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.

Practice

1. What does priority scheduling in operating systems primarily use to decide which process runs first?

easy

A. The time each process has already run

B. The importance level assigned to each process

C. The size of the process in memory

D. The order in which processes arrive

Solution

1. Step 1: Understand priority scheduling basics

   Priority scheduling chooses processes based on their assigned importance or priority level.

2. Step 2: Compare with other scheduling criteria

   Unlike first-come-first-served or shortest job first, priority scheduling uses priority, not arrival time or size.

3. Final Answer:

   The importance level assigned to each process -> Option B

4. Quick Check:

   Priority scheduling = importance level [OK]

Hint: Remember: priority means importance, not arrival time [OK]

Common Mistakes:

• Confusing priority with arrival order
• Thinking process size affects scheduling
• Assuming time already run decides priority

2. Which of the following is the correct way to describe a preemptive priority scheduling system?

easy

A. A system that ignores priority and runs processes randomly

B. A system where processes run to completion without interruption

C. A system that schedules processes based on their arrival time only

D. A system where a running process can be interrupted if a higher priority process arrives

Solution

1. Step 1: Define preemptive scheduling

   Preemptive scheduling allows interruption of a running process if a more important one arrives.

2. Step 2: Match with priority scheduling

   In priority scheduling, preemptive means higher priority processes can interrupt lower priority ones.

3. Final Answer:

   A system where a running process can be interrupted if a higher priority process arrives -> Option D

4. Quick Check:

   Preemptive priority = interrupt for higher priority [OK]

Hint: Preemptive means interrupt if higher priority comes [OK]

Common Mistakes:

• Confusing preemptive with non-preemptive
• Thinking processes always run to completion
• Ignoring priority in scheduling decisions

3. Consider three processes with priorities: P1=2, P2=1, P3=3 (1 is highest priority). If all arrive at the same time, which order will they be scheduled in a non-preemptive priority scheduling?

medium

A. P3, P1, P2

B. P1, P2, P3

C. P2, P1, P3

D. P1, P3, P2

Solution

1. Step 1: Identify priority order

   Priority 1 is highest, so P2 (priority 1) runs first, then P1 (2), then P3 (3).

2. Step 2: Apply non-preemptive scheduling

   Since all arrive together, processes run fully in priority order without interruption.

3. Final Answer:

   P2, P1, P3 -> Option C

4. Quick Check:

   Lower number = higher priority, run in that order [OK]

Hint: Lower priority number runs first in non-preemptive [OK]

Common Mistakes:

• Mixing priority numbers with arrival order
• Assuming preemption changes order here
• Confusing priority 1 as lowest priority

4. A priority scheduling system is implemented but processes with lower priority sometimes run before higher priority ones. What is the most likely cause?

medium

A. The system is using non-preemptive scheduling and a low priority process started first

B. The priority numbers are assigned incorrectly with higher numbers meaning higher priority

C. The system is ignoring arrival times completely

D. The CPU is idle and no processes are running

Solution

1. Step 1: Understand non-preemptive behavior

   In non-preemptive priority scheduling, once a process starts, it runs to completion even if a higher priority process arrives later.

2. Step 2: Explain why lower priority runs first

   If a low priority process starts first, it will finish before the higher priority process can run.

3. Final Answer:

   The system is using non-preemptive scheduling and a low priority process started first -> Option A

4. Quick Check:

   Non-preemptive lets running process finish first [OK]

Hint: Non-preemptive means no interruption once started [OK]

Common Mistakes:

• Assuming priority numbers are reversed
• Ignoring scheduling type (preemptive vs non-preemptive)
• Thinking arrival time is always ignored

5. You have four processes arriving at different times with priorities: P1(2, arrives at 0s), P2(1, arrives at 1s), P3(3, arrives at 2s), P4(1, arrives at 3s). Using preemptive priority scheduling, which process runs at time 3 seconds?

hard

A. P2

B. P4

C. P1

D. P3

Solution

1. Step 1: Track process arrivals and priorities

   At 0s: P1(2) starts. At 1s: P2(1) arrives and preempts P1 since higher priority (lower number). At 2s: P3(3) arrives, lower priority than P2(1), so P2 continues. At 3s: P4(1) arrives, same priority as running P2.

2. Step 2: Determine which process runs at 3s

   In preemptive priority scheduling, preemption occurs only if a strictly higher priority process arrives. Since P4 has the same priority (1) as P2, P2 is not preempted and continues running at time 3s.

3. Final Answer:

   P2 -> Option A

4. Quick Check:

   Preempt only for higher priority; same priority continues [OK]

Hint: Preempt only if strictly higher priority [OK]

Common Mistakes:

• Thinking same priority causes preemption
• Assuming new arrivals always preempt
• Confusing tie-breaker rules (usually FCFS for same priority)