Name: Omofade Olusegun
Matriculation number: 13/SCI01/018
Department: Computer Science
College: Science
Date: 21/12/2014
Course: CSC 203
Course lecturer: Mrs. Gbemisola Balalola
GROUP 1
Table of Contents:
Page
1.0 Introduction
2
1.1 Definition
3
1.2 Merits and Demits
4
1.3 My analogy
5
1.4 Examples
6
1.4.1 Example 1
1.4.2 Example 2
1.4.3 Exampl3 3
1.5 Summary
9
1.6 References
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FIRST COME, FIRST SERVE.
Topic:
1.0 Introduction:
The first come, first served (commonly called FIFO ‒ first in, first out) processscheduling algorithm is the simplest process-scheduling algorithm. The FirstCome-First-Served algorithm is rarely used as a master scheme in modern
operating systems but it is often embedded within other schemes.
It is used in Batch Processing.
Other names of this algorithm are:

First-In-First-Out (FIFO)

Run-to-Completion
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
Run-Until-Done
1.1 Definition:
What does first come, first served (FCFS) mean?
First come, first served (FCFS) is an operating system process scheduling
algorithm mechanism that automatically executes queued requests and
processes by the order of their arrival. With first come, first served, what
comes first is handled first; the next request in line will be executed once the
one before it is complete.
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The first operation is processed; the next process in queue is handled once
the one before is complete.
FCFS is very simple - Just a FIFO queue, like customers waiting in line at the
bank or the post office or at a copying machine.
1.2 Merits and demerits:
Perhaps, First-Come-First-Served algorithm is the simplest scheduling
algorithm is the simplest scheduling algorithm. Processes are dispatched
according to their arrival time on the ready queue. Being a non-preemptive
discipline, once a process has a CPU, it runs to completion.
The FCFS scheduling is fair in the formal sense or human sense of fairness
but it is unfair in the sense that long jobs make short jobs wait and
unimportant jobs make important jobs wait.
FCFS is more predictable than most of other schemes since it offers time.
FCFS scheme is not useful in scheduling interactive users because it cannot
guarantee good response time. The code for FCFS scheduling is simple to
write and understand.
Advantages/Merits:
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
Easy to understand

FIFO scheduling is simple to implement.

It is also intuitively fair (the first one in line gets to run first).

It is the simplest CPU scheduling algorithm.
Disadvantages/Demerits:

This scheduling method is non preemptive, that is, the process will run
until it finishes. Because of this non-preemptive scheduling, It is not
suitable for interactive jobs.

In addition, short processes, which are at the back of the queue, have
to wait for the long process at the front to finish execution. Which
mean that a long-running process will delay all jobs behind it.

The average waiting time under FCFS is long.

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.
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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.
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.
In general, a convoy effect happens when a se 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.
1.3 My analogy of FCFS:
An example is to use a printer; several documents are to be printed from a
computer system, which are sent to the printer. The printer queues the
documents to be printed upon arrival and prints the first document that
arrived following the others in line.
Steps:
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1. Here the “Open Printer Queue” button is selected.
2. Here 3 Documents are sent to the printer to be printed.
3. Here the 3 documents are printed by First come, first serve.
ORDER OF
EXECUTION
1
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2
3
Explanation:
The first document (OPERATION) OMOFADE SEGUN.docx is printed first
(EXECUTED), following is the second Document (OPERATION) Document
1 which is printed (EXECUTED) after the completion of the printing of the
document before it, lastly to be printed (EXECUTED) is the document
(OPERATION) TRAVERSING OF AN ARRAY.docx which comes last in
order.
1.4 Examples:
Let's take a look at how FCFS process scheduling works.
1.4.1 Example 1:
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Suppose there are three processes in a queue: P1, P2 and P3.
P1 is placed in the processing register with a waiting time of zero seconds
and 10 seconds for complete processing.
The next process, P2, must wait 10 seconds and is placed in the processing
cycle until P1 is processed. Assuming that P2 will take 15 seconds to
complete,
The final process, P3, must wait 25 seconds to be processed.
In first come first serve scheduling the jobs are served in the order they arrive.
1.4.2 Example 2:
Let’s make a diagrammatic representation of how the jobs will look in a
relative time scale. Since Job A was the first one to arrive so Job A is
scheduled first. At time t=5, job A finishes its execution. The second job to
arrive was job B so job B is scheduled then and so on till all the jobs execute
and exit. This is represented graphically as shown below:
This graphical representation of the execution of jobs is called a Gantt chart.
FCFS may not be the fastest process-scheduling algorithm, as it does not
check for priorities associated with processes. These priorities may depend on
the processes' individual execution times.
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1.4.3:
a.
b.
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Possibly the most straightforward approach to scheduling processes is to
maintain a FIFO (first-in, first-out) run queue. New processes go to the end of
the queue. When the scheduler needs to run a process, it picks the process
that is at the head of the queue.
The scheduler is non-preemptive. If the process has to block on I/O, it enters
the waiting state and the scheduler picks the process from the head of the
queue. When I/O is complete and the process is ready to run again, it gets
put at the end of the queue.
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1.5 Summary:
FCFS provides an efficient, simple and error-free process-scheduling algorithm
that saves valuable CPU resources. It uses non-preemptive scheduling in
which a process is automatically queued and processing occurs according to
an incoming request or process order. FCFS derives its concept from real-life
customer service where the first person is attended to following the second
person and so on. In FCFS, a process that arrives first will be
served first. When it arrives at the same time, the services performed by the
order they are in the queue. It does not matter whether they
are short or a long process. To be able to be serviced by the
processor, process the queue behind must wait until all processes carried
out in front of it completed.
1.6 References:

http://naharhasyim.blogspot.com/2013/03/algorithm-first-come-firstserved.html

http://www.os-concepts.thiyagaraaj.com/scheduling-algorithms/firstcome-first-served--fcfs--scheduling

http://tanmayonrun.blogspot.com/2011/12/explain-and-solve-firstcome-first.html

http://www.tutorialspoint.com/operating_system/os_process_schedul
ing_algorithms.htm

http://www.gitam.edu/eresource/comp/gvr(os)/5.3.htm

http://en.wikipedia.org/wiki/Scheduling_(computing)
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Merry Christmas.
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