shortest job first example code

SJF.java

package os.coursework;

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;

/**
 * Shortest Job First (SJF) scheduling model
 * reads in a set of processes from CSV file,
 * one process per line in format ^ID, CBT, AAT$
 * and computes various metrics including:
 *  - mean wait time
 *  - mean turnaround time
 *
 * Note that this is a non-preemptive SJF 
 * implementation, so:
 *  turnaround time == wait time + CPU burst time
 */

public class SJF {

  public static String inputFileName;

  public static void main(String [] args) {

	  if (args.length != 1) {
	    System.err.println("usage: SJF FILENAME.csv");
	    System.exit(-1);
	  }

	  inputFileName = args[0];

	  int currentTime = 0;
	  int totalTimeCPUBusy = 0;
	  int totalWaitingTime = 0;
	  int totalTurnaroundTime = 0;
	  int numTasks = 0;

	  ArrayList<Process> l = new ArrayList<>();
	
	  try (BufferedReader r = new BufferedReader(new FileReader(inputFileName))) {

	    String line = r.readLine();
	    while (line != null) {
	   	  // each line has 3 comma separated values
		    String [] data = line.split(",");
		    assert (data.length == 3);
		    numTasks++;

		    Process p = new Process(
				  	Integer.parseInt(data[0].trim()),
				  	Integer.parseInt(data[1].trim()),
				  	Integer.parseInt(data[2].trim()));

		    // put process in list
		    l.add(p);
		    line = r.readLine();
	    }
	  }
	  catch (IOException e) {
	    e.printStackTrace();
	  }

	  assert (l.size() == numTasks);
	  Collections.sort(l);

	  // time to do simulation of scheduling

	  while (l.size() > 0) {
	    // pseudo-code
	    // iterate over l, look for first task that is schedulable
	    Process firstProcess = null;
	    boolean processWasScheduled = false;
	    for (Process p : l) {
		    if (firstProcess == null ||
		        firstProcess.aat > p.aat) {
		      firstProcess = p;
		    }
		
		    if (p.aat <= currentTime) {
		      int waitingTime, turnaroundTime;
		      waitingTime = (currentTime - p.aat);
		      totalWaitingTime += waitingTime;
		      // schedule p
		      l.remove(p);
		      // add its CBT to totalExecutionTime
		      totalTimeCPUBusy+= p.cbt;
		      currentTime += p.cbt;
		      processWasScheduled = true;
		      turnaroundTime = (currentTime - p.aat); 
		      // == (waitingTime + p.cbt)
		      totalTurnaroundTime += turnaroundTime;
		      System.out.printf("PID %d, WT %d TT %d\n", p.id, waitingTime, turnaroundTime);
		      break;
		    }
		    if (!processWasScheduled) {
		      assert (firstProcess != null);
		      // fast-forward currentTime
		      // since there are no processes ready
		      // to execute
		      currentTime = firstProcess.aat;
		    }
		
	    }
	  }
	  System.out.println("average waiting time (AWT): " +
	  		   totalWaitingTime / numTasks);
	
  	System.out.println("average turnaround time (ATT): " +
	  		   totalTurnaroundTime / numTasks);
  }

} /* SJF */