This document discusses the First Come First Served (FCFS) CPU scheduling algorithm. FCFS is a non-preemptive algorithm that allocates the CPU to the process that requests it first. It is simple to implement using a first-in first-out queue, but it is not optimal and can result in longer waiting times for shorter processes. While easy to understand, FCFS is not suitable for time-sharing systems due to its non-preemptive nature.

1. ‫الرحيم‬ ‫الرحمن‬ ‫هللا‬ ‫بسم‬
Elmashreq University
Faculty of engineering
Electronics engineering dept
Prepared by: supervisor:
1.

2. Introduction:
Process that requests the CPU FIRST is allocated the CPU FIRST.
It’s simplest to understand and the simplest to implement .
Other names of this algorithm are:
• First-In-First-Out (FIFO) .
• Run-to-Completion .
• Run-Until-Done .
 This is a non-preemptive algorithm.
 Example on real life such as: Buying tickets !
 Example on computer system likes: sharing printers.
 Implementation:
• FIFO queues.
• A new process enters the tail of the queue.
• The schedule selects from the head of the queue.

4. Scheduling Algorithms:
First-Come, First-Served (FCFS)
• Example: Three processes arrive in order P1, P2, P3.
– P1 burst time: 24
– P2 burst time: 3
– P3 burst time: 3
• Waiting Time
– P1: 0
– P2: 24
– P3: 27
• Completion Time:
– P1: 24
– P2: 27
– P3: 30
• Average Waiting Time: (0+24+27)/3 = 17
• Average Completion Time: (24+27+30)/3 = 27
P1 P2 P3
0 24 27 30

5. Problems with FCFS:
o Non-preemptive.
o Not optimal AWT.
 Cannot utilize resources in parallel:
• Assume 1 process CPU bounded and many I/O
bounded processes.
 result: Convoy effect, low CPU and I/O Device
utilization.
 a convoy effect happens when a set of processes
need to use a resource for a short time, and one
process holds the resource for a long time, blocking
all of the other processes. Essentially, it causes poor
utilization of the other resources in the system.

6. Advantages and Disadvantages:
Advantages:
• simple
• easy to understand
• first come, first served
Disadvantages:
• Non-preemptive, that is, the process will run until it
finishes.
• short processes which are at the back of the queue
have to wait for the long process at the front to
finish.
• Not good for time sharing systems.
• Because of its simplicity, FCFS is not very efficient.