chapter6cpuscheduling-免費(fèi)閱讀
【正文】 2022 Operating System Concepts – 9th Edition Algorithm Evaluation (Optional) ? How to select CPUscheduling algorithm for an OS? ? Determine criteria, then evaluate algorithms. ? Four evaluation methods are discussed: ? Deterministic modeling: Type of Analytic evaluation. ? Given a predetermined load, putes performance of each algorithm for it. For example, given the processes and their CPU bursts, one can pute the average waiting times and evaluate which algorithm gives least waiting times. ? This type of modelling is simple and fast ? Most cases, processes running on a system vary, hence this type of modelling is not useful. Silberschatz, Galvin and Gagne 169。2022 Operating System Concepts – 9th Edition MultipleProcessor Scheduling ? CPU scheduling more plex when multiple CPUs are available ? Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing ? Symmetric multiprocessing (SMP) – each processor is selfscheduling, all processes in mon ready queue, or each has its own private queue of ready processes ? Currently, most mon Silberschatz, Galvin and Gagne 169。2022 Operating System Concepts – 9th Edition Example of SJF ProcessArrival Time CPU Burst Time P1 6 P2 8 P3 7 P4 3 ? SJF scheduling chart ? Average waiting time = (3 + 9 + 16 + 0) / 4 = 7 P 30 3 2 4P 4 P 11 69P 2 Silberschatz, Galvin and Gagne 169。2022 Operating System Concepts – 9th Edition Scheduling Criteria ? CPU utilization – keep the CPU as busy as possible ? Throughput – of processes that plete their execution per time unit ? Turnaround time – amount of time to execute a particular process ? Waiting time – amount of time a process has been waiting in the ready queue ? Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for timesharing environment) ? Scheduling Algorithm Optimization Criteria ? Max CPU utilization ? Max throughput ? Min turnaround time ? Min waiting time ? Min response time Silberschatz, Galvin and Gagne 169。Silberschatz, Galvin and Gagne 169。2022 Operating System Concepts – 9th Edition First Come, FirstServed (FCFS) Scheduling ? Process requesting the CPU first is allocated the CPU first. ? The implementation of the FCFS policy can be achieved with a FIFO queue. ? FCFS scheduling algorithm is non preemptive. ? Disadvantage: Average CPU waiting time for a process to use CPU is long. ? Example: Suppose processes P1, P2, P3 have CPU burst time 24,3,3 respectively. ? Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: ? The waiting time for P1 = 0。2022 Operating System Concepts – 9th Edition
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