Day 01Part 01- Operating System for gate Computer Science – Introduction to scheduling algorithm.

Lui Kreyszig

2 weeks ago

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1 Day 01 – Part 01: Operating System for GATE CSE

Day 01 – Part 01: Operating System for GATE CSE

Topic: Introduction to Scheduling Algorithms

1. What is CPU Scheduling?

CPU Scheduling is the process of selecting one process from the ready queue to execute next on the CPU.

In a multiprogramming system, multiple processes are in memory, and the CPU scheduler picks one of the processes to run.

Why Scheduling is Important in OS (GATE Point of View):

Improves CPU utilization
Minimizes waiting time, turnaround time
Maximizes throughput and responsiveness

2. Types of Schedulers in OS:

Scheduler Type	Role
Long-Term	Decides which jobs are admitted to the system
Short-Term	Picks from ready queue → allocated to CPU (CPU Scheduling)
Medium-Term	Suspends/resumes processes (used for swapping)

3. Scheduling Criteria (Very Important for GATE MCQs)

Criterion	Description
CPU Utilization	Keep CPU as busy as possible (ideal: 100%)
Throughput	No. of processes completed per unit time
Turnaround Time	Completion time – Arrival time
Waiting Time	Turnaround Time – Burst Time
Response Time	First response – Arrival time
Fairness	No starvation, all processes get chance

4. Classification of Scheduling Algorithms

Type	Examples
Non-Preemptive	FCFS, SJF (non-preemptive), Priority (non-preemptive)
Preemptive	SRTF, Round Robin, Priority (preemptive), Multilevel Queue

5. Brief Intro to Common Scheduling Algorithms

Algorithm	Key Idea
FCFS	First Come First Serve – easy but can cause convoy effect
SJF	Shortest Job First – optimal avg. waiting time, may cause starvation
SRTF	Preemptive version of SJF
Priority	Processes scheduled based on priority
Round Robin	Each process gets a time quantum – good for time-sharing systems

GATE Question Examples

Q (GATE 2017):
Which of the following scheduling algorithms could result in starvation?

Answer: SJF (non-preemptive) and Priority (if lower priority never gets CPU)

Formula Quick Recap

Turnaround Time (TAT) = Completion Time – Arrival Time
Waiting Time = TAT – Burst Time
Response Time = First Response – Arrival Time

Recommended for GATE CSE Preparation

Practice 10–15 scheduling problems with Gantt charts
Focus on edge cases (e.g., same arrival time, preemption mid-process)
Learn to compute average waiting and turnaround time

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<img src="https://s.w.org/images/core/emoji/13.0.1/72x72/1f9e0.png" alt="🧠" class="wp-smiley amp-wp-843f19c" width="72" height="72" data-amp-original-style="height: 1em; max-height: 1em;"> Day 01 – Part 01: Operating System for GATE CSE

<img src="https://s.w.org/images/core/emoji/13.0.1/72x72/1f3af.png" alt="🎯" class="wp-smiley amp-wp-843f19c" width="72" height="72" data-amp-original-style="height: 1em; max-height: 1em;"> Topic: Introduction to Scheduling Algorithms

<img src="https://s.w.org/images/core/emoji/13.0.1/72x72/1f4cc.png" alt="📌" class="wp-smiley amp-wp-843f19c" width="72" height="72" data-amp-original-style="height: 1em; max-height: 1em;"> 1. What is CPU Scheduling?

<img src="https://s.w.org/images/core/emoji/13.0.1/72x72/1f3af.png" alt="🎯" class="wp-smiley amp-wp-843f19c" width="72" height="72" data-amp-original-style="height: 1em; max-height: 1em;"> Why Scheduling is Important in OS (GATE Point of View):

<img src="https://s.w.org/images/core/emoji/13.0.1/72x72/1f504.png" alt="🔄" class="wp-smiley amp-wp-843f19c" width="72" height="72" data-amp-original-style="height: 1em; max-height: 1em;"> 2. Types of Schedulers in OS:

<img src="https://s.w.org/images/core/emoji/13.0.1/72x72/1f9ee.png" alt="🧮" class="wp-smiley amp-wp-843f19c" width="72" height="72" data-amp-original-style="height: 1em; max-height: 1em;"> 3. Scheduling Criteria (Very Important for GATE MCQs)

<img src="https://s.w.org/images/core/emoji/13.0.1/72x72/2699.png" alt="⚙" class="wp-smiley amp-wp-843f19c" width="72" height="72" data-amp-original-style="height: 1em; max-height: 1em;"> 4. Classification of Scheduling Algorithms

<img src="https://s.w.org/images/core/emoji/13.0.1/72x72/1f4d8.png" alt="📘" class="wp-smiley amp-wp-843f19c" width="72" height="72" data-amp-original-style="height: 1em; max-height: 1em;"> 5. Brief Intro to Common Scheduling Algorithms

<img src="https://s.w.org/images/core/emoji/13.0.1/72x72/1f4dd.png" alt="📝" class="wp-smiley amp-wp-843f19c" width="72" height="72" data-amp-original-style="height: 1em; max-height: 1em;"> GATE Question Examples

<img src="https://s.w.org/images/core/emoji/13.0.1/72x72/1f527.png" alt="🔧" class="wp-smiley amp-wp-843f19c" width="72" height="72" data-amp-original-style="height: 1em; max-height: 1em;"> Formula Quick Recap

<img src="https://s.w.org/images/core/emoji/13.0.1/72x72/1f4da.png" alt="📚" class="wp-smiley amp-wp-843f19c" width="72" height="72" data-amp-original-style="height: 1em; max-height: 1em;"> Recommended for GATE CSE Preparation