Terminology for CPU Scheduling
Duration: 9 min
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This educational video provides a comprehensive lecture on Operating Systems, specifically focusing on the fundamental terminology used in process scheduling algorithms. The instructor systematically defines key performance metrics such as Arrival Time (AT), Burst Time (BT), Completion Time (CT), Turn Around Time (TAT), and Waiting Time (WT). The lecture transitions from theoretical definitions to a practical application, where the instructor solves a sample problem involving two processes, P1 and P2. Through a step-by-step walkthrough, he demonstrates how to construct a Gantt chart-like timeline and populate a data table to calculate the final scheduling metrics, ensuring students understand the mathematical relationships between these variables. The visual aids, including the slide and handwritten notes, serve as primary references for the formulas and calculations presented.
Chapters
0:00 – 2:00 00:00-02:00
The session begins with the instructor presenting a slide titled "Terminology" which lists five critical definitions essential for understanding process scheduling. He explains that Arrival Time (AT) is the specific moment a process enters the ready state, while Burst Time (BT) represents the total amount of CPU time required for a process to complete its execution. He further defines Completion Time (CT) as the time when the process finishes. Crucially, he writes out the formulas for Turn Around Time (TAT) as Completion Time minus Arrival Time, and Waiting Time as Turn Around Time minus Burst Time. To prepare for the example, he draws a table with columns for Process, AT, and BT, populating it with data for Process P1 (AT=0, BT=4) and Process P2 (AT=2, BT=6). The instructor gestures towards the slide to emphasize the definitions as he speaks.
2:00 – 5:00 02:00-05:00
The instructor moves to the practical application phase by drawing a timeline on the right side of the screen to visualize the execution order. He explains that since P1 arrives at time 0, it is the first to execute. He marks the start of P1 at 0 and, given its burst time of 4, extends the block to time 4. He highlights that P2 arrives at time 2 but cannot start immediately because the CPU is occupied by P1. Consequently, P2 waits in the ready queue. Once P1 completes at time 4, P2 begins its execution. With a burst time of 6, P2 runs from time 4 to time 10. The instructor then updates the main table, filling in the Completion Time (CT) column, recording 4 for P1 and 10 for P2, establishing the finish times for both processes. He uses a pen to point at the timeline and the table simultaneously to connect the visual representation with the data.
5:00 – 8:45 05:00-08:45
In the final segment, the instructor calculates the Turn Around Time (TAT) and Waiting Time (WT) for both processes using the formulas previously defined. He applies the formula TAT = CT - AT. For P1, he calculates 4 - 0 = 4. For P2, he calculates 10 - 2 = 8. He then uses the relationship TAT = Waiting Time + Burst Time to find the waiting time. For P1, the waiting time is 4 - 4 = 0, indicating it ran immediately without delay. For P2, the waiting time is 8 - 6 = 2, showing it waited for P1 to finish. He points to the slide to emphasize that Waiting Time is the difference between Turn Around Time and Burst Time. The video concludes with a fully populated table showing all calculated values, reinforcing the concepts of process scheduling efficiency and the impact of arrival times on waiting periods.
The video effectively bridges the gap between theoretical definitions and practical calculation in process scheduling. By starting with clear definitions on a slide and immediately applying them to a two-process example, the instructor creates a cohesive learning experience. The visual progression from a blank table to a filled timeline and finally to a complete data table helps students visualize the flow of time and resource allocation. The explicit calculation of waiting time highlights the cost of queuing, a key concept in operating systems. The instructor's methodical approach ensures that students can replicate the process for more complex scenarios involving multiple processes.