Priority Scheduling | CPU Scheduling | Examples

Priority Scheduling-

 

In Priority Scheduling,

  • Out of all the available processes, CPU is assigned to the process having the highest priority.
  • In case of a tie, it is broken by FCFS Scheduling.

 

 

  • Priority Scheduling can be used in both preemptive and non-preemptive mode.

 

Advantages-

 

  • It considers the priority of the processes and allows the important processes to run first.
  • Priority scheduling in preemptive mode is best suited for real time operating system.

 

Disadvantages-

 

  • Processes with lesser priority may starve for CPU.
  • There is no idea of response time and waiting time.

 

Important Notes-

 

Note-01:

 

  • The waiting time for the process having the highest priority will always be zero in preemptive mode.
  • The waiting time for the process having the highest priority may not be zero in non-preemptive mode.

 

Note-02:

 

Priority scheduling in preemptive and non-preemptive mode behaves exactly same under following conditions-

  • The arrival time of all the processes is same
  • All the processes become available

 

PRACTICE PROBLEMS BASED ON PRIORITY SCHEDULING-

 

Problem-01:

 

Consider the set of 5 processes whose arrival time and burst time are given below-

 

Process Id Arrival time Burst time Priority
P1 0 4 2
P2 1 3 3
P3 2 1 4
P4 3 5 5
P5 4 2 5

 

If the CPU scheduling policy is priority non-preemptive, calculate the average waiting time and average turn around time. (Higher number represents higher priority)

 

Solution-

 

Gantt Chart-

 

 

Now, we know-

  • Turn Around time = Exit time – Arrival time
  • Waiting time = Turn Around time – Burst time

 

Also read- Various Times of Process

 

Process Id Exit time Turn Around time Waiting time
P1 4 4 – 0 = 4 4 – 4 = 0
P2 15 15 – 1 = 14 14 – 3 = 11
P3 12 12 – 2 = 10 10 – 1 = 9
P4 9 9 – 3 = 6 6 – 5 = 1
P5 11 11 – 4 = 7 7 – 2 = 5

 

Now,

  • Average Turn Around time = (4 + 14 + 10 + 6 + 7) / 5 = 41 / 5 = 8.2 unit
  • Average waiting time = (0 + 11 + 9 + 1 + 5) / 5 = 26 / 5 = 5.2 unit

 

Problem-02:

 

Consider the set of 5 processes whose arrival time and burst time are given below-

 

Process Id Arrival time Burst time Priority
P1 0 4 2
P2 1 3 3
P3 2 1 4
P4 3 5 5
P5 4 2 5

 

If the CPU scheduling policy is priority preemptive, calculate the average waiting time and average turn around time. (Higher number represents higher priority)

 

Solution-

 

Gantt Chart-

 

 

Now, we know-

  • Turn Around time = Exit time – Arrival time
  • Waiting time = Turn Around time – Burst time

 

Process Id Exit time Turn Around time Waiting time
P1 15 15 – 0 = 15 15 – 4 = 11
P2 12 12 – 1 = 11 11 – 3 = 8
P3 3 3 – 2 = 1 1 – 1 = 0
P4 8 8 – 3 = 5 5 – 5 = 0
P5 10 10 – 4 = 6 6 – 2 = 4

 

Now,

  • Average Turn Around time = (15 + 11 + 1 + 5 + 6) / 5 = 38 / 5 = 7.6 unit
  • Average waiting time = (11 + 8 + 0 + 0 + 4) / 5 = 23 / 5 = 4.6 unit

 

To gain better understanding about Priority Scheduling,

Watch this Video Lecture

 

Next Article- Practice Problems On CPU Scheduling Algorithms

 

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Summary
Priority Scheduling | CPU Scheduling | Examples
Article Name
Priority Scheduling | CPU Scheduling | Examples
Description
Priority Scheduling is a CPU Scheduling Algorithm that assigns CPU to the process having the highest priority. Priority Scheduling Preemptive and Non-preemptive Examples.
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Gate Vidyalay
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