Scheduling Algorithms in Operating Systems

In a multitasking environment, where multiple processes require the CPU and other resources simultaneously, an operating system (OS) must decide the execution order of these processes. This decision-making process is governed by scheduling algorithms, which play a crucial role in managing system performance and resource utilization.

This blog explores the concept of scheduling in operating systems, its importance, and various scheduling algorithms.

What Are Scheduling Algorithms?

Scheduling algorithms are a set of rules and techniques used by an operating system to determine which process gets access to the CPU and when. They aim to achieve key goals such as:

• Maximizing CPU utilization.
• Minimizing process waiting time.
• Ensuring fairness among processes.
• Balancing response time and throughput.

Types of Scheduling

1. Long-Term Scheduling: Decides which processes are admitted into the system for execution.
2. Short-Term Scheduling: Determines which process in the ready queue gets the CPU next.
3. Medium-Term Scheduling: Suspends and resumes processes to optimize system performance.

Types of Scheduling Algorithms

1. First-Come, First-Served (FCFS)

• Description: Processes are executed in the order they arrive in the ready queue.
• Advantages: Simple and easy to implement.
• Disadvantages: Leads to the convoy effect, where short processes are delayed by longer ones.

2. Shortest Job Next (SJN)

• Description: Executes the process with the shortest burst time first.
• Advantages: Minimizes average waiting time.
• Disadvantages: Requires knowledge of process burst times in advance, making it impractical for some scenarios.

3. Round Robin (RR)

• Description: Each process is assigned a fixed time slice (quantum) to execute in a cyclic order.
• Advantages: Ensures fairness and responsiveness, especially in time-sharing systems.
• Disadvantages: Performance depends on the size of the time quantum.

4. Priority Scheduling

• Description: Processes are executed based on their priority level. Higher-priority processes are executed first.
• Advantages: Handles critical tasks efficiently.
• Disadvantages: May lead to starvation, where low-priority processes are indefinitely delayed.

5. Multilevel Queue Scheduling

• Description: Divides processes into multiple queues based on characteristics (e.g., priority or type) and schedules each queue differently.
• Advantages: Segregates tasks for optimized performance.
• Disadvantages: Requires careful configuration of queues and scheduling rules.

6. Multilevel Feedback Queue Scheduling

• Description: Similar to multilevel queue scheduling but allows processes to move between queues based on their behavior.
• Advantages: Dynamically adjusts to process needs, ensuring better resource utilization.
• Disadvantages: Complex to implement and configure.

7. Shortest Remaining Time First (SRTF)

• Description: A preemptive version of SJN where the process with the shortest remaining burst time is executed next.
• Advantages: Reduces waiting time and maximizes efficiency.
• Disadvantages: Requires accurate knowledge of burst times.

Criteria for Choosing a Scheduling Algorithm

The choice of a scheduling algorithm depends on various factors, such as:

1. System Type: Interactive, batch, or real-time.
2. Process Characteristics: Burst time, priority, and arrival time.
3. Performance Metrics: CPU utilization, throughput, waiting time, and response time.

Conclusion

Scheduling algorithms are vital for the efficient functioning of operating systems, ensuring processes are executed fairly and resources are utilized optimally. While no single algorithm is perfect for all scenarios, understanding their strengths and limitations helps system designers choose or combine algorithms suited to specific requirements.

By balancing performance, fairness, and complexity, scheduling algorithms form the backbone of a responsive and efficient operating system.