Average wait time for pre-emptive priority scheduling

Question

Given the following table for calculating and average waiting time for the processes for priority based preemptive scheduling.

Process     Burst Time       Priority          
P1             10               3  
P2             1                1  
P3             2                4
P4             1                5
P5             5                2

The gantt chart would be as follows:

| P2 | P5 | P1 | P3 | P4  |
0    1    6    16   18   19

I have following questions:

1) Is the turn around time = 19 units?

2) How do i calculate average waiting time? Is there a formula?

3) What if few processes have the same priority?

I am new to OS. I have viewed some other similar questions but I did not get exactly how to do it.

Solution

Given the data,before you have to implement priority based preemptive scheduling, you should know the following facts :-

1. priorities are usually numeric over a range
2. high numbers may indicate low priority (system dependent)
3. associate a priority with each process, allocate the CPU to the process with the highest priority
4. any 2 processes with the same priority are handled FCFS

Proceeding with this much of knowledge,the required Gantt chart would be the same as what you've drawn:-

| P2 | P5 | P1 | P3 | P4  |
0    1    6    16   18   19

1) Is the turn around time = 19 units?

No, the turnaround time will be 16 + 1 + 18 + 19 + 6 = 60. Average turnaround time = 60 / 5 = 12.

2) How do i calculate average waiting time? Is there a formula?

Average waiting time is defined as the sum of total time waited before starting of the processes divided by the total number of processes.

Here, average waiting time = (6 + 0 + 16 + 18 + 1) / 5 = 41 / 5 = 8.2.

3) What if few processes have the same priority?

If the few processes will have same priority then the scheduling would be handled using First-Come First-Serve (FCFS) as mentioned in the 4th point above. So, everywhere including Gantt chart, the process coming first will be scheduled first and the other similar-priority process would be scheduled late as it came arrived late.

I hope it is crystal clear from my steps and doesn't need any further explanation.

Comments

[Anant Vaibhav]

Thanks for in depth explanation. Could you please tell me what more variation could be done in these questions other than same priorities? Like Another column of 'ARRIVAL TIME' is added. How do I tackle that?

[Am_I_Helpful]

The only change to be done is that processes to be arranged should be arranged in order of their incoming order. Everything rest willremain same. I'd encourage you to post a new question regarding that if you face a new problem. I'll surely lend my help.