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

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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.