Question 37

Preemptive Shortest Remaining Time First (SRTF):

• $t = 0 \to 2$: A runs for $2$ units (remaining burst $= 8$). At $t = 2$, B arrives with burst $6 < 8$ and preempts A.
• $t = 2 \to 4$: B runs for $2$ units (remaining burst $= 4$). At $t = 4$, C arrives with burst $3 < 4$ and preempts B.
• $t = 4 \to 7$: C runs to completion at $t = 7$ (D arrives at $t = 6$ with burst $7$).
• $t = 7 \to 11$: B (remaining $4$) runs to completion at $t = 11$.
• $t = 11 \to 18$: D (burst $7 < 8$) runs to completion at $t = 18$.
• $t = 18 \to 26$: A (remaining $8$) runs to completion at $t = 26$.
• Waiting times ($\text{WT} = \text{CT} - \text{AT} - \text{BT}$):
  • $\text{WT}_A = 26 - 0 - 10 = 16$
  • $\text{WT}_B = 11 - 2 - 6 = 3$
  • $\text{WT}_C = 7 - 4 - 3 = 0$
  • $\text{WT}_D = 18 - 6 - 7 = 5$
• Average WT (SRTF) $= \frac{16 + 3 + 0 + 5}{4} = \frac{24}{4} = 6$.

Non-Preemptive Shortest Job First (NP-SJF):

• $t = 0 \to 10$: A starts at $t = 0$ and runs to completion at $t = 10$ ($\text{WT}_A = 0$).
• At $t = 10$, B (burst $6$), C (burst $3$), and D (burst $7$) are all in the ready queue; they execute in order of burst time: C ($10 \to 13$), B ($13 \to 19$), D ($19 \to 26$).
• Waiting times:
  • $\text{WT}_A = 0$
  • $\text{WT}_C = 10 - 4 = 6$
  • $\text{WT}_B = 13 - 2 = 11$
  • $\text{WT}_D = 19 - 6 = 13$
• Average WT (NP-SJF) $= \frac{0 + 6 + 11 + 13}{4} = \frac{30}{4} = 7.5$.