Round Robin MCQ Quiz - Objective Question with Answer for Round Robin - Download Free PDF

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

Key Points

Scheduling Algorithm: First Come First Serve (FCFS)

Gantt chart:

0 5 8 10 13 14

Process Table:

Average Turnaround Time: \({{5+6+8+8+8} \over 5} = 7.0\)

 Important Points:

TAT = CT - AT

The correct answer is option 2.

CONCEPT:

Highest Response Ratio Next (HRN):

HRN is a non-preemptive scheduling algorithm where the process with the highest response ratio is selected next. The response ratio is calculated as:

Response Ratio = (Waiting Time + Burst Time) / Burst Time

Formula:

Turnaround Time (TAT) = Completion Time (CT) - Arrival Time (AT)

Weighted TAT = Turnaround Time / Burst Time

Steps:

1. Select the process with the highest response ratio among all arrived processes.
2. Compute completion time, TAT, and weighted TAT.
3. Repeat until all processes are scheduled.

Calculated weighted turnaround times:

Average Weighted Turnaround Time = \({{1+ 2.5+2.67+2.67+8} \over 5 } = 2.834\)

∴ Hence, the correct answer is 2.834 ms

• 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.

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.

Concepts:

• First come first serve automatically executes queued requests and processes in order of their arrival.
•  Round robin: In RR, each ready task runs turn by turn only in a cyclic queue for a limited time slice.
• Shortest –job-first: In which the process having the smallest execution time present in ready queue is chosen for the next execution.
• Shortest-remaining-time-first: In this scheduling algorithm, the process with the smallest amount of time remaining until completion is selected to execute next.

Data:

The given scheduling algorithm assigns priority proportional to the waiting time of a process and every process starts with priority zero (the lowest priority). The scheduler re-evaluates the process priorities every T.

Explanation

With options elimination method

• The scheduler re-evaluates the process priorities every T time units and decides the next process to schedule means here the algorithm using- pre-emptive approach. By this analogy we can eliminate the option 1(FCFS) and 3(SJF).
• This means there must be a time quantum T exists without time quantum scheduler cannot re-evaluates the process priorities and the scheduling algorithm assigns priority proportional to the waiting time of a process instead the less service time.
• From this it is quite clear that the algorithm cannot be Shortest-remaining-time-first and it will equivalent to Round-robin algorithms. 

FCFS:

Gantt chart

0                   5                   12                  15

Round – robin with time quantum = 2:

Gantt chart

0        2          4          6         8         10        11       12       14      15

The correct answer is "option 3".

CONCEPT:

The circular queue is best known for the Round Robin CPU scheduling algorithm.

A circular Queue is a linear data structure based on the FIFO principle and the last position of the queue is connected back to the first position.

In Round Robin scheduling, when the timer goes off the process then swapped out & goes to the end of the ready queue.

Hence, the correct answer is "option 3".

Circular Queue

 Additional Information

1. Queue is a linear data structure used to store data on the basis of the FIFO (First In First Out) principle.

2. Stack is a linear data structure based on the LIFO (Last In First Out) principle.

3. A tree is a non-linear data structure that store data in hierarchical form.

4. LIFO means the data entered last in the data structure will be removed first. 

5. FIFO means the data entered first in the data structure will be removed first. 

Turnaround time = completion time – arrival time

Since arrival time = 0

∴ Turnaround time = completion time

Average Turnaround time = (sum of completion time) ÷ (number of process)

Gantt chart for SJF

0              2                6                13          21

\({\rm{Average\;turnaround\;time}} = \frac{{2 + 6 + 13 + 21}}{4} = 10.5\)

Gantt chart for RR with time quantum = 4

0              4                8                10          14          18          21

\({\rm{Average\;turnaround\;time}} = \frac{{21 + 18 + 14 + {\rm{\;}}10}}{4} = \frac{{63}}{4} = 15.75\)

Concept:

Time quantum is defined in round robin scheduling algorithm.

Explanation:

• The period of time for which a process is allowed to run in a pre-emptive multitasking system is generally called the time slice or quantum.
• Each process is assigned a fixed time (Time Quantum/Time Slice) in cyclic way. It is designed especially for the time-sharing system. 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.
• If time quantum for Round Robin scheduling is very large, then it behaves same as FCFS scheduling.

The correct answer is varies irregularly.

Key Points

• The Round-Robin is known as a CPU scheduling algorithm.
• The Round-Robin method mainly used the time-sharing method. The time frame for which a method or task is permitted to run in a pre-emptive technique is known as time quantum.
• In Round Robin CPU scheduling, as the time quantum is increased, the average turn-around time varies irregularly.

Thus the correct answer is varies irregularly.

Additional Information

• The main characteristics of the Round-Robin method are-
  • It is very easy to implement in the system.
  • In this process, time is shared equally for each process.
• The main advantage of the Round Robin Scheduling is time slicing. If any new process is created, then it is always added at the end of the queue.
• The main disadvantage of the Round Robin Scheduling is waiting time is more than the response time. It has very low throughput.

Concept:

Round - robin scheduling uses time slices for each process. Each process will be assigned a time quantum to execute. If time quantum is too large, then it will behave as FCFS.

Explanation:

Given time quantum = 2 units

There is only one job that requires 14-time units to execute completely.

So, the job queue will be :

0     2     4      6     8     10   12    14

Context switches at 2, 4, 6, 8, 10, 12

In this, there is a total of 6 context switches, as first and last will not be the context switch.

Concept:

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.

Time quantum = 5 units

Gantt Chart:

0       5        10     15       20      25      30      35     40       45      50      55      60      65

Calculation

Average turn around time = \(\frac{50 + 35 + 65 + 20 + 60}{5} = 46\)