- Asked 4/19
- 2078 Chaitra
Round robin scheduling (53, 17, 68, 24 ms)
Short answer
Round-robin with quantum 4 ms yields waiting times: P1 93, P2 52, P3 94, P4 65 ms; average waiting time = 76 ms, average turnaround time = 116.5 ms.
Consider three processes with process IDs P1, P2, P3 and P4 with estimated completion time 53, 17, 68, 24 ms respectively, enters the ready queue together in order P1, P2, P3, P4. Calculate waiting time and turn around time for each process and average waiting times and average turn around time in Round Robin algorithm with time slice 4 ms. Assume there is no I/O waiting for the process.
Round robin with a time slice of 4 ms; all four processes arrive at 0 in the order P1, P2, P3, P4. The Gantt chart is long, so the rounds are summarised below it.
Gantt chart
| P1 | P2 | P3 | P4 | P1 | P2 | P3 | P4 | P1 | P2 | P3 |
0 4 8 12 16 20 24 28 32 36 40 44
| P4 | P1 | P2 | P3 | P4 | P1 | P2 | P3 | P4 | P1 | P3 |
44 48 52 56 60 64 68 69 73 77 81 85
| P4 | P1 | P3 | P1 | P3 | P1 | P3 | P1 | P3 | P1 | P3 |
85 89 93 97 101 105 109 113 117 121 125 129
| P1 | P3 | P1 | P3 | P1 | P3 | P3 | P3 | P3 |
129 133 137 141 145 146 150 154 158 162
Rounds
- Rounds 1-4 (0-64): P1, P2, P3, P4 run 4 ms each per round (16 ms per round). Remaining after round 4: P1 37, P2 1, P3 52, P4 8.
- Round 5 (64-77): P1 64-68, P2 68-69 (needs only 1 ms, so it finishes at 69), P3 69-73, P4 73-77.
- Round 6 (77-89): P1 77-81, P3 81-85, P4 85-89 (P4 finishes at 89).
- From 89 only P1 (29 ms left) and P3 (44 ms left) remain and alternate in 4 ms slices: P1 89-93, P3 93-97, and so on.
- P1 finishes at 146 (its last slice is 1 ms, 145-146). P3 then runs alone from 146 to 162.
Calculation
TAT = completion time - arrival time; WT = TAT - burst time.
| Process | AT | BT | CT | TAT = CT - AT | WT = TAT - BT |
|---|---|---|---|---|---|
| P1 | 0 | 53 | 146 | 146 | 93 |
| P2 | 0 | 17 | 69 | 69 | 52 |
| P3 | 0 | 68 | 162 | 162 | 94 |
| P4 | 0 | 24 | 89 | 89 | 65 |
AWT = (93 + 52 + 94 + 65) / 4 = 304/4 = 76 ms
ATAT = (146 + 69 + 162 + 89) / 4 = 466/4 = 116.5 ms
Answer: AWT = 76 ms, ATAT = 116.5 ms.
✓ Answer checked by Claude Opus 5.5 (high effort)
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