and the burst time, HRRN balances the needs of both short and long processes, mitigating the risk of starvation.

Additional Information

• The HRRN algorithm dynamically adjusts the priority of processes based on their waiting time, making it more flexible and fair compared to static scheduling algorithms.
• This algorithm is particularly useful in systems where processes have varying burst times, as it ensures a more equitable distribution of CPU time.
• Other scheduling algorithms like Round Robin and Priority Scheduling also aim to address different aspects of process scheduling, but HRRN specifically targets the starvation problem in SJF.
• HRRN is a non-preemptive scheduling algorithm, meaning once a process starts executing, it runs to completion before the next process is selected.

Scheduling Algorithm: Non-preemptive Shortest Job First (SJF)

Gantt chart:

0       5       7       8       11       14

Process Table:

Average waiting time = \(\frac{{0 + 3 + 1 + 6 + 6}}{5} = 3.2\)

Important Points:

WT = TAT - BT

The correct answer is "option 3".

CONCEPT:

FCFS is a scheduling algorithm in which the process that scheduled first will execute first completely.

FCFS stands for First Come First Serve.

Round Robin is a scheduling algorithm in which the process executes for a fixed CPU quantum time then the next process gets executed then the next process and goes on.

CALCULATION:

The GANTT chart for the FCFS scheduling algorithm is:

The completion order for the FCFS scheduling algorithm is: P1 P2 P3

The GANTT chart for the RR scheduling algorithm is:

The completion order for the RR scheduling algorithm is: P1 P3 P2

Hence, the correct answer is "option 3".

• 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.