LRTF

Duration: 4 min

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  1. About Course: About the Course
  2. CS | 01 | Digital Logic and Circuits and Discrete Mathematical Structures: Digital Systems & Boolean Basics, Number System, Number Representation, Logic Gates & Hardware, Boolean Expression, Boolean Minimization, Combinational Circuit, Sequential Circuits, Set Theory, Relations, Functions, Graph Theory, Group Theory, Propositional and Predicate Logic, Permutation and Combination
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AI summary & chapters

AI Summary

An AI-generated summary of this video lecture.

This lecture covers CPU scheduling algorithms, specifically Longest Job First (LJF) and its pre-emptive variant, Longest Remaining Time First (LRTF). The instructor defines LJF, noting on-screen text that "Process having longest Burst Time will get scheduled first." He demonstrates a non-preemptive example using a table of five processes (P1-P5) with varying arrival and burst times. He constructs a Gantt chart, showing how P1 runs first, followed by P4, P5, P3, and finally P2, based on the longest available burst time. The second half introduces LRTF, emphasizing its pre-emptive nature where the process with the longest *remaining* time is chosen, even if it was already running.

Chapters

  1. 0:00 – 2:00 00:00-02:00

    The instructor introduces the Longest Job First algorithm, highlighting on-screen text that states "Process having longest Burst Time will get scheduled first." He presents a table with five processes (P1-P5) and their Arrival/Burst Times. He then draws a timeline (Gantt chart) to solve a non-preemptive example. He starts with P1 at time 0, runs it for 3 units (finishing at t=3). At t=3, he compares available processes P2 (BT=2), P3 (BT=4), and P4 (BT=5), selecting P4 because it has the longest burst time. He continues this logic, selecting P5 (BT=6) which runs from t=8 to t=14, then P3 (BT=4) from t=14 to t=18, and finally P2 (BT=2) from t=18 to t=20.

  2. 2:00 – 3:42 02:00-03:42

    The topic shifts to Longest Remaining Time First (LRTF), described as pre-emptive. A table shows three processes (P1-P3) arriving at time 0 with burst times 2, 4, and 8. The instructor draws a Gantt chart showing P3 running initially. He updates the remaining burst times in the table (crossing out original values and writing new ones, e.g., P3's BT changes from 8 to 4). He demonstrates preemption by switching between P3 and P2 as their remaining times fluctuate. At time 8, when remaining times for P1, P2, and P3 are all equal (2 units), he switches to P1, then P2, then P3, finishing the schedule at time 11. This illustrates how LRTF handles ties and preemption dynamically.

The lecture distinguishes between non-preemptive and pre-emptive scheduling based on job length. LJF minimizes average waiting time but can cause starvation for short jobs. LRTF adds pre-emption, allowing shorter jobs to run if their remaining time becomes the longest relative to others, though the example shows frequent context switching. Both algorithms prioritize longer tasks, contrasting with Shortest Job First. The visual Gantt charts and table updates provide clear evidence of the scheduling logic.

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