Preemptive scheduling – Gate 2025 – TOC previous year solved question Paper.
Preemptive scheduling – Gate 2025 – TOC previous year solved question Paper.
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Preemptive Scheduling – GATE 2025 | TOC Previous Year Solved Question
Topic: Operating System (OS) – Preemptive CPU Scheduling
Exam Focus: GATE CSE / IT 2025
Category: Theory of Computation (TOC often refers to OS in GATE context when dealing with scheduling topics)
क्या होता है Preemptive Scheduling?
Preemptive Scheduling एक CPU scheduling तकनीक है जिसमें running process को बीच में रोककर किसी दूसरे higher priority या ready process को CPU दिया जाता है।
Key Features:
| Feature | Description |
|---|---|
| Preemption | हाँ (Yes) – Process को बीच में interrupt किया जा सकता है |
| Control | OS के पास होता है |
| Response Time | बेहतर |
| Context Switching | ज़्यादा होता है (CPU overhead बढ़ता है) |
Common Preemptive Algorithms:
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Round Robin (RR)
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Shortest Remaining Time First (SRTF)
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Preemptive Priority Scheduling
GATE Previous Year Solved Question (Example)
GATE CSE Question (Simplified):
GATE Year: 2017
Question Type: Numerical
Topic: Preemptive Scheduling – SRTF
Q:
Consider the following set of processes with arrival time and burst time:
| Process | Arrival Time | Burst Time |
|---|---|---|
| P1 | 0 ms | 7 ms |
| P2 | 2 ms | 4 ms |
| P3 | 4 ms | 1 ms |
| P4 | 5 ms | 4 ms |
Use Shortest Remaining Time First (Preemptive) scheduling.
What is the average waiting time?
Solution Step-by-Step:
Step 1: Gantt Chart
| Time | Running Process |
|---|---|
| 0-2 | P1 (7→5) |
| 2-4 | P2 (4→2) |
| 4-5 | P3 (1) |
| 5-6 | P2 (2→1) |
| 6-7 | P2 (1→0) |
| 7-11 | P4 (4) |
| 11-13 | P1 (5→3) |
| 13-15 | P1 (3→1) |
| 15-17 | P1 (1→0) |
Completion Time (CT)
| Process | CT | TAT (CT – AT) | WT (TAT – BT) |
|---|---|---|---|
| P1 | 17 | 17 – 0 = 17 | 17 – 7 = 10 |
| P2 | 7 | 7 – 2 = 5 | 5 – 4 = 1 |
| P3 | 5 | 5 – 4 = 1 | 1 – 1 = 0 |
| P4 | 11 | 11 – 5 = 6 | 6 – 4 = 2 |
Average Waiting Time:
= (10 + 1 + 0 + 2) / 4
= 13 / 4 = 3.25 ms
Concept Check:
Preemptive = Process को interrupt किया जा सकता है Waiting Time = Turnaround Time – Burst Time SRTF always selects the process with minimum remaining time
Summary:
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GATE Relevance: Highly important topic
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Common Question Format: Gantt chart-based, waiting time, turnaround time
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Pro Tip: Practice SRTF with multiple process arrivals
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