5.1 A CPU-scheduling algorithm determines an order for the execution of its scheduled processes. Given n processes to be scheduled on one processor, how many different schedules are possible? Give a formula in terms of n. Answer: n! (n factorial = n × n – 1 × n – 2 × ... × 2 × 1). Explain the difference between preemptive and nonpreemptive scheduling. Answer: Preemptive scheduling allows a process to be interrupted in the midst of its execution, taking the CPU away and allocating it to another process. Nonpreemptive scheduling ensures that a process relinquishes control of the CPU only when it ﬁnishes with its current CPU burst. Suppose that the following processes arrive for execution at the times indicated. Each process will run for the amount of time listed. In answering the questions, use nonpreemptive scheduling, and base all decisions on the information you have at the time the decision must be made. Process P1 P2 P3 a. b. c. Arrival Time 0.0 0.4 1.0 Burst Time 8 4 1
What is the average turnaround time for these processes with the FCFS scheduling algorithm?
What is the average turnaround time for these processes with the SJF scheduling algorithm?
The SJF algorithm is supposed to improve performance, but notice that we chose to run process P1 at time 0 because we did not know that two shorter processes would arrive soon. Compute what the average turnaround time will be if the CPU is left idle for the ﬁrst 13
Chapter 5 CPU Scheduling
1 unit and then SJF scheduling is used. Remember that processes P1 and P2 are waiting during this idle time, so their waiting time may increase. This algorithm could be known as future-knowledge scheduling. Answer: a. b. c. 10.53 9.53 6.86
Remember that turnaround time is ﬁnishing time minus arrival time, so you have to subtract the arrival times to compute the turnaround times. FCFS is 11 if you forget to subtract arrival time. 5.4 What...
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