Non-Pre-emptive Priority Scheduling
Duration: 12 min
This video lesson is available to enrolled students.
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This educational video provides a comprehensive lecture on Priority Scheduling in Operating Systems. The instructor begins by defining the algorithm, explaining that each process is assigned a priority number, and the CPU is allocated to the process with the highest priority. He clarifies that 'highest priority' can be represented by either the highest or lowest number depending on the system's convention. The lecture covers tie-breaking mechanisms using First-Come-First-Served (FCFS) order and notes that the algorithm supports both pre-emptive and non-pre-emptive versions. The second half of the video features a detailed numerical example involving six processes (P0 to P5). The instructor constructs a Gantt chart to determine the execution sequence based on priority values and arrival times. He then calculates the Completion Time (CT), Turn Around Time (TAT), and Waiting Time (WT) for each process using standard formulas, filling out a data table to demonstrate the final results.
Chapters
0:00 – 2:00 00:00-02:00
The instructor introduces the concept of Priority Scheduling. The slide text states: 'Here a priority is associated with each process. At any instance of time out of all available process, CPU is allocated to the process which possess highest priority (may be higher or lower number).' He explains that a priority number is assigned to every process. He writes 'P1 - 5' and 'P2 - 3' on the screen to illustrate that priority is just a number. He mentions that if priorities are equal, the tie is broken using FCFS order. He also states that there is no importance to seniority or burst time in this algorithm.
2:00 – 5:00 02:00-05:00
The instructor elaborates on the characteristics of Priority Scheduling. He draws a diagram of a queue to visualize how processes wait. He emphasizes that the algorithm supports both non-pre-emptive and pre-emptive versions. He explains that the CPU is always given to the process with the highest priority among the available ones. He clarifies that 'highest priority' might mean the highest number (e.g., 8 is higher than 5) or the lowest number (e.g., 1 is higher than 5), depending on the specific implementation. He reiterates that burst time does not influence the scheduling decision, only the priority value matters.
5:00 – 10:00 05:00-10:00
The lecture transitions to a numerical example. A table appears with columns: P. No, AT (Arrival Time), BT (Burst Time), Priority, CT, TAT, and WT. The data includes processes P0 through P5 with varying priorities. P3 has a priority of 8(H), indicating High priority. The instructor draws a Gantt chart at the bottom of the screen. He determines the execution order: P0 starts at t=1. At t=5, P3 (Priority 8) is selected. At t=10, P2 (Priority 7) is selected. At t=13, P5 (Priority 6) is selected. For P1 and P4, which both have Priority 5, he applies FCFS, so P1 runs before P4. He writes the sequence P0, P3, P2, P5, P1, P4 on the timeline.
10:00 – 11:39 10:00-11:39
The instructor completes the numerical solution by calculating the metrics for each process. He fills the CT column: P0=5, P3=10, P2=13, P5=15, P1=17, P4=18. He then calculates TAT using the formula 'TAT = CT - AT'. For P0, TAT is 5-1=4. For P1, TAT is 17-2=15. He proceeds to calculate WT using 'WT = TAT - BT'. For P0, WT is 4-4=0. For P1, WT is 15-2=13. He fills the entire table row by row, pointing to the values and explaining the arithmetic for each process to ensure students understand how the final metrics are derived from the Gantt chart.
The video effectively bridges theory and practice in Priority Scheduling. It starts by establishing the core rule: CPU allocation is driven by priority numbers, not burst time or arrival time, though arrival time dictates availability. The instructor clarifies the ambiguity of 'highest priority' (high number vs low number) and the role of FCFS in tie-breaking. The practical application involves a step-by-step execution of a Gantt chart where the scheduler dynamically picks the highest priority process available at each time unit. The final segment focuses on performance evaluation, calculating Turn Around Time and Waiting Time for each process, reinforcing the formulas TAT = CT - AT and WT = TAT - BT. This progression ensures students understand both the logic of the scheduler and the method for evaluating its efficiency.