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GATE 2014 (Set 2) - 42 - Three processes A, B and C each

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 ___________.

Answer

The Gantt chart for Round robin algorithm for the first iteration execution for each of the 3 processes is as follows:

After finishing tc CPU ms at time 500 ms, C goes for I/O operation, that needs 500 ms more, so the time at which process C would complete its first I/O operations is 500 + 500 = 1000 ms.

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