In CPU Scheduling, we often need to find the average Turnaround and Waiting Time with the help of Arrival, Burst and Completion Time. Let’s have a brief look of them: Turnaround Time (TAT):
- It is the time interval from the time of submission of a process to the time of the completion of the process.
- The difference b/w Completion Time and Arrival Time is called Turnaround Time.
Completion Time (CT): This is the time when the process completes its execution. Arrival Time (AT): This is the time when the process has arrived in the ready state.
TAT = CT - AT
Waiting Time (WT):
- The time spent by a process waiting in the ready queue for getting the CPU.
- The time difference b/w Turnaround Time and Burst Time is called Waiting Time.
Burst Time (BT): This is the time required by the process for its execution.
WT = TAT - BT
Now with Waiting Time and Burst Time, we can also calculate Turn Around Time via:
TAT = BT + WT
Example:
| Process | Burst Time (in sec.) |
|---|---|
| P1 | 24 |
| P2 | 3 |
| P3 | 4 |
Solution: Figure – Gantt Chart
Avg. TAT = (24 + 27 + 31) / 3 = 27.33 sec Avg. WT = (0 + 24 + 27) / 3 = 17.0 sec
Let’s see the difference between Turnaround Time and Waiting Time:
| Turn Around Time | Waiting Time |
|---|---|
| The time since the process entered into ready queue for execution till the process completed it’s execution. | The time process spent in the ready queue and for I/O completion. |
| Different CPU Scheduling algorithms produce different TAT for the same set of processes. | CPU Scheduling Algorithm doesn’t affect the amount of time during which a process executes or does I/O but only the amount of time that a process spends waiting in the ready queue. |
| The turnaround time is generally limited by the speed of the output device. | Waiting time has no such major effect. |
| Turn Around Time = Burst time + Waiting time. | Waiting time = Turn Around – Burst time. |
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