Lecture Operating systems: Lesson 18 - Dr. Syed Mansoor Sarwar. The main topics covered in this lesson include: UNIX System V scheduling and UNIX system V scheduling algorithm, algorithm evaluation, process synchronization, round robin scheduling and process priorities, . | Operating Systems Lecture 18 Syed Mansoor Sarwar Agenda for Today Review of previous lecture UNIX System V scheduling Algorithm evaluation Process synchronization Recap of lecture 28 April 2022 Copyright Virtual University of Pakistan Review of Lecture 17 Multi-level queues scheduling Multi-level feedback queues scheduling UNIX System V scheduling algorithm 28 April 2022 Copyright Virtual University of Pakistan UNIX System V Scheduling Algorithm Every second the priority number of all those processes that are in the main memory and ready to run is updated by using the following formula Priority Recent CPU Usage 2 Thr. Pri. nice Threshold priority and nice values are always positive to prevent a user from migrating out of its assigned group 28 April 2022 Copyright Virtual University of Pakistan UNIX System V Example PA PB PC CPU CPU CPU Time Priority Count Priority Count Priority Count 60 0 60 0 0 60 0 1 60 75 30 60 0 1 1 60 0 30 60 2 67 15 75 30 60 0 1 30 15 60 3 63 7 67 75 8 30 67 30 4 76 33 63 7 8 67 15 67 28 April 5 2022 68 16 Copyright Virtual 76 University33of 7 63 Pakistan Round Robin Scheduling and Process Priorities 60 Higher Priority B B A A A 1 2 3 B A B B A runs first A 28 April 2022 Copyright Virtual University of 4 5Pakistan 6 Algorithm Evaluation Analytic Evaluation The algorithm and some system workload are used to produce a formula or number which gives the performance of the algorithm for that workload. Deterministic modeling Queuing models Implementation 28 April 2022 Copyright Virtual University of Pakistan Deterministic Modeling Predetermined workload and performance of each algorithm for that workload. Use of Gantt charts. Simple and fast Exact numbers for comparison Requires exact input Performance figures may not be true in general 28 April 2022 Copyright Virtual University of Pakistan Deterministic Modeling Process Arrival Time Burst Time P1 7 P2 4 P3 1 P4 4 Gantt chart P1 P2 P3 P2 P4 P1 0 2 4 5 7 11 16 28Average April 2022
