First Come First Serve

FIRST COME FIRST SERVE
SUBMITTED BY:
Harmanjit Dhillon(B130010121)
Manav Khurana (B130010122)
Leepika Kapoor (B130010123)
Priyanka (B130010125)
Kanav Sood (B130010126)

INTRODUCTION :
• First-Come First-Serve (FCFS)
– One ready queue;
– OS runs the process at head of the queue;
– New processes come in at the end of the queue;
– Running process does not give up the CPU until it
terminates or it performs IO.
• Round Robin
– Process runs for one time slice, then moved to back of the
queue;
– Each process gets equal share of the CPU.

• Shortest Time to Completion (STCF)
– Process with shortest computation time left is picked;
– Varianted by preemption;
– Requires knowledge of the future.
• Exponential Queue (Multi-level Feedback)
– Gives newly runnable processes a high priority and a very
short time slice;
– If process uses up the time slice without blocking then
decrease priority by one and double time slice for next time;
– Solves both efficiency and response time problems.

Scheduling Algorithms
First-Come First-Serve
• Non-preemptive FCFS (no priority scheme)
– Simplest implementation of scheduling
algorithms
– Used on timeshared systems (with timer
interruption)
• Non-preemptive FCFS (with priority scheme)
– Next highest priority process is picked
when CPU is yielded

 Once process grabs CPU,
former keeps latter until
completion
Rarely used in real-time
 Preemptive FCFS (with
priority scheme)

Round Robin
• Used mostly on timeshared
systems
• Allows multiple users slices
of the CPU on a “round
robin” basis
• Majority of users have the
same priority
• Not a popular scheme with
dynamic priority systems

Shortest Time to Completion
• Priorities are assigned in inverse order of time needed for completion
of the entire job
• Minimizes average turnaround time
• Exponential averaging is used to estimate the process’ burst duration
• A job exceeding the resource estimation is aborted
• A job exceeding the time estimation is preempted
• Store estimated value in PCB for the current burst, and compare with
actual value

Exponential Queues
• Popular in interactive systems
• A single queue is maintained for each
priority level
• A new process is added at the end of the
highest priority queue
– It is alloted a single time quantum when
it reaches the front
• If it yields the CPU within the time
quantum, it is moved to the rear
• If not, it is placed at the rear of the next
queue down
• Dispatcher selects the head of the
highest priority queue
– A job that “succeeds” moves up
– A job that “fails” moves down

Evaluated Program And
Parameters
The Program helps us find the total
time a person has worked on to set
up his career and waited to work at
the position he desired to in his
life.

Parameters:
1.Number Of Processes: The total number of
tasks he has performed to achieve his job and
his job as well
2.Process Name: The process he has worked on
in his entire life.
3.Process Time: The time he has spent on one
process.
4.Waiting Time: The time he has waited to
enter every task.

5. Total Waiting Time:The Total amount of
time waited for this event, in hundredths of
a second.
6. Average Waiting Time:The total amount of
waiting time divided by the number of
incoming calls answered.

Program Worked On
• #include<stdio.h>
• #include<conio.h>
• int main()
• {
• char p[100][100];
• int tot=0,wt[10],pt[10],z,n,i;
• float avg=0;
• printf("n enter number of process :");
• scanf("%d",&n);
• wt[0]=0;

• for(i=1;i<=n;i++)
• {
• printf("n enter process %d name : ",i);
• scanf("%s",&p[i-1]);
• printf("enter process time : ");
• scanf("%d",&pt[i-1]);
• wt[i]=wt[i-1]+pt[i-1];
• tot=tot+wt[i];
• }
• avg=(float)tot/n;
• printf("n p-nametp-timetw-time n");

• for (i=0;i<n;i++)
• printf("%st%dt%dn",p[i],pt[i],wt[i]);
• printf("n total waiting time : %d n avg
wating time : %f nn ",tot ,avg);
• getch();
• }

Implementation - Analysis
• Direct analysis
– Pick a task set and observe results
• Apply queuing theory to obtain results
– Multi-level feedback queue scheme
• Simulations of scheme implementations
– FCFS, RR, STCF
• Innovations and projections
– “Lottery” scheduling and “own” algorithm

References
1) Cooling, J.E. Software Design for Real-Time Systems. Chapman &
Hall, London, UK: 1995.
2) Stallings, William. Operating Systems: Internals and Design
Principles. Upper Saddle River, NJ: Prentice Hall, 1998.
3) http://www.cs.wisc.edu/~bart/537/lecturenotes/s11.html - viewed on
03/24/2000
4) Savitzky, Stephen. Real-Time Microprocessor Systems. Van Nostrand
Reinhold Company, N.Y.: 1985.
5) Undergraduate Operating System Course Notes (Ottawa University,
1998)

First Come First Serve

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