Process Scheduling MCQ Quiz - Objective Question with Answer for Process 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.

The correct answer is: option 1: 6.5 ms

Concept:

We are using the Preemptive Shortest Job First (SJF) scheduling algorithm, also known as Shortest Remaining Time First (SRTF). In this approach, the process with the shortest remaining burst time is scheduled next, even if it means preempting the currently running process.

Given process data:

Step-by-step execution using Gantt Chart (Preemptive SJF):

Timeline:

• 0–1: P0 runs (remaining 7)
• 1–5: P1 arrives (shortest), so preempts P0 and finishes
• 5–10: P3 (shorter than P0 & P2)
• 10–17: P0 resumes (remaining 7) and finishes
• 17–26: P2 runs and finishes

Completion Times:

• P0: CT = 17
• P1: CT = 5
• P2: CT = 26
• P3: CT = 10

Turnaround Time (TAT) = Completion Time – Arrival Time

• P0: 17 – 0 = 17
• P1: 5 – 1 = 4
• P2: 26 – 2 = 24
• P3: 10 – 3 = 7

Waiting Time (WT) = TAT – Burst Time

• P0: 17 – 8 = 9
• P1: 4 – 4 = 0
• P2: 24 – 9 = 15
• P3: 7 – 5 = 2

Total Waiting Time = 9 + 0 + 15 + 2 = 26

Average Waiting Time = 26 / 4 = 6.5

Hence, the correct answer should be: option 2: 6.5 ms

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.

 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.

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.

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

Linux operating system supports both preemptive priority scheduling and time sharing CPU scheduling.

Linux scheduler is a priority preemptive scheduler with two priority ranges – Real time from 0 to 99 and a nice range from 100 to 140.

Explanation:

Preemptive priority scheduling: It maintains two array: active array and expired array.  Schedule the processes based on their static and dynamic priorities.

1)  It finds the highest priority queue with a runnable process and find the first process on that queue. Calculate its time quantum and then run. After time quantum, it put that process on expire array list and repeat the process again.

2) When priority level of active queue is empty, it looks for next highest queue, after running all active queues, it swaps active and expired queues.

Diagram:

The correct answer is option 2.

Concept: 

Priority Scheduling:

 Priority Scheduling is a method of scheduling processes that are based on priority. In this algorithm, the scheduler selects the tasks to work as per the priority.

Consider arrival time= 0

The Gantt chart:

TAT(Turnaround time)= CT-AT

Waiting time= TAT-BT

Average waiting time= $\frac{(6+0+16+18+1)}{5}=\frac{41}{5}=8.2ms$  

Hence the correct answer is 8.2 ms.

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. 

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.

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