Question 57

Construct the timeline of process execution under preemptive SRTF scheduling:

• At $t = 0$: $P_1$ arrives with burst 10. $P_1$ starts executing.
• At $t = 3$: $P_2$ arrives with burst 6. Remaining burst of $P_1$ is $10 - 3 = 7$. Since remaining time $P_2 (6) < P_1 (7)$, $P_1$ is preempted and $P_2$ runs.
• At $t = 7$: $P_3$ arrives with burst 1. $P_2$ has executed for 4 units, so its remaining burst is $6 - 4 = 2$. Compare remaining bursts: $P_3 (1) < P_2 (2) < P_1 (7)$. Thus, $P_2$ is preempted and $P_3$ runs.
• At $t = 8$: $P_3$ completes ($C_3 = 8$). Concurrently, $P_4$ arrives with burst 3. Ready queue remaining bursts: $P_2 = 2, P_4 = 3, P_1 = 7$. Smallest is $P_2$ (2), so $P_2$ resumes.
• At $t = 10$: $P_2$ completes ($C_2 = 10$). Ready queue remaining bursts: $P_4 = 3, P_1 = 7$. Smallest is $P_4$ (3), so $P_4$ runs.
• At $t = 13$: $P_4$ completes ($C_4 = 13$). Only $P_1$ remains with 7 units.
• At $t = 20$: $P_1$ finishes execution ($C_1 = 20$).

Calculate turnaround time ($TAT = \text{Completion Time} - \text{Arrival Time}$) for each process:

• $P_1$: $TAT_1 = 20 - 0 = 20\text{ ms}$
• $P_2$: $TAT_2 = 10 - 3 = 7\text{ ms}$
• $P_3$: $TAT_3 = 8 - 7 = 1\text{ ms}$
• $P_4$: $TAT_4 = 13 - 8 = 5\text{ ms}$

Calculate the average turnaround time: