GATE CS 2014 Set 2 — Question 42
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Operating System → Process Scheduling → Round Robin
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Question
Three processes A, B and C each execute a loop of 100 iterations. In each iteration of the loop, a process performs a single computation that requires CPU milliseconds and then initiates a single I/O operation that lasts for milliseconds. It is assumed that the computer where the processes execute has sufficient number of I/O devices and the OS of the computer assigns different I/O devices to each process. Also, the scheduling overhead of the OS is negligible. The processes have the following characteristics:
The processes A, B, and C are started at times 0, 5 and 10 milliseconds respectively, in a pure time sharing system (round robin scheduling) that uses a time slice of 50 milliseconds. The time in milliseconds at which process C would complete its first I/O operation is ___________.
| Process id | ||
|---|---|---|
| A | 100 ms | 500 ms |
| B | 350 ms | 500 ms |
| C | 200 ms | 500 ms |
Correct answer
1000 to 1000
Solution
We trace the execution using Round Robin scheduling with Time Slice () = 50 ms.Arrivals: A at 0, B at 5, C at 10.
CPU Bursts: A=100, B=350, C=200.
I/O Duration: 500 for all.Gantt Chart Trace:
CPU Bursts: A=100, B=350, C=200.
I/O Duration: 500 for all.Gantt Chart Trace:
1.0 - 50: Process A runs (A arrives at 0). A remaining: . Queue at 50: B (arrived 5), C (arrived 10), A (preempted).
2.50 - 100: Process B runs. B remaining: . Queue at 100: C, A, B.
3.100 - 150: Process C runs. C remaining: . Queue at 150: A, B, C.
4.150 - 200: Process A runs. A remaining: . A finishes CPU and starts I/O at 200. Queue at 200: B, C.
5.200 - 250: Process B runs. B remaining: . Queue at 250: C, B.
6.250 - 300: Process C runs. C remaining: . Queue at 300: B, C.
7.300 - 350: Process B runs. B remaining: . Queue at 350: C, B.
8.350 - 400: Process C runs. C remaining: . Queue at 400: B, C.
9.400 - 450: Process B runs. B remaining: . Queue at 450: C, B.
10.450 - 500: Process C runs. C remaining: . C finishes CPU and starts I/O at 500.
Process C I/O:- Starts at ms.
- Duration ms.
- Completion time = ms.
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