Page 82 - DCAP403_Operating System

Unit 5: Scheduling

5.3.1 First-Come, First-Served (FCFS) Notes

This is a Non-Preemptive scheduling algorithm. FCFS strategy assigns priority to processes in

the order in which they request the processor. The process that requests the CPU first is allocated

the CPU first. When a process comes in, add its PCB to the tail of ready queue. When running
process terminates, dequeue the process (PCB) at head of ready queue and run it.
Figure 5.2: FCFS Scheduling

P P P P P P
1 2 3 4 5 6
3 time 4 time 2 time 3 time 3 time 5 time
units units units units units units

Advantage

Very simple

Disadvantages

1. Long average and worst-case waiting times

2. Poor dynamic behavior (convoy effect - short process behind long process)
Figure 5.3: Convey Effects

CPU

I/O
empty!

CPU-bound
I/O-bound
CPU
empty!

I/O

Example:

Process Burst Time
P 24
1
P 3
2
P 3 3

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