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CAVITE STATE UNIVERSITY
T3 CAMPUS
Department of Information Technology ITEC 55 – Platform Technologies

Non-preemptive Scheduling
A scheduling is non-preemptive if, once the CPU has been assigned to a process and
the process starts executing, the CPU cannot be taken away from that process.
Following are some of the characteristics of non-preemptive scheduling:
1. In non-preemptive system, short jobs are made to wait by longer jobs but the overall
treatment of all processes is fair.
2. In non-preemptive system, response times are more predictable because incoming
high priority jobs cannot displace waiting jobs.
3. In non-preemptive scheduling, a schedular executes jobs in the following two
situations:
a. When a process switches from running state to the waiting state.
b. When a process terminates.

Preemptive Scheduling
In preemptive scheduling, even though the CPU has been assigned to a process and
the process is already executing, the CPU scheduler may decide to assign the CPU to another
process in the ready queue.
The strategy of allowing processes that are logically runnable to be temporarily
suspended is called Preemptive Scheduling and it is contrast to the “run to completion”
method.

First-Come-First-Serve (FCFS) Scheduling
It is a non-preemptive scheduling algorithm wherein the one that enters the ready
queue first gets to be executed by the CPU first. In choosing the next process to be executed,
the CPU scheduler selects the process at the front of the ready queue. Processes enter at the
rear of the ready queue.
First-Come-First-Serve algorithm is the simplest scheduling algorithm. Processes are
dispatched according to their arrival time on the ready queue. Being a non-preemptive
scheduling, once a process has the CPU, it runs to completion.

Let’s take an example of the FCFS scheduling algorithm. In the following schedule,
there are 5 processes with process ID A, B, C, D and E. A arrives at time 0, B arrives at time
1, C arrives at time 2, D arrives at time 3, and E arrives at time 4 in the ready queue. The
processes and their respective Arrival Time and Burst Time are given in the following table.
To calculate the completion time, turnaround time, waiting time, average waiting time
and average turnaround time, use the following formula:

Completion Time = Total Amount of time spent by the process
Turnaround Time = Completion Time – Arrival Time
Waiting Time = Turnaround Time – Burst Time
Average Turnaround Time = Sum of Turnaround Tome / No. of Process
Average Waiting Time = Sum of Waiting Time / No. of Process

Completion Turnaround Waiting
Process ID Arrival Time Burst Time
Time Time Time
A 0 2 2 2 0
B 1 6 8 7 1
C 2 4 12 10 6
D 3 9 21 18 9
E 4 12 33 29 17

A B C D E

Gantt Chart

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