Question

Consider three processes P1, P2, and P3, all ready to run at time = 0. Each process has two CPU bursts, separated by a single I/O burst. When its first CPU burst is completed, each process requests a blocking I/O operation on an I/O device. It becomes ready for the second CPU burst only when its I/O burst completes.

The CPU scheduling policy is non-preemptive Shortest-Job-First (SJF). The system has only one CPU.
The lengths of the CPU bursts of the processes are given below:
P₁: 2 ms (each CPU burst)
P2: 4 ms (each CPU burst)
P3: 5 ms (each CPU burst)
When a process starts receiving service for I/O, its I/O burst takes 10 ms.

Give the average turnaround time for these processes, assuming each process P₁ performs I/O on a separate I/O device D₁ Show your work (including Gantt charts for the CPU and all I/O devices). Assume FCFS scheduling on all I/O devices.

Answer 1

The average turnaround time for processes P1, P2, and P3 scheduled using non-preemptive SJF CPU scheduling is approximately 17.33 milliseconds. This was calculated using their respective CPU and I/O burst times under the assumption that each process immediately performs I/O on a separate device after its first CPU burst.

Step-by-step explanation:

The question is about calculating the average turnaround time for three processes P1, P2, and P3 under a non-preemptive Shortest-Job-First (SJF) CPU scheduling policy with separate I/O devices for each process. Since all processes are ready at time = 0 and the system uses SJF scheduling, P1 will run first as it has the shortest CPU burst of 2 ms.

Gantt Chart for CPU:

• Time 0-2ms: P1's first CPU burst
• Time 2-6ms: P2's first CPU burst
• Time 6-11ms: P3's first CPU burst
• Time 12-14ms: P1's second CPU burst (after 10 ms I/O)
• Time 14-18ms: P2's second CPU burst (after 10 ms I/O)
• Time 18-23ms: P3's second CPU burst (after 10 ms I/O)

Assuming each process performs I/O on a separate I/O device immediately after its first CPU burst, the process turnaround times are as follows:

• P1 Turnaround Time: 2ms (1st CPU) + 10ms (I/O) + 2ms (2nd CPU) = 14ms
• P2 Turnaround Time: 4ms (1st CPU) + 10ms (I/O) + 4ms (2nd CPU) = 18ms
• P3 Turnaround Time: 5ms (1st CPU) + 10ms (I/O) + 5ms (2nd CPU) = 20ms

To find the average turnaround time, we add the turnaround times of all processes and divide by the number of processes: (14ms + 18ms + 20ms) / 3 = 52ms / 3 ≈ 17.33ms

Thus, the average turnaround time for the processes is approximately 17.33 milliseconds.