Priority scheduling in Operating Systems - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Priority Scheduling Basics

In priority scheduling, processes are assigned priorities. The CPU is allocated to the process with the highest priority first.

Which statement best describes what happens if two processes have the same priority?

A The CPU randomly selects one of the processes.

B The process with the longer burst time is scheduled first.

C The process that arrived first is scheduled first.

D Both processes run simultaneously.

💡 Hint

Think about how operating systems handle ties in priority.

When two processes have the same priority, the scheduler usually uses the arrival time to decide which process runs first, following a first-come, first-served approach.

📋 Factual

intermediate

Preemptive vs Non-preemptive Priority Scheduling

Which of the following correctly distinguishes preemptive priority scheduling from non-preemptive priority scheduling?

A In non-preemptive scheduling, processes are interrupted by higher priority processes; in preemptive, they run to completion.

B Preemptive scheduling ignores process priority when scheduling.

C Both preemptive and non-preemptive scheduling always run processes to completion without interruption.

D In preemptive scheduling, a running process can be interrupted if a higher priority process arrives; in non-preemptive, it cannot.

💡 Hint

Consider what happens when a higher priority process arrives while another is running.

Preemptive priority scheduling allows interruption of the running process if a higher priority process arrives. Non-preemptive scheduling waits for the running process to finish before switching.

🔍 Analysis

advanced

Calculating Average Waiting Time

Given three processes with arrival times and priorities:

• P1: arrival=0, burst=4, priority=2
• P2: arrival=1, burst=3, priority=1
• P3: arrival=2, burst=1, priority=3

Using preemptive priority scheduling (lower number means higher priority), what is the average waiting time?

A 2.67 units

B 2.0 units

C 1.0 units

D 0.67 units

💡 Hint

Simulate the process execution timeline considering preemption by higher priority processes.

Execution timeline:
- 0-1: P1 (1 unit)
- 1-4: P2 (3 units, preempts P1 due to higher priority)
- 4-7: P1 resumes (remaining 3 units)
- 7-8: P3 (1 unit)

Waiting times:
- P1: 3 units (waits 1-4)
- P2: 0 units
- P3: 5 units (waits 2-7)

Average waiting time = (3 + 0 + 5) / 3 = 8/3 ≈ 2.67 units.

❓ Comparison

advanced

Effect of Aging in Priority Scheduling

What is the main purpose of aging in priority scheduling?

A To assign fixed priorities based on process arrival time.

B To increase the priority of waiting processes over time to prevent starvation.

C To randomly change priorities to balance CPU usage.

D To decrease the priority of all processes to speed up execution.

💡 Hint

Think about how aging helps processes that wait too long.

Aging gradually increases the priority of processes that wait in the queue to ensure they eventually get CPU time, preventing starvation.

❓ Reasoning

expert

Identifying Starvation Scenario

Consider a priority scheduling system where priorities range from 1 (highest) to 5 (lowest). If a low priority process arrives and there is a continuous stream of higher priority processes, what will happen to the low priority process?

A It will wait indefinitely and may never get CPU time (starvation).

B It will run immediately after arrival regardless of other processes.

C It will preempt higher priority processes after a fixed time.

D It will be assigned a higher priority automatically.

💡 Hint

Consider what happens when higher priority processes keep arriving.

In priority scheduling without aging, low priority processes can be starved if higher priority processes keep arriving, preventing them from running.