SJF Scheduling

Duration: 10 min

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AI summary & chapters

AI Summary

An AI-generated summary of this video lecture.

The video lecture provides a comprehensive overview of CPU scheduling algorithms, specifically focusing on Shortest Job First (SJF) and Shortest Remaining Time First (SRTF). The instructor begins by defining the core principle of these algorithms: selecting the process with the smallest burst time for CPU execution. He distinguishes between the non-preemptive nature of SJF and the preemptive nature of SRTF, noting that SRTF is a 'purely greedy approach.' The lecture then transitions into a detailed numerical problem involving five processes. The instructor demonstrates how to construct a Gantt chart, calculate Completion Time (CT), Turn Around Time (TAT), and Waiting Time (WT) for each process, and finally compute the average waiting time. This practical application reinforces the theoretical concepts introduced at the start of the session.

Chapters

  1. 0:00 – 2:00 00:00-02:00

    The video opens with a slide titled 'Shortest Job First (SJF)(non-pre-emptive)' and 'Shortest Remaining Time First (SRTF)/ (Shortest Next CPU Burst) (Pre-emptive)'. The instructor explains the fundamental rule: 'Whenever we make a decision of selecting the next process for CPU execution, out of all available process, CPU is assigned to the process having smallest burst time requirement.' He clarifies that if there is a tie in burst times, First Come First Serve (FCFS) is used to break the tie. He also notes that SRTF is a 'purely greedy approach' because it supports both non-preemptive and preemptive versions. The instructor emphasizes that the decision is made based on the smallest burst time requirement among available processes. The slide text explicitly states: 'When the CPU is available, it is assigned to the process that has the smallest next CPU burst.'

  2. 2:00 – 5:00 02:00-05:00

    The instructor focuses on the non-preemptive nature of SJF. He writes on the whiteboard: 'In Shortest Job First (SJF)(non-pre-emptive) once a decision is made and among the available process, the process with the smallest CPU burst is scheduled on the CPU, it cannot be pre-empted even if a new process with the smaller CPU burst requirement then the remaining CPU burst of the running process enter in the system.' He draws a small example table with processes P1, P2, and P3, listing their Arrival Time (AT) and Burst Time (BT) to illustrate the concept. He explains that once a process starts, it runs to completion regardless of new arrivals with shorter burst times. This section sets the theoretical groundwork for the numerical problem that follows. He specifically writes 'P1 0 5', 'P2 0 4', 'P3 1 1' on the board to show a scenario where P1 starts but P3 arrives later with a smaller burst.

  3. 5:00 – 9:42 05:00-09:42

    The instructor solves a numerical problem with five processes (P0 to P4). A table shows Arrival Times (1, 2, 3, 4, 5) and Burst Times (7, 5, 1, 2, 8). He draws a Gantt chart at the bottom. P0 arrives at t=1 and runs for 7 units, finishing at t=8. At t=8, P2 (BT=1) is selected as it has the smallest burst time among available processes (P1, P2, P3, P4). P2 runs from 8 to 9. Next, P3 (BT=2) is selected, running from 9 to 11. Then P1 (BT=5) runs from 11 to 16. Finally, P4 (BT=8) runs from 16 to 24. He calculates Completion Time (CT), Turn Around Time (TAT = CT - AT), and Waiting Time (WT = TAT - BT) for each process. For example, P0 has CT=8, TAT=7, WT=0. P1 has CT=16, TAT=14, WT=9. P2 has CT=9, TAT=6, WT=5. P3 has CT=11, TAT=7, WT=5. P4 has CT=24, TAT=19, WT=11. He concludes by calculating the average waiting time. The formulas are clearly written on the slide: 'Turn Around Time (TAT) = CT - AT' and 'Waiting Time (WT) = TAT - BT'.

The lecture effectively transitions from theoretical definitions to practical application. It starts by defining SJF and SRTF, highlighting the key difference in preemption. The instructor then demonstrates the non-preemptive logic with a small board example before tackling a comprehensive numerical problem. The step-by-step calculation of CT, TAT, and WT, along with the Gantt chart, provides a clear method for solving scheduling problems. This progression ensures students understand both the 'why' and the 'how' of CPU scheduling algorithms. The 'purely greedy approach' mentioned is key to understanding why SRTF minimizes waiting time.

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