GATE CS 2015 Set 1 — Question 52
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Operating System → Process Scheduling → Priority Scheduling
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Question
Consider a uniprocessor system executing three tasks and , 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 and requires an execution time of 1, 2 and 4 milliseconds, respectively. Given that all tasks initially arrive at the beginning of the millisecond and task preemptions are allowed, the first instance of completes its execution at the end of __________ milliseconds.
Correct answer
12 to 12
Solution
We are given three tasks with periods and execution times :
- . Priority: High (1/3).
- . Priority: Medium (1/7).
- . Priority: Low (1/20).
- t=0: arrive. runs (highest priority).
- t=1: finishes. runs.
- t=3: finishes (ran for 2ms). arrives again. runs.
- t=4: finishes. runs (first time).
- t=6: has run for 2ms (needs 2 more). arrives. is preempted. runs.
- t=7: finishes. arrives. runs.
- t=9: finishes (ran for 2ms). arrives. runs.
- t=10: finishes. resumes.
- t=12: runs for 2ms (total 2+2=4ms). finishes.
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