Priority scheduling in Operating Systems - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the process with the highest priority.

Operating Systems

highest_priority_process = min(processes, key=lambda p: p[1])

Drag options to blanks, or click blank then click option'

A.burst_time

B.pid

C.arrival_time

D.priority

Attempts:

3 left

💡 Hint

Common Mistakes

Using burst time or arrival time instead of priority.

Selecting the process with the maximum priority number.

The process with the smallest priority number is selected first in priority scheduling.

2fill in blank

medium

Complete the code to check if a process is preempted by a higher priority process.

Operating Systems

if new_process.priority [1] current_process.priority:

Drag options to blanks, or click blank then click option'

A>

B==

C<

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using greater than sign instead of less than.

Checking for equality instead of priority comparison.

In priority scheduling, a process with a lower priority number preempts one with a higher number.

3fill in blank

hard

Fix the error in the code that calculates waiting time in non-preemptive priority scheduling.

Operating Systems

waiting_time = start_time - process[1]

Drag options to blanks, or click blank then click option'

Aarrival_time

Bcompletion_time

Cburst_time

Dpriority

Attempts:

3 left

💡 Hint

Common Mistakes

Using burst time or completion time instead of arrival time.

Subtracting in the wrong order causing negative waiting time.

Waiting time is the difference between the start time and the arrival time of the process.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps process IDs to their priorities for processes with priority less than 5.

Operating Systems

{p[1]: p[2] for p in processes if p.priority < 5}

Drag options to blanks, or click blank then click option'

A.pid

B.priority

C.arrival_time

D.burst_time

Attempts:

3 left

💡 Hint

Common Mistakes

Using burst time or arrival time instead of priority as values.

Using priority as keys instead of process IDs.

This comprehension creates a dictionary with process IDs as keys and their priorities as values, filtering for priorities less than 5.

5fill in blank

hard

Fill all three blanks to create a dictionary of processes with their waiting times, only for processes with waiting time greater than 0.

Operating Systems

waiting_times = {p[1]: p[2] - p[3] for p in processes if p[2] - p[3] > 0}

Drag options to blanks, or click blank then click option'

A.pid

B.start_time

C.arrival_time

D.priority

Attempts:

3 left

💡 Hint

Common Mistakes

Using priority instead of arrival time or start time.

Including processes with zero or negative waiting time.

This dictionary comprehension maps process IDs to their waiting times, calculated as start time minus arrival time, only including positive waiting times.

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)