Examples of Round Robin CPU Scheduling algorithm

 Evaluate Round Robin  CPU Scheduling algorithm for given Problem Time slice =3 ms.

Process	P1	P2	P3	P4
Process Time	10	5	18	6
Arrival Time	5	3	0	4

Given:

• Time Slice (Quantum): 3 ms
• Processes:
  • P1: Burst Time = 10 ms, Arrival Time = 5 ms
  • P2: Burst Time = 5 ms, Arrival Time = 3 ms
  • P3: Burst Time = 18 ms, Arrival Time = 0 ms
  • P4: Burst Time = 6 ms, Arrival Time = 4 ms

Step 1: Order of Execution

We will simulate the round-robin scheduling process:

1. Process Arrival:

   • P3 arrives at time 0 ms. Burst Time = 18 ms
   • P2 arrives at time 3 ms. Burst Time = 5 ms
   • P4 arrives at time 4 ms. Burst Time = 6 ms
   • P1 arrives at time 5 ms. Burst Time = 10 ms

2. Time Chart: We will execute each process for up to 3 ms (time slice) in a round-robin manner, continuing until all processes are completed.

   Time (ms)	Process Executed	Remaining Time of Process
   0 - 3	P3	P3 (15 ms left)
   3 - 6	P2	P2 (2 ms left)
   6 - 9	P4	P4 (3 ms left)
   9 - 12	P1	P1 (7 ms left)
   12 - 15	P3	P3 (12 ms left)
   15 - 17	P2	P2 (0 ms, completed)
   17 - 20	P4	P4 (0 ms, completed)
   20 - 23	P1	P1 (4 ms left)
   23 - 26	P3	P3 (9 ms left)
   26 - 29	P1	P1 (1 ms left)
   29 - 32	P3	P3 (6 ms left)
   32 - 33	P1	P1 (0 ms, completed)
   33 - 36	P3	P3 (3 ms left)
   36 - 39	P3	P3 (0 ms, completed)

Step 2: Calculate Completion Time, Turnaround Time, and Waiting Time

Completion Time (CT): The time at which each process completes execution.

Turnaround Time (TAT): The total time taken from arrival to completion of a process.
$\text{TAT} = \text{Completion Time} - \text{Arrival Time}$

Waiting Time (WT): The total time the process spends waiting in the ready queue.
$\text{WT} = \text{Turnaround Time} - \text{Burst Time}$

Process	Arrival Time (AT)	Burst Time (BT)	Completion Time (CT)	Turnaround Time (TAT)	Waiting Time (WT)
P1	5 ms	10 ms	33 ms	28 ms	18 ms
P2	3 ms	5 ms	17 ms	14 ms	9 ms
P3	0 ms	18 ms	39 ms	39 ms	21 ms
P4	4 ms	6 ms	20 ms	16 ms	10 ms

Step 3: Summary of Metrics

Step 3: Summary of Metrics

• Average Turnaround Time (TAT): $$\text{Average TAT} = \frac{28 + 14 + 39 + 16}{4} = 24.25 \text{ ms}$$
  
• Average Waiting Time (WT): $$\text{Average WT} = \frac{18 + 9 + 21 + 10}{4} = 14.5 \text{ ms}$$
  

• Completion Order: P2, P4, P1, P3
• Average Turnaround Time: 24.25 ms
• Average Waiting Time: 14.5 ms

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Consider 3 processes P1, P2 and P3, which require 5, 7 and 4 time units and arrive at time 0, 1 and 3. Draw the Gant chart, process completion sequence and average waiting time for. Round robin scheduling with CPU quantum of 2 time units.

Given:

• Processes:
  • P1: Burst Time = 5 ms, Arrival Time = 0 ms
  • P2: Burst Time = 7 ms, Arrival Time = 1 ms
  • P3: Burst Time = 4 ms, Arrival Time = 3 ms
• Time Quantum: 2 ms

Step 1: Gantt Chart

1. Initial State:

   • P1 arrives at 0 ms.
   • P2 arrives at 1 ms.
   • P3 arrives at 3 ms.

2. Execution Sequence:

   • Time 0 - 2: P1 executes (Remaining Burst Time: P1 = 3 ms).
   • Time 2 - 4: P2 executes (Remaining Burst Time: P2 = 5 ms).
   • Time 4 - 6: P3 executes (Remaining Burst Time: P3 = 2 ms).
   • Time 6 - 8: P1 executes (Remaining Burst Time: P1 = 1 ms).
   • Time 8 - 10: P2 executes (Remaining Burst Time: P2 = 3 ms).
   • Time 10 - 12: P3 executes (Remaining Burst Time: P3 = 0 ms, completed).
   • Time 12 - 14: P1 executes (Remaining Burst Time: P1 = 0 ms, completed).
   • Time 14 - 16: P2 executes (Remaining Burst Time: P2 = 1 ms).
   • Time 16 - 17: P2 completes (Remaining Burst Time: P2 = 0 ms).

Gantt Chart:

Step 2: Completion Time (CT), Turnaround Time (TAT), and Waiting Time (WT)

Completion Time (CT): The time at which each process completes its execution.

Turnaround Time (TAT): The total time taken from arrival to completion. $\text{TAT} = \text{Completion Time (CT)} - \text{Arrival Time (AT)}$

Waiting Time (WT): The total time a process spends waiting in the ready queue. $\text{WT} = \text{Turnaround Time (TAT)} - \text{Burst Time (BT)}$

Process	Arrival Time (AT)	Burst Time (BT)	Completion Time (CT)	Turnaround Time (TAT)	Waiting Time (WT)
P1	0 ms	5 ms	14 ms	14 ms	9 ms
P2	1 ms	7 ms	17 ms	16 ms	9 ms
P3	3 ms	4 ms	12 ms	9 ms	5 ms

Step 3: Average Waiting Time

• Average Waiting Time (WT): $$\text{Average WT} = \frac{9 + 9 + 5}{3} = 7.67 \text{ ms}$$
  

• Completion Time (CT): P1 = 14 ms, P2 = 17 ms, P3 = 12 ms

• Average Turnaround Time (TAT): 13 ms

• Average Waiting Time (WT): 7.67 ms