Understanding Schedulers and Scheduling algorithm

Welcome! Ever wondered what all things RTOS require to function properly? An RTOS depends on a scheduler, IPC, memory management, interrupts, timers, resource management, and hardware drivers to ensure real-time, deterministic behavior. Today, Let’s explore what scheduler is, why it’s important, and how different scheduling algorithms work.

What is a Scheduler?

A scheduler is like a traffic controller for your CPU—it decides which process gets CPU time and when. In multitasking environments, multiple processes compete for CPU resources, and the scheduler ensures they execute efficiently.

Why is Scheduling Important?

Without scheduling, your system would be chaotic! Here’s why scheduling matters:

1. Maximizes CPU Usage – No CPU cycles go to waste.
2. Better Response Time – Faster interactions in real-time systems.
3. Higher Throughput – More tasks completed per second.
4. Prevents Resource Starvation – Ensures all processes get CPU time.

Types of Scheduling

Scheduling can be broadly categorized into two types:

1. Non-Preemptive Scheduling

Once a process starts execution, it cannot be interrupted until it finishes or voluntarily yields control.

• Pros: Simpler to implement, avoids frequent context switches.
• Cons: May cause long wait times if a long process blocks others.
• Example Algorithms: First-Come, First-Served (FCFS), Shortest Job Next (SJN), Non-Preemptive Priority Scheduling.

2. Preemptive Scheduling

The scheduler can interrupt a running process if a higher-priority task arrives or a time slice expires.

• Pros: Ensures better responsiveness, prevents CPU monopolization.
• Cons: More complex, requires efficient context switching.
• Example Algorithms: Round Robin (RR), Preemptive Priority Scheduling, Multilevel Queue Scheduling with Preemption.

Common Scheduling Algorithms:-

Below mentioned are few scheduling algorithm types which different system employs.

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

A non-preemptive scheduling method where tasks execute in arrival order.

• Pros: Simple, easy to implement.
• Cons: Long wait times for short tasks (convoy effect).
• Example Usage: Batch processing, print spoolers.

2. Shortest Job Next (SJN)

This scheduling method executes the shortest task first to reduce overall waiting time.

• Pros: Minimizes average waiting time.
• Cons: Requires job length prediction, risks starving long tasks.
• Example Usage: Scientific computing, CPU-intensive applications.

3. Round Robin (RR)

In this scheduing method each task gets a fixed CPU time slice before switching.

• Pros: Fair allocation, reduces response time.
• Cons: High context-switching overhead.
• Example Usage: Multi-user systems (Linux, Windows).

4. Priority Scheduling

This scheduling method executes the highest priority task first.

• Pros: Ensures critical tasks execute sooner.
• Cons: Low-priority tasks may starve.
• Example Usage: Emergency response systems, embedded applications.

5. Preemptive Priority Scheduling

It is a preemptive version of priority scheduling where a higher-priority task can interrupt a lower-priority task.

• Pros: Ensures high-priority tasks run immediately.
• Cons: Risk of starvation for low-priority tasks, requires priority handling techniques.
• Example Usage: Real-time systems, embedded controllers.

6. Multilevel Queue Scheduling

This scheduling method categorizes processes into different queues with separate scheduling policies.

• Pros: Differentiates between real-time and background tasks.
• Cons: Complex implementation, workload balancing challenges.
• Example Usage: Unix-like OS (interactive vs. batch processes).

Conclusion:

Schedulers play a crucial role in managing CPU time and ensuring system efficiency. The choice of scheduling algorithm depends on system requirements, balancing factors like response time, fairness, and throughput. Selecting the right scheduler is essential for ensuring optimal performance, particularly in real-time systems where meeting deadlines is critical. We will talk more about scheduling algorithms in details in next posts. Until then Stay tuned!!