FIRST-COME,FIRST-SERVED SCHEDULING

Scheduling Algorithms:

 

CPU scheduling deals with deciding which processes in the ready queue to be allocated the CPU.

1. First -Come.First-Serve Scheduling

2. Shortest Job-First Scheduling

3.Priority Scheduling

4.Round-Robin Scheduling

5.Multilevel Queue Scheduling

6.Multilevel Feedback Queue Scheduling

First -Come.First-Serve Scheduling:

• FCFS is very simple - Just like a FIFO queue
• code for FCFS scheduling is simple to write and understand
• is non pre-emptive
• FCFS is troublesome for time sharing systems.
• FCFS can take very long waiting times,particularly if the first process to get there takes a long time.

For example, consider the following three processes:

Process	Burst Time
P1	24
P2	3
P3	3

If the processes arrive in the order P1,P2,P3 and are served in FCFS order,we get the result in the following Gantt Chart:

P1

P2

P3

0                                     24               27              30

The Waiting time for process P1 is 0 milliseconds.

The Waiting time for process P2 is 24 milliseconds.

The Waiting time for process P3 is 27 milliseconds.

The Average Waiting time is(0+24+27)/3=17 milliseconds.

Convey Effect:

 

FCFS can also block the system in a busy dynamic system in another way, known as the convoy effect. When one CPU intensive process blocks the CPU, a number of I/O intensive processes can get backed up behind it, leaving the I/O devices idle. When the CPU hog finally relinquishes the CPU, then the I/O processes pass through the CPU quickly, leaving the CPU idle while everyone queues up for I/O, and then the cycle repeats itself when the CPU intensive process gets back to the ready queue.