Introduction
CPU scheduling is a fundamental concept in operating systems that plays a crucial role in managing the execution of processes. At its core, CPU scheduling determines which process in the ready queue should be allocated the CPU time optimization next. This decision-making process is vital for multitasking, resource management, and ensuring a responsive computing environment. In this article, we’ll explore what CPU scheduling is, why it matters, and the various scheduling algorithms that drive modern operating systems.
What is CPU Scheduling?
CPU scheduling is the method by which an operating system decides the order in which processes are executed by the processor. When multiple processes are in the "ready" state, the operating system uses a scheduling algorithm to select one of them to run next.
The goal is to maximize CPU utilization, throughput (number of processes completed per unit time), and responsiveness, while minimizing waiting time, turnaround time, and context switching.
Why is CPU Scheduling Important?
In a multitasking system, many processes compete for the CPU. Efficient scheduling ensures:
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Fairness – Each process gets an equitable share of CPU time.
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Efficiency – The CPU remains as busy as possible.
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Responsiveness – Users experience minimal delay.
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Predictability – The system performs consistently over time.
Without proper CPU scheduling, some processes might starve (wait indefinitely), and overall system performance would degrade.
Types of Scheduling Algorithms
CPU scheduling algorithms fall into two broad categories:
1. Non-Preemptive Scheduling
In non-preemptive scheduling, once a process is assigned the CPU, it runs to completion or until it voluntarily yields control. Examples include:
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First-Come, First-Served (FCFS): Processes are executed in the order they arrive. It is simple but can lead to the "convoy effect" where short processes wait behind long ones.
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Shortest Job Next (SJN): Also known as Shortest Job First (SJF), this algorithm selects the process with the shortest estimated run-time. It minimizes average waiting time but requires prior knowledge of execution time.
2. Preemptive Scheduling
In preemptive scheduling, the CPU can be taken away from a process if a higher-priority process arrives. Examples include:
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Round Robin (RR): Each process is given a fixed time slice (quantum). After the quantum expires, the process is preempted and moved to the back of the queue. This is widely used in time-sharing systems.
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Shortest Remaining Time First (SRTF): A preemptive version of SJF, this algorithm selects the process with the shortest remaining execution time. It offers optimal average turnaround time.
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Priority Scheduling: Each process is assigned a priority. The CPU is allocated to the process with the highest priority. In preemptive versions, a higher-priority process can interrupt a running one. Starvation can occur but is often handled using "aging," which increases a process’s priority over time.
Key Metrics in CPU Scheduling
When evaluating a scheduling algorithm, several metrics are important:
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CPU Utilization – Percentage of time the CPU is actively processing.
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Throughput – Number of processes completed per unit time.
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Turnaround Time – Total time taken from submission to completion.
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Waiting Time – Time spent in the ready queue.
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Response Time – Time from submission until the first response (especially critical in interactive systems).
Conclusion
CPU scheduling is essential to the performance and efficiency of any multitasking operating system. By selecting the right scheduling algorithm based on system goals—such as responsiveness, fairness, or throughput—developers can significantly enhance user experience and resource utilization. From simple FCFS to advanced preemptive techniques like Round Robin and SRTF, each method has strengths and trade-offs. As computing environments grow more complex, adaptive and hybrid scheduling approaches continue to evolve to meet the dynamic demands of modern applications.
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