GATE Papers >> CSE >> 2015 >> Question No 52

Question No. 52 CSE | GATE 2015

Consider a uniprocessor system executing three tasks T1, T2 and T3, each of which is composed of an infinite sequence of jobs (or instances) which arrive periodically at intervals of 3, 7 and 20 milliseconds, respectively. The priority of each task is the inverse of its period, and the available tasks are scheduled in order of priority, with the highest priority task scheduled first. Each instance of T1, T2 and T3 requires an execution time of 1, 2 and 4 milliseconds, respectively. Given that all tasks initially arrive at the beginning of the 1st millisecond and task preemptions are allowed, the first instance of T3 completes its execution at the end of ______ milliseconds.


Answer : 12


Solution of Question No 52 of GATE 2015 CSE Paper

Periodic arrival times of T1: 0, 3, 6, 9, 12, 15 18, 21,...

Priority of T1=1/3, service time of T1 = 1.

Periodic arrival times of T2: 0, 7, 14, 21,...

Priority of T2= 1/7, service time of T2 = 2.

Periodic arrival times of T3: 0, 20, 40,...

Priority of T3=1/20, service time of T3 = 4.

T1 has highest priority and T3 has lowest priority.

First instance of T3 (4 units) completed at the end of 12 ms.

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