chapter6cpuscheduling-文庫(kù)吧資料
2025-07-21 18:06本頁(yè)面
【正文】 4 Process Burst Time P1 24 P2 3 P3 3 ? The Gantt chart is: ? Typically, higher average turnaround than SJF, but better response ? q should be large pared to context switch time ? q usually 10ms to 100ms, context switch 10 usec P P P1 1 10 1 8 3 02 61 44 7 1 0 2 2P 2 P 3 P 1 P 1 P 1 Silberschatz, Galvin and Gagne 169。2022 Operating System Concepts – 9th Edition Example of Priority Scheduling ProcessAarri Burst Time(ms)T Priority P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2 ? Priority scheduling Gantt Chart ? Average waiting time = (0 + 1 + 6 + 16 + 18 )/5 = ms (milliseconds) P2 P5 P1 P3 P4 Silberschatz, Galvin and Gagne 169。2022 Operating System Concepts – 9th Edition Example of Shortestremainingtimefirst ? Now we add the concepts of varying arrival times and preemption to the analysis ProcessAarri Arrival TimeT Burst Time P1 0 8 P2 1 4 P3 2 9 P4 3 5 ? Preemptive SJF Gantt Chart ? Average waiting time = [(101)+(11)+(172)+53)]/4 = 26/4 = msec P 40 1 2 6P 1 P 21 0P 3P 15 1 7 Silberschatz, Galvin and Gagne 169。2022 Operating System Concepts – 9th Edition ShortestJobFirst (SJF) Scheduling ? Associated with each process is the length of its next CPU burst ? Use these lengths to schedule the process with the shortest time ? SJF is optimal – gives minimum average waiting time for a given set of processes ? The difficulty is knowing the length of the next CPU request, and therefore it is generally not implemented at the level of shortterm CPU scheduling. ? Can however estimate the length of the next CPU burst. It is done by exponential averaging. ? SJF algorithm can either be preemptive or nonpreemptive ? Preemptive version of SJF is called shortestremainingtimefirst Silberschatz, Galvin and Gagne 169。 P2 = 0。 P3 = 27 ? Average waiting time: (0 + 24 + 27)/3 = 17 P P P1 2 30 2 4 3 02 7 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。 this involves: ? switching context ? switching to user mode ? jumping to the proper location in the user program to restart that program ? Dispatch latency – time it takes for the dispatcher to stop one process and start another running Silberschatz, Galvin an
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