Why Scheduling criteria (turnaround time, waiting time, throughput) in Operating Systems? - Purpose & Use Cases

The Big Idea

What if you could make every waiting minute count less and get more done smoothly?

The Scenario

Imagine you are managing a busy restaurant kitchen where multiple orders come in at the same time. You try to cook each dish one by one without any plan, just picking orders randomly.

The Problem

This random approach causes some dishes to take too long, customers get frustrated waiting, and the kitchen gets overwhelmed. You lose track of which order should be done first, leading to wasted time and unhappy customers.

The Solution

Scheduling criteria like turnaround time, waiting time, and throughput help organize the cooking order efficiently. They guide you to finish orders faster, reduce waiting, and serve more customers smoothly.

Before vs After

✗ Before

Cook orders as they come, no plan
Order1 -> Order3 -> Order2

✓ After

Use scheduling criteria
Order1 -> Order2 -> Order3 (minimize waiting and turnaround time)

What It Enables

It enables smooth, fair, and efficient handling of multiple tasks so everything gets done faster and customers stay happy.

Real Life Example

In a hospital, scheduling patient tests and treatments using these criteria ensures patients spend less time waiting and more time getting care, improving overall service.

Key Takeaways

Scheduling criteria help prioritize tasks to reduce delays.

They improve overall system efficiency and fairness.

Understanding these helps manage resources better in real life and computers.

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