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GATE | GATE-CS-2014-(Set-2) | Question 42

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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 tc CPU milliseconds and then initiates a single I/O operation that lasts for tio 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:

 Process id      tc           tio
     A        100 ms    500 ms
     B        350 ms    500 ms
     C        200 ms    500 ms

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 ___________.

(A)

500

(B)

1000

(C)

2000

(D)

10000


Answer: (B)

Explanation:

There are three processes A, B and C that run in 
round robin manner with time slice of 50 ms.

Processes start at 0, 5 and 10 milliseconds.

The processes are executed in below order
A, B, C, A 
50 + 50 + 50 + 50 (200 ms passed)

Now A has completed 100 ms of computations and 
goes for I/O now

B, C, B, C, B, C
50 + 50 + 50 + 50 + 50 + 50 (300 ms passed)

C goes for i/o at 500ms and it needs 500ms to
finish the IO.

So C would complete its first IO at 1000 ms


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Last Updated : 06 Sep, 2021
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