Process Scheduling Algorithms Explained: FCFS, SJF, RR, Priority & Multilevel Queue
Category: OPERATING SYSTEM | 21st May 2025, Wednesday
Certainly! Here’s A Detailed Blog Post On Process Scheduling Algorithms, Covering FCFS, SJF, RR, Priority Scheduling, And Multilevel Queue, Along With questions And Answers To Solidify Understanding.
Process Scheduling Is A Crucial Aspect Of Operating Systems, Ensuring Efficient Execution Of Tasks. The CPU Scheduler Selects Which Process To Run Next Based On Different Scheduling Algorithms.
Let’s Explore Five Major Scheduling Algorithms:
- First-Come, First-Served (FCFS)
- Shortest Job First (SJF)
- Round Robin (RR)
- Priority Scheduling
- Multilevel Queue Scheduling
Each Algorithm Has Unique Properties Impacting Performance And Fairness.
1. First-Come, First-Served (FCFS) Scheduling
How It Works
- The Process That Arrives First gets Executed First.
- It Follows A queue-based Approach (FIFO Structure).
- The CPU Processes Tasks In The Order They Arrive.
Advantages
? Simple & Easy To Implement.
? Works Well For Batch Systems.
Disadvantages
? Can Cause Long Waiting Times (for Larger Processes).
? Suffers From The Convoy Effect (one Long Process Can Delay All Others).
Example Calculation
| Process | Arrival Time | Burst Time | Completion Time | Waiting Time | Turnaround Time |
|---|---|---|---|---|---|
| P1 | 0 | 5 | 5 | 0 | 5 |
| P2 | 1 | 3 | 8 | 4 | 7 |
| P3 | 2 | 8 | 16 | 6 | 14 |
2. Shortest Job First (SJF) Scheduling
How It Works
- The Process With The smallest Burst Time Gets Executed First.
- Reduces waiting Time, Improving CPU Utilization.
Types
- Preemptive SJF (Shortest Remaining Time First)
- Non-preemptive SJF (Pure Shortest Job Selection)
Advantages
? Minimizes Average Waiting Time.
? Efficient For Small Processes.
Disadvantages
? Longer Processes May Suffer Starvation.
? Requires Precise Knowledge Of Burst Time.
Example
Processes Executed In Order Of Their Burst Time:
- P2 (3 Units)
- P1 (5 Units)
- P3 (8 Units)
3. Round Robin (RR) Scheduling
How It Works
- Each Process Gets A Fixed Time Slice (quantum) To Execute.
- If A Process Isn’t Finished, It goes Back In The Queue.
Advantages
? Ensures Fairness In Time-sharing Systems.
? Avoids Starvation Issues.
Disadvantages
? High Context Switching Overhead.
? Choosing An Optimal Time Quantum Is Tricky.
Example
For A time Quantum Of 4, Processes Execute In Small Chunks Until Completion.
| Process | Burst Time | Execution Order |
|---|---|---|
| P1 | 10 | P1 → P2 → P3 → P1 → P3 |
| P2 | 4 | P2 (completed) |
| P3 | 8 | P3 → P3 (completed) |
4. Priority Scheduling
How It Works
- Each Process Is Assigned A priority.
- The Highest Priority process Executes First.
Types
- Preemptive Priority Scheduling (interrupts Lower Priority Tasks).
- Non-preemptive Priority Scheduling (executes Without Interruption).
Advantages
? Good For Real-time Systems.
? Ensures Critical Processes Are Executed First.
Disadvantages
? May Cause Starvation (low-priority Processes May Never Execute).
? Requires Priority Assignment Strategy.
Example
If Priority Is Assigned As: | Process | Priority | Execution Order | |---------|----------|-----------------| | P1 | 3 | P2 → P3 → P1 | | P2 | 1 | P2 (completed) | | P3 | 2 | P3 (completed) |
5. Multilevel Queue Scheduling
How It Works
- Processes Are Divided Into multiple Priority Queues (e.g., System Tasks, Interactive Tasks).
- High-priority Queues Are Served First, Lower Ones Later.
Advantages
? Efficient Management Of Different Process Types.
? Used In Modern Operating Systems.
Disadvantages
? Complex To Implement.
? Not Always Fair If Low-priority Tasks Get Delayed Indefinitely.
Example Structure
| Queue Type | Process Category |
|---|---|
| High | System Processes |
| Medium | Interactive Processes |
| Low | Background Tasks |
FAQs On Process Scheduling Algorithms
Q1: Which Scheduling Algorithm Is Best For Real-time Systems?
A: Priority Scheduling Ensures Important Processes Execute Without Delay.
Q2: Why Is FCFS Inefficient For Interactive Systems?
A: It Causes Long Wait Times, As Larger Processes Can Block Smaller Ones.
Q3: What Is The Main Drawback Of SJF Scheduling?
A: It Suffers From starvation, Where Long Processes May Never Execute.
Q4: How Does Round Robin Ensure Fairness?
A: By Allotting equal Time Slices To Each Process.
Q5: What Happens If The Time Quantum Is Too Small In Round Robin?
A: Context Switching Overhead Increases, Reducing Efficiency.
Q6: Why Is Multilevel Queue Scheduling Used In Modern OS?
A: It categorizes Tasks, Improving Responsiveness For System-critical Operations.
Q7: Which Algorithm Is Most Commonly Used In Windows And Linux?
A: Multilevel Feedback Queue Scheduling Is Often Implemented For Better Task Management.
Information - Process Scheduling Algorithms
Process Scheduling Algorithms Play A Crucial Role In Determining System Performance. Here’s How Each Affects Efficiency, Responsiveness, And Resource Utilization:
1. First-Come, First-Served (FCFS)
- Impact On Performance: This Simple Approach Works Well For Batch Systems But Is Inefficient For interactive Tasks.
- Drawback: Long Processes Can Delay Smaller Ones, Leading To The convoy Effect, Where Overall Response Time Suffers.
- Best Used For: Systems With Predictable Workloads That Don’t Require Rapid Switching.
2. Shortest Job First (SJF)
- Impact On Performance: SJF Minimizes Average Waiting Time, boosting System Throughput By Prioritizing Shorter Tasks.
- Drawback: Starvation Can Occur If Long Processes Are Constantly Pushed Back.
- Best Used For: Workloads Where Burst Times Are Known In Advance, Ensuring Optimal CPU Utilization.
3. Round Robin (RR)
- Impact On Performance: RR Ensures fairness, Preventing Any Single Process From Dominating CPU Time.
- Drawback: Frequent context Switching Introduces Overhead, Reducing Efficiency For Tasks With Short Execution Times.
- Best Used For: Time-sharing Systems, Ensuring Balanced Process Execution.
4. Priority Scheduling
- Impact On Performance: High-priority Tasks Get Processed First, Improving Responsiveness For real-time Operations.
- Drawback: Low-priority Tasks May Experience starvation If Priorities Are Poorly Managed.
- Best Used For: Systems Where task Urgency Varies, Such As Embedded Or Real-time Applications.
5. Multilevel Queue Scheduling
- Impact On Performance: Categorizes Tasks Into Different Queues, Ensuring optimized Execution For System-critical Processes.
- Drawback: Complexity In Managing Multiple Queues Can Lead To unequal Distribution Of Resources.
- Best Used For: Modern Operating Systems Requiring Diverse Process Handling.
Overall Takeaway
The Right Scheduling Algorithm Depends On Workload Characteristics. SJF Improves Efficiency, RR Ensures Fairness, Priority Scheduling Is Great For Urgent Tasks, And Multilevel Queue Enables Modern OS Performance Optimization.
Conclusion
Process Scheduling Optimizes CPU Efficiency And Responsiveness. While FCFS Is Simple, SJF Minimizes Wait Time, RR Ensures Fairness, Priority Scheduling Supports Real-time Tasks, And Multilevel Queue Manages Diverse Workloads. Choosing The Best Algorithm Depends On The nature Of Tasks And System Requirements.
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Process Scheduling Algorithms
