Scheduling criteria (turnaround time, waiting time, throughput) in Operating Systems - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Turnaround Time

Which of the following best defines turnaround time in process scheduling?

A The number of processes completed per unit time.

B The time a process spends waiting in the ready queue.

C The time taken by the CPU to execute the process instructions.

D The total time taken from process submission to completion.

💡 Hint

Think about the entire duration a process is in the system.

Turnaround time measures the total time from when a process enters the system until it finishes execution, including waiting and running times.

🧠 Conceptual

intermediate

Waiting Time Explanation

What does waiting time represent in CPU scheduling?

A The time a process spends executing on the CPU.

B The time a process spends waiting in the ready queue before execution.

C The total time from process arrival to completion.

D The time taken to switch between processes.

💡 Hint

Consider the time before the process actually starts running.

Waiting time is the total time a process spends waiting in the ready queue before it gets CPU time.

🔍 Analysis

advanced

Calculating Throughput

Given 5 processes completed in 20 seconds total, what is the throughput?

A 0.25 processes per second

B 20 processes per second

C 5 processes per second

D 4 processes per second

💡 Hint

Throughput is the number of processes completed divided by total time.

Throughput = Number of processes completed / Total time = 5 / 20 = 0.25 processes per second.

❓ Comparison

advanced

Comparing Scheduling Criteria

Which scheduling criterion focuses on maximizing the number of processes completed in a given time?

A Throughput

B Waiting time

C Turnaround time

D Response time

💡 Hint

Think about the rate of process completion.

Throughput measures how many processes finish per unit time, so maximizing throughput means completing more processes quickly.

❓ Reasoning

expert

Impact of Scheduling on Waiting Time

Consider three processes with burst times 4, 3, and 2 units arriving at the same time. Which scheduling method will minimize the average waiting time?

A First-Come, First-Served (FCFS)

B Round Robin with quantum 1

C Shortest Job First (SJF)

D Priority Scheduling with arbitrary priorities

💡 Hint

Think about which method runs shorter processes first.

SJF schedules the shortest processes first, reducing waiting time for shorter jobs and minimizing average waiting time overall.

Practice

1. Which scheduling criterion measures the total time taken from the arrival of a process to its completion?

easy

A. Turnaround time

B. Waiting time

C. Throughput

D. Response time

Solution

1. Step 1: Understand the definition of turnaround time

   Turnaround time is the total time from when a process arrives until it finishes execution.

2. Step 2: Compare with other criteria

   Waiting time is only the time a process waits before starting, throughput is number of processes completed per time, and response time is time until first response.

3. Final Answer:

   Turnaround time -> Option A

4. Quick Check:

   Turnaround time = total process duration [OK]

Hint: Turnaround = arrival to finish total time [OK]

Common Mistakes:

• Confusing waiting time with turnaround time
• Mixing throughput with time durations
• Thinking response time equals turnaround time

2. Which of the following correctly defines waiting time in process scheduling?

easy

A. Number of processes completed per unit time

B. Time from process arrival to completion

C. Time a process spends in the ready queue before execution

D. Time taken by CPU to execute the process

Solution

1. Step 1: Define waiting time

   Waiting time is the time a process spends waiting in the ready queue before it starts running on the CPU.

2. Step 2: Eliminate other options

   Time from process arrival to completion describes turnaround time, C describes throughput, and D is CPU burst time, not waiting time.

3. Final Answer:

   Time a process spends in the ready queue before execution -> Option C

4. Quick Check:

   Waiting time = time before execution [OK]

Hint: Waiting time = time before process runs [OK]

Common Mistakes:

• Confusing waiting time with turnaround time
• Thinking waiting time includes execution time
• Mixing throughput with waiting time

3. Consider three processes with the following completion times (in seconds): P1=10, P2=15, P3=20. If all arrived at time 0, what is the throughput if the total time observed is 20 seconds?

medium

A. 0.20 processes per second

B. 0.15 processes per second

C. 0.10 processes per second

D. 0.25 processes per second

Solution

1. Step 1: Calculate total processes completed

   All three processes (P1, P2, P3) completed within 20 seconds, so total completed = 3.

2. Step 2: Calculate throughput

   Throughput = number of processes completed / total time = 3 / 20 = 0.15 processes per second.

3. Final Answer:

   0.15 processes per second -> Option B

4. Quick Check:

   Throughput = 3/20 = 0.15 [OK]

Hint: Throughput = completed tasks ÷ total time [OK]

Common Mistakes:

• Dividing total time by number of processes instead of reverse
• Counting incomplete processes
• Using average completion time instead of total time

4. A scheduler reports the following for a process: Arrival time = 0, Start time = 5, Completion time = 12. The waiting time is incorrectly calculated as 7 seconds. What is the correct waiting time?

medium

A. 5 seconds

B. 7 seconds

C. 12 seconds

D. 0 seconds

Solution

1. Step 1: Understand waiting time formula

   Waiting time = Start time - Arrival time = 5 - 0 = 5 seconds.

2. Step 2: Identify error in reported waiting time

   The reported waiting time of 7 seconds is incorrect because it does not match the formula.

3. Final Answer:

   5 seconds -> Option A

4. Quick Check:

   Waiting time = start - arrival = 5 [OK]

Hint: Waiting time = start time minus arrival time [OK]

Common Mistakes:

• Using completion time instead of start time
• Adding instead of subtracting times
• Confusing waiting time with turnaround time

5. A system runs 4 processes with arrival times and burst times as follows:
P1: arrival=0, burst=4
P2: arrival=1, burst=3
P3: arrival=2, burst=1
P4: arrival=3, burst=3
If the scheduler uses First-Come-First-Serve (FCFS), what is the average turnaround time?

hard

A. 3.5 seconds

B. 4.5 seconds

C. 5.0 seconds

D. 6.0 seconds

Solution

1. Step 1: Calculate completion times using FCFS

   Process order by arrival: P1(0), P2(1), P3(2), P4(3).
   P1 completes at 0+4=4,
   P2 starts at 4, completes at 4+3=7,
   P3 starts at 7, completes at 7+1=8,
   P4 starts at 8, completes at 8+3=11.

2. Step 2: Calculate turnaround times

   Turnaround = completion - arrival:
   P1: 4-0=4,
   P2: 7-1=6,
   P3: 8-2=6,
   P4: 11-3=8.
   Average turnaround = (4+6+6+8)/4 = 24/4 = 6.0 seconds.

3. Final Answer:

   6.0 seconds -> Option D

4. Quick Check:

   Average turnaround = 24/4 = 6.0 [OK]

Hint: Turnaround = completion - arrival; average = sum/number [OK]

Common Mistakes:

• Calculating turnaround as burst time only
• Ignoring arrival times in scheduling order
• Mixing waiting time with turnaround time