By applying this scheduling algorithm , the CPU makes sure that the process which is run by user are lined
in queue , just like the queue for buying tickets of movie . The person who comes first , will have the chance to get the ticket , similarly , if CPU is idle and CPU is using First come and First Serve algorithm then ,it
executes the process which arrives first .
Read Also : Langton's Ant : Java Program Code Explain
Here , User can calculate the average turnaround time and average waiting time along with the starting and finishing time of each process
Turnaround time : Its the total time taken by the process between starting and the completion
Waiting time : Its the time for which process is ready to run but not executed by CPU scheduler
for example ,
we have three processes
Burst time Waiting time Turnaround time
P1 18 0 18
P2 5 18 23
P3 7 23 30
So here we can see the turnaround time for the process 1 is 18 while 23 and 30 for 2nd and 3rd process
Gantt chart for the turnaround time is
|----------------------------------------------------|------------------|------------------------|
| P1 | P2 | P3 |
|----------------------------------------------------|------------------|------------------------|
0 18 23 30
A Gantt chart is a chart which shows the start and finish times of all the processes .
Also first come first serve algorithm is non preemptive so if the process starts then the other process has to wait in the queue till the executing process finishes .
The major features of the First come first serve algorithm is that
* Throughput is low as the large process is holding up the Central processing unit for execution .
* The main disadvantage of FCFS is starving . As long as all processes completes the execution then we
dont have any trouble, But the problem starts when any of the process fails to complete . The incomplete
execution of any process leads to starvation .
* Queuing is done without using any prioritization of the processes.
Demo :
import java.util.ArrayList; import java.util.Collection; import java.util.Iterator; import java.util.List; import java.util.Queue; /* implement this class for all three strategies */ public abstract class AllocationStrategy { protected List<Job> Jobs; protected ArrayList<Job> Queue; public AllocationStrategy(List<Job> jobs) { super(); Jobs = jobs; } public abstract void run(); // update current job by 1 tick // check if the job queue might need to be changed. // check for jobs to add to the queue } FirstComeFirstServed.java import java.util.ArrayList; import java.util.List; public class FirstComeFirstServed extends AllocationStrategy { int temp; int proceessArrivalTime; int waitingTime; double avgWaitingTime; double avgTurnAroundTime; public FirstComeFirstServed(List<Job> jobs) { super(jobs); } @Override public void run() { } public void run(List<Job> jobList) { int count = 0; System.out.println("============================================ "); System.out.println("Process ID | Turnaround time | Waiting time "); System.out.println("============================================ "); for(Job job:jobList){ if(count==0){ job.processArrivalTime = job.getArrivalTime(); job.ProcessCompletionTime = job.getArrivalTime()+job.getCpuTime(); }else{ job.processArrivalTime = temp-job.getArrivalTime(); job.ProcessCompletionTime = temp+job.getCpuTime(); } temp = job.ProcessCompletionTime; job.turnAroundTime = temp-job.getArrivalTime(); job.waitingTime = job.turnAroundTime-job.getCpuTime(); count++; avgWaitingTime = avgWaitingTime+job.waitingTime; avgTurnAroundTime = avgTurnAroundTime+job.turnAroundTime; System.out.println(" "+job.getProcessId()+" | "+" "+job.turnAroundTime+" | "+" "+job.waitingTime+" "); System.out.println("----------------------------------------"); } System.out.println("==============================================="); System.out.println("Avg waiting time:"+avgWaitingTime/jobList.size()); System.out.println("==============================================="); System.out.println("Avg turn around time:"+avgTurnAroundTime/jobList.size()); System.out.println("==============================================="); } } Job.java public class Job { private int id, submitTime, CPUTime, CPUTimeLeft; private int startTime = 0, endTime = 0; public int ProcessCompletionTime; public int processArrivalTime; public int waitingTime; public int turnAroundTime; private JobFinishEvent evt; private int arrivalTime,cpuTime,processId; public Job(int id, int submitTime, int CPUTime, JobFinishEvent evt) { super(); this.id = id; this.submitTime = submitTime; this.CPUTime = CPUTime; this.CPUTimeLeft = CPUTime; this.evt = evt; } public Job(int processId, int arrivalTime, int cpuTime) { this.processId = processId; this.arrivalTime = arrivalTime; this.cpuTime = cpuTime; } public void start(int sysTime) { startTime = sysTime; } public void tick(int sysTime) { CPUTimeLeft --; if (CPUTimeLeft <= 0){ endTime = sysTime; evt.onFinish(this); } } public int getId() { return id; } public void setId(int id) { this.id = id; } public int getSubmitTime() { return submitTime; } public void setSubmitTime(int submitTime) { this.submitTime = submitTime; } public int getCPUTime() { return CPUTime; } public void setCPUTime(int cPUTime) { CPUTime = cPUTime; } public int getCPUTimeLeft() { return CPUTimeLeft; } public void setCPUTimeLeft(int cPUTimeLeft) { CPUTimeLeft = cPUTimeLeft; } public int getStartTime() { return startTime; } public void setStartTime(int startTime) { this.startTime = startTime; } public int getEndTime() { return endTime; } public void setEndTime(int endTime) { this.endTime = endTime; } public int getArrivalTime() { return arrivalTime; } public void setArrivalTime(int arrivalTime) { this.arrivalTime = arrivalTime; } public int getCpuTime() { return cpuTime; } public void setCpuTime(int cpuTime) { this.cpuTime = cpuTime; } public int getProcessId() { return processId; } public void setProcessId(int processId) { this.processId = processId; } } JobFinishEvent.java public interface JobFinishEvent { public void onFinish(Job j); } Question1.java import java.io.BufferedReader; import java.io.FileReader; import java.io.IOException; import java.util.ArrayList; import java.util.List; import java.util.Scanner; /** * Application class for Assignment 1, Question 1, compsci215 2013 * * @author Sonia
* */ public class Question1 { public static void main(String[] args) { // Process command line arguments // read the file Scanner sc = new Scanner(System.in); Scanner sc1 = new Scanner(System.in); Scanner sc2 = new Scanner(System.in); String filename ; String allocationStrategy; int quantum=20; /*filename = args[0]; allocationStrategy = args[1];*/ filename = "testing.txt"; allocationStrategy = "FCFS"; //filename = sc.nextLine(); if(args.length==3){ quantum = new Integer(args[2]); } BufferedReader br = null; try { String sCurrentLine; br = new BufferedReader(new FileReader("C://Users/Sonia/Desktop/"+filename)); //System.out.println("processId arrivalTime cpuTime"); List<Job> jobList = new ArrayList<Job>(); while ((sCurrentLine = br.readLine()) != null) { String a[] = sCurrentLine.split(","); int processId = new Integer(a[0]); int arrivalTime = new Integer(a[1]); int cpuTime = new Integer(a[2]); Job job = new Job(processId,arrivalTime,cpuTime); jobList.add(job); //System.out.println(processId+" "+ arrivalTime+" " + cpuTime); } if("FCFS".equalsIgnoreCase(allocationStrategy)){ FirstComeFirstServed firstComeFirstServed = new FirstComeFirstServed(jobList); firstComeFirstServed.run(jobList); }else if("SRT".equalsIgnoreCase(allocationStrategy)){ ShortestRemainingTime shortestRemainingTime = new ShortestRemainingTime(jobList); shortestRemainingTime.run(jobList); }else if("RR".equalsIgnoreCase(allocationStrategy)){ RoundRobin roundRobin = new RoundRobin(); roundRobin.run(jobList,quantum); } } catch (IOException e) { e.printStackTrace(); } finally { try { if (br != null)br.close(); } catch (IOException ex) { ex.printStackTrace(); } } JobFinishEvent callback = new JobFinishEvent() { @Override public void onFinish(Job j) { // this will be called when a job is finished. } }; /*// example job addition: ArrayListjobs = new ArrayList jobs.add(new Job(1, 0, 2, callback)); jobs.add(new Job(2, 1, 3, callback)); FirstComeFirstServed fcfs = new FirstComeFirstServed(jobs); fcfs.run(); */ } }();