Consider the following set of processes, the length of the CPU burst time given in milliseconds.
Process | Burst time
P1 | 6
P2 | 8
P3 | 7
P4 | 3
Assuming the above process being scheduled with the SJF scheduling algorithm, what is the waiting time for process P1?
A. 16 ms
B. 9 ms
C. 0 ms
D. 3 ms
Answer (2)
Sort processes by burst time using SJF algorithm.
Determine the execution order: P4, P1, P3, P2.
Calculate completion times for each process.
Calculate waiting time for P1 as Completion Time - Burst Time = 9 − 6 = 3 ms. The waiting time for process P1 is 3 ms.
Explanation
Understanding the Problem We are given four processes, P1, P2, P3, and P4, with burst times of 6, 8, 7, and 3 milliseconds, respectively. We need to determine the waiting time for process P1 using the Shortest Job First (SJF) scheduling algorithm. The SJF algorithm schedules processes based on their burst times, executing the shortest job first.
Determining Execution Order First, we need to determine the order in which the processes will be executed. According to SJF, we sort the processes by their burst times in ascending order:
P4 (3 ms)
P1 (6 ms)
P3 (7 ms)
P2 (8 ms)
Calculating Completion Times Next, we calculate the completion time for each process. The completion time is the time at which a process finishes its execution.
P4 completes at 3 ms.
P1 completes at 3 + 6 = 9 ms.
P3 completes at 9 + 7 = 16 ms.
P2 completes at 16 + 8 = 24 ms.
Calculating Waiting Times Now, we calculate the waiting time for each process. The waiting time is the time a process spends waiting in the ready queue before it starts its execution. It is calculated as Completion Time - Burst Time.
Waiting time for P4 = 3 - 3 = 0 ms
Waiting time for P1 = 9 - 6 = 3 ms
Waiting time for P3 = 16 - 7 = 9 ms
Waiting time for P2 = 24 - 8 = 16 ms
Final Answer The waiting time for process P1 is 3 ms. Therefore, the correct answer is d.
Examples
SJF scheduling is like managing tasks based on how quickly they can be done. Imagine you have a few errands to run: one takes 3 minutes, another 6, a third 7, and the last 8. If you use SJF, you'd do the 3-minute errand first, then the 6-minute one, and so on. This way, the average time you spend waiting to complete each errand is minimized. This is useful in many real-life scenarios, such as managing tasks in a factory or scheduling appointments to reduce overall waiting times.
The waiting time for process P1, calculated using the Shortest Job First scheduling algorithm, is 3 ms. Therefore, the correct answer is D. This is determined by first sorting the processes by their burst times, then calculating completion and waiting times accordingly.
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