Pre-emptive Vs Non-Pre-emptive Scheduling

Duration: 5 min

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AI summary & chapters

AI Summary

An AI-generated summary of this video lecture.

The video lecture focuses on the fundamental types of process scheduling in operating systems, specifically contrasting Non-Preemptive and Preemptive scheduling strategies. The instructor begins by defining Non-Preemptive scheduling, emphasizing that once a process is allocated the CPU, it retains control until it voluntarily releases it. This is illustrated through a state transition diagram where a process moves from Running to Terminated upon completion or to Blocked when waiting for I/O. The lecture then transitions to Preemptive scheduling, where the operating system can forcibly remove a process from the CPU. This section details four specific scenarios for CPU release, including completion, voluntary I/O requests, high-priority process arrival, and time quantum expiration. The instructor uses a modified state diagram to visually represent these transitions, highlighting the additional paths from Running back to Ready that characterize preemptive behavior.

Chapters

  1. 0:00 – 2:00 00:00-02:00

    The instructor introduces Non-Preemptive scheduling, defining it as a strategy where a process keeps the CPU until it releases it willingly. On-screen text explicitly states: 'Under Non-Pre-emptive scheduling, once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU willingly.' He explains that a process leaves the CPU only in two specific cases: first, when it completes execution (Termination state), and second, when it voluntarily performs I/O operations (Blocked state). He draws a state transition diagram showing the flow from Start to Ready, then to Running. From Running, he draws an arrow labeled '1' pointing to Terminated and another arrow labeled '2' pointing to Blocked, which then loops back to Ready. This visual aid reinforces the concept that the process is not forced out during execution, ensuring it runs to completion or until it explicitly requests a resource.

  2. 2:00 – 4:48 02:00-04:48

    The lecture shifts to Preemptive scheduling, where a process can be forced out of the CPU. The on-screen text reads: 'Under Pre-emptive scheduling, once the CPU has been allocated to a process, A process will leave the CPU willingly or it can be forced out.' The instructor lists four conditions for leaving the CPU: completion of execution, voluntary I/O operations, arrival of a high-priority process, and time quantum expiration. He updates the state diagram to show arrows labeled '3' and '4' moving from Running back to Ready, representing the forced preemption scenarios. He explains that unlike the previous model, the OS can interrupt the running process if a higher priority task arrives or if the time slice expires, forcing the process back into the Ready queue. This highlights the dynamic nature of preemptive systems where the scheduler has more control over resource allocation.

The video effectively contrasts two core scheduling philosophies. Non-preemptive scheduling relies on the process's voluntary cooperation to release the CPU, limiting transitions to completion or I/O waits. Preemptive scheduling introduces OS control, allowing the system to interrupt running processes for higher priority tasks or time limits. The visual progression from a simple state diagram to a more complex one with additional feedback loops clearly demonstrates the increased flexibility and complexity of preemptive systems. Understanding these distinctions is crucial for analyzing system performance and responsiveness in operating system design.

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