Scheduling criteria (turnaround time, waiting time, throughput) in Operating Systems - Step-by-Step Execution

Concept Flow - Scheduling criteria (turnaround time, waiting time, throughput)

Start: Processes arrive

↓

Schedule processes

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Calculate Turnaround Time

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Calculate Waiting Time

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Calculate Throughput

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Evaluate performance

Processes arrive and are scheduled; then turnaround time, waiting time, and throughput are calculated to evaluate scheduling performance.

Execution Sample

Operating Systems

Process | Arrival | Burst
P1      | 0       | 4
P2      | 1       | 3
P3      | 2       | 1

Calculate turnaround, waiting, throughput

This example schedules three processes and calculates their turnaround time, waiting time, and throughput.

Analysis Table

Step	Process	Start Time	Finish Time	Turnaround Time	Waiting Time	Throughput (Processes Completed)
1	P1	0	4	4 - 0 = 4	0 - 0 = 0	1
2	P2	4	7	7 - 1 = 6	4 - 1 = 3	2
3	P3	7	8	8 - 2 = 6	7 - 2 = 5	3
4	-	-	-	-	-	Throughput = 3 processes / 8 time units = 0.375

💡 All processes completed by time 8; throughput calculated as total processes divided by total time.

State Tracker

Variable	Start	After P1	After P2	After P3	Final
Start Time	-	0	4	7	-
Finish Time	-	4	7	8	-
Turnaround Time	-	4	6	6	-
Waiting Time	-	0	3	5	-
Throughput	0	1	2	3	0.375

Key Insights - 3 Insights

Why is waiting time different from turnaround time?

Waiting time is the time a process waits before execution (see rows 1-3 in execution_table), while turnaround time includes both waiting and execution time.

How is throughput calculated?

Throughput is total processes completed divided by total time taken (see step 4 in execution_table).

Why does process P3 have a longer waiting time despite a short burst time?

Because P3 arrives later and waits for P1 and P2 to finish (see waiting time values in execution_table).

Visual Quiz - 3 Questions

Look at the execution_table at step 2: What is the waiting time for process P2?

A 4

B 1

C 3

D 0

💡 Hint

Check the Waiting Time column for P2 in execution_table row 2.

At which step does the throughput reach 3 processes completed?

A Step 1

B Step 3

C Step 4

D Step 2

💡 Hint

Look at the Throughput column in execution_table rows.

If process P3 arrived at time 0 instead of 2, how would its waiting time change?

A It would decrease

B It would increase

C It would stay the same

D It would be zero

💡 Hint

Refer to variable_tracker Waiting Time values and consider arrival times.

Concept Snapshot

Scheduling criteria:
- Turnaround Time = Finish Time - Arrival Time
- Waiting Time = Turnaround Time - Burst Time
- Throughput = Number of processes completed / Total time
These metrics help evaluate how well a scheduler performs.

Full Transcript

Scheduling criteria measure how well an operating system schedules processes. Turnaround time is the total time from process arrival to completion. Waiting time is how long a process waits before it starts running. Throughput is how many processes finish per unit time. In the example, three processes arrive at different times and run one after another. We calculate each process's turnaround and waiting times and then compute throughput as total processes divided by total time. These values help us understand the efficiency of scheduling.

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