What is scheduling?

Scheduling in an operating system is the process of determining which task (process or thread) will run at any given time. It manages the execution of processes to ensure efficient utilization of the CPU and other resources.

Real-World Example

Imagine a bank with one teller and multiple customers waiting in line. The teller decides the order in which customers are served based on predefined rules, such as "first-come, first-served" or "priority to senior citizens." Similarly, a scheduler determines which process runs on the CPU.

Goals of Scheduling

1. Maximize CPU Utilization: Ensure the CPU is always performing useful tasks.
2. Minimize Turnaround Time: Reduce the total time a process takes to complete.
3. Fairness: Allocate resources fairly among processes.
4. Minimize Waiting Time: Reduce the time processes spend in the ready queue.
5. Meet Deadlines: For real-time systems, ensure tasks are completed within their time constraints.

Types of Scheduling

1. Long-Term Scheduling

• Decides which processes are admitted to the system for processing.
• Affects system load and the degree of multiprogramming.

2. Medium-Term Scheduling

• Temporarily removes processes from memory (swapping) to reduce load and later reintroduces them.
• Improves system responsiveness during heavy loads.

3. Short-Term Scheduling (CPU Scheduling)

• Determines which process in the ready queue gets the CPU next.
• Directly affects system performance.

Common CPU Scheduling Algorithms

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

   • Processes are executed in the order they arrive.
   • Simple but can cause a "convoy effect" where long processes delay others.

2. Shortest Job Next (SJN)

   • Processes with the shortest execution time are given priority.
   • Efficient but may lead to starvation of longer processes.

3. Round Robin (RR)

   • Each process gets a fixed time slice (quantum). If it doesn’t finish, it goes back to the queue.
   • Ensures fairness and is commonly used in time-sharing systems.

4. Priority Scheduling

   • Processes with higher priority are executed first.
   • May cause starvation for lower-priority processes.

5. Multilevel Queue Scheduling

   • Processes are divided into different queues based on priority or type, with each queue having its own scheduling policy.

6. Multilevel Feedback Queue Scheduling

   • Similar to multilevel queue scheduling but allows processes to move between queues based on behavior.

Importance of Scheduling

Efficient scheduling ensures optimal system performance by balancing resource allocation, maximizing throughput, and minimizing latency. Poor scheduling can lead to resource underutilization, high response times, or even deadlocks.

For deeper insights into scheduling and process management, explore Grokking Multithreading and Concurrency for Coding Interviews. Understanding scheduling is essential for designing efficient, multitasking systems.