Q152: CPU Idle time in SRTF scheduling

Posted on by bytesoftheday

Question 152: Consider 3 processes, all arriving at time zero with total execution time of 10, 20, and 30 units respective each process spends the first 20% of execution time doing I/O, the next 70% of time doing computation and last 10% of time doing I/O again. OS uses SRTF scheduling algorithm. Assuming I/O of processes can be overlapped, what percentage of CPU time will remain idle?

Options:

  1. 0%
  2. 30%
  3. 6%
  4. 4%

Solution: The correct answer is the third one. Processes will run as follows:

0-2: Idle

2-9: P1

9-23: P2

23-44: P3

44-47: Idle

Idle time = 5 / 47 * 100 = 10.6%