Shortest Remaining Time First: Java Multithreading

Question

I am trying to simulate CPU scheduling algorithms in java and am using multithreading. I have successfully implemented FCFS(First Come First Serve) and SJF(Shortest Job First). But the problem is when i start to think of SRTF(Shortest Remaining Time First) which is a preemptive form of SJF. I am using the following model:

• A thread for CPU, which has a CLOCK variable, which keeps ticking(a simple clock increment) every 100ms . I have a boolean isAvailable; flag for the processes to check if the CPU is available before starting execution.
• A thread for Long Term Scheduler(LTS), which pushes the process from the process list to a Ready Queue.
• A thread for Short Term Scheduler(STS), which takes a process from the ReadyQueue and assigns it to the CPU.
• Once a process is removed from the ReadyQueue by STS for execution, the process checks for the isAvailable flag of CPU. If true , it sets the flag to false and starts its execution (for which i am just making the thread to sleep for (100 * burstTime) ms as this is just a simulation). Otherwise, the process just keeps busy waiting : while(CPU.isAvailable != true); .

I have the list of processes along with their arrival and burst times before hand. It is ok till i am simulatiing non-preemptive scheduling(FCFS and SJF). But as i try for SRTF, I am unable to find a way to preempt the currently running process thread.

For SRTF, i know the way ahead to select the next process from ReadyQueue. I can try to set the isAvailable flag to false once i select a process from the queue, but then how am I to know which thread was originally executing? And since I am not using much of synchronization b/w threads, I will have multiple processes using the CPU thread. Its getting a little messed up. Please help. Thanks!

This is the code for a Process:

enum State {ARRIVED, WAITING, READY, RUNNING, EXECUTED}
public class Process implements Runnable
{
    int pid;
    int arrTime;
int burstTime;
int priority;
long startTime;
long endTime;
State procState = null;

Process(int pid, int arrTime, int burstTime, int priority)
{
    this.pid = pid;
    this.arrTime = arrTime;
    this.burstTime = burstTime;
    this.priority = priority;
    this.procState = State.ARRIVED;
    this.startTime = 0;


    this.endTime = 0;    /* I also considered adding a timeElapsedUnderExecution
 attribute to the process. So I can check after every cycle if the CPU is still available
 and keep incrementing the time elapsed. Once the timeElapsed becomes same as burstTime, i
 stop the process. Or if after a cycle, the CPU is not available, i know from where to
 resume my Process. Is this the way to go ? */

    }

boolean isReady()
{
    if((this.arrTime <= CPU.CLOCK) && (this.procState == State.ARRIVED))
        return true;
    else return false;
}

@Override
public void run() {
    // TODO Auto-generated method stub
    if(this.procState == State.READY)
        this.procState = State.WAITING;

    while(!CPU.isAvailable());

    try 
    {
        this.procState = State.RUNNING;
        System.out.println("Process " + pid + " executing...");
        this.startTime = CPU.CLOCK;
        System.out.println("Process " + this.pid + ": Begins at " + this.startTime);
        Thread.sleep(this.burstTime * 100);
        this.endTime = CPU.CLOCK;
        System.out.println("Process " + this.pid + ": Ends at " + this.endTime);
        this.procState = State.EXECUTED;

    }
    catch (InterruptedException e) 
    {
        // TODO Auto-generated catch block
        System.out.println("Interrupted: " + pid);
        e.printStackTrace();
    }
    }
}

The code for CPU:

    import java.util.LinkedList;
    import java.util.Queue;

    public class CPU implements Runnable

{
    static Long CLOCK = new Long(0);
    static LinkedList<Process> ReadyQ = new LinkedList<Process>();
private static boolean isAvailable = true;
static boolean done = false;

public static boolean isAvailable() {
    return isAvailable;
}

public static void setAvailable(boolean isAvailable) {
    CPU.isAvailable = isAvailable;
}

static void incrementCLOCK()
{
    LTS.checkArrival();
    CPU.CLOCK++;
    try {
        Thread.sleep(100);
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    System.out.println("Clock Tick: " + CPU.CLOCK);
}

@Override
public void run() {
    // TODO Auto-generated method stub
    System.out.println("CPU starts.!!!");
    while(CPU.done != true)
        synchronized(CPU.CLOCK)
        {
            incrementCLOCK();
            }
    }
}

The code for LTS:

public class LTS implements Runnable 
{
    private static Process[] pList = null;
    private final int NUM;
    static Integer procStarted;
    static Integer procFinished;
    static boolean STSDone = false;


LTS(Process[] pList, int num)
{
    this.NUM = num;
    LTS.pList = pList;
}

static void checkArrival()
{
    if(pList == null) return;
    for(int i = 0; i < pList.length; i++)
        if(pList[i].isReady())
        {
            pList[i].procState = State.READY;
            System.out.println("Process " + pList[i].pid + " is now ready.");
            CPU.ReadyQ.add(pList[i]);
        }
}

@Override
public void run() {
    // TODO Auto-generated method stub
    System.out.println("Long Term Scheduler starts.!!!");
    while(LTS.STSDone != true)
    {
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    System.out.println(LTS.STSDone);
    System.out.println("LTS ends.!!!");
        CPU.done = true;
    }
}

Answer 1

Problem number 1 is that your shared state is not thread-safe. Even simple things like booleans need correct threading primitives to ensure visibility across threads (aka "volatile").