Round Robin scheduling in Operating Systems - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the time quantum used in Round Robin scheduling.

Operating Systems

time_quantum = [1]

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

A10

B20

C5

D15

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a very large number as time quantum, which defeats the purpose of Round Robin.

Using zero or negative values.

The time quantum is the fixed time slice each process gets in Round Robin scheduling. Here, it is set to 5 units.

2fill in blank

medium

Complete the code to select the next process in the Round Robin queue.

Operating Systems

next_process = ready_queue.[1](0)

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

Aremove

Bappend

Cinsert

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using append() which adds to the list instead of removing.

Using remove() without specifying the element.

In Round Robin, the next process is taken from the front of the ready queue using pop() (usually pop(0)), which removes and returns the first process.

3fill in blank

hard

Fix the error in the code to add the current process back to the end of the queue after its time slice.

Operating Systems

ready_queue.[1](current_process)

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

Aappend

Binsert

Cremove

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which removes elements.

Using remove() which removes by value.

After a process uses its time slice, it is added back to the end of the ready queue using append().

4fill in blank

hard

Fill both blanks to check if the process has finished and either remove it or add it back to the queue.

Operating Systems

if current_process.[1] == 0:
    [2]  # Process finished, do not add back

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

Aremaining_time

Bpass

Cappend(current_process)

Dremove(current_process)

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to append the finished process back to the queue.

Using incorrect attribute names.

We check if the process's remaining time is zero. If yes, we do nothing (pass) because the process is finished and should not be added back.

5fill in blank

hard

Fill all three blanks to update the remaining time, add unfinished process back, and move to the next process.

Operating Systems

current_process.[1] -= [2]
if current_process.[3] > 0:
    ready_queue.append(current_process)

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

Aremaining_time

Btime_quantum

Dtotal_time

Attempts:

3 left

💡 Hint

Common Mistakes

Using total_time instead of remaining_time.

Not subtracting the time quantum correctly.

The process's remaining time is reduced by the time quantum. If it still has time left, it is added back to the queue.

Practice

1. What is the main idea behind Round Robin scheduling in operating systems?

easy

A. The shortest job runs first until completion.

B. Processes are run based on their priority levels.

C. Each process gets an equal fixed time slice to run in turns.

D. Processes run only when they request CPU time.

Solution

1. Step 1: Understand Round Robin scheduling basics

   Round Robin scheduling assigns each process a fixed time slice called a quantum, and processes run in a cyclic order.

2. Step 2: Compare options with the definition

   Only "Each process gets an equal fixed time slice to run in turns." correctly describes this fixed time slice and cyclic turn-taking approach.

3. Final Answer:

   Each process gets an equal fixed time slice to run in turns. -> Option C

4. Quick Check:

   Round Robin = fixed time slice per process [OK]

Hint: Round Robin means equal time slices in a cycle [OK]

Common Mistakes:

• Confusing Round Robin with priority scheduling
• Thinking shortest job runs first
• Assuming processes run only on request

2. Which of the following is the correct way to represent the time quantum in Round Robin scheduling?

easy

A. A fixed time interval each process runs before switching.

B. The total time a process needs to complete.

C. The priority level assigned to a process.

D. The time a process waits before starting.

Solution

1. Step 1: Define time quantum in Round Robin

   The time quantum is the fixed time interval given to each process to run before the CPU switches to the next process.

2. Step 2: Eliminate incorrect options

   Options B, C, and D describe other concepts like total burst time, priority, and waiting time, not the time quantum.

3. Final Answer:

   A fixed time interval each process runs before switching. -> Option A

4. Quick Check:

   Time quantum = fixed run time per process [OK]

Hint: Time quantum is the fixed run time slice [OK]

Common Mistakes:

• Mixing time quantum with total process time
• Confusing quantum with priority
• Thinking quantum is waiting time

3. Consider three processes P1, P2, and P3 with burst times 5, 3, and 8 units respectively. Using Round Robin scheduling with a time quantum of 3 units, what is the order of process execution in the first two cycles?

medium

A. P1, P3, P2, P1, P2, P3

B. P3, P1, P2, P3, P1, P2

C. P2, P1, P3, P2, P1, P3

D. P1, P2, P3, P1, P3, P3

Solution

1. Step 1: Calculate first cycle execution

   Each process runs for 3 units or less if burst time is less. P1 runs 3 (remaining 2), P2 runs 3 (done), P3 runs 3 (remaining 5).

2. Step 2: Calculate second cycle execution

   Next, P1 runs remaining 2 (done), P3 runs 3 (remaining 2), then P3 runs remaining 2 (done).

3. Final Answer:

   P1, P2, P3, P1, P3, P3 -> Option D

4. Quick Check:

   Round Robin cycles through processes with quantum 3 [OK]

Hint: Run each process max quantum, repeat until done [OK]

Common Mistakes:

• Not updating remaining burst times correctly
• Mixing process order in cycles
• Assuming processes finish in one quantum

4. A Round Robin scheduler has a time quantum of 4 units. A process with burst time 6 units is scheduled. The process runs for 6 units without interruption. What is the likely error in the scheduling?

medium

A. The process voluntarily gave up CPU before quantum ended.

B. The time quantum was ignored; process should have been preempted after 4 units.

C. The scheduler used priority instead of Round Robin.

D. The process was too short to be preempted.

Solution

1. Step 1: Understand expected Round Robin behavior

   With quantum 4, a process running longer than 4 units should be preempted after 4 units to allow others to run.

2. Step 2: Analyze the given scenario

   The process ran full 6 units without interruption, which means the scheduler did not preempt it as expected.

3. Final Answer:

   The time quantum was ignored; process should have been preempted after 4 units. -> Option B

4. Quick Check:

   Quantum ignored means no preemption [OK]

Hint: Process must be preempted after quantum expires [OK]

Common Mistakes:

• Assuming short processes don't get preempted
• Confusing voluntary yield with scheduler preemption
• Ignoring time quantum enforcement

5. In a Round Robin system, if the time quantum is set too large, what is the most likely effect on system performance?

hard

A. It behaves like First-Come-First-Served, causing longer wait times for some processes.

B. Processes switch too frequently, increasing overhead.

C. All processes finish faster due to longer CPU bursts.

D. The system becomes unfair by always running the shortest job first.

Solution

1. Step 1: Understand effect of large time quantum

   If the quantum is very large, each process runs almost to completion before switching, similar to First-Come-First-Served scheduling.

2. Step 2: Analyze performance impact

   This causes longer wait times for other processes and reduces the fairness and responsiveness of Round Robin.

3. Final Answer:

   It behaves like First-Come-First-Served, causing longer wait times for some processes. -> Option A

4. Quick Check:

   Large quantum = FCFS behavior, longer waits [OK]

Hint: Large quantum makes Round Robin act like FCFS [OK]

Common Mistakes:

• Thinking large quantum reduces overhead
• Assuming all processes finish faster
• Confusing with shortest job first scheduling