GATE - 2015 | OS | Consider a uniprocessor system executing three tasks T1, T2 and T3

GATE - 2015 [Operating System]

Question:

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.

A	13
B	12
C	15
D	16

  

Solution:

Option (B) is Correct.

All the processes or tasks are available at the beginning of 1st millisecond, which means at t=0. So, the Gantt chart will be as follows:

∴ At the end of 12 milliseconds, 1st instance of T₃ will complete its execution.

Thank You