Question

I am having simple code of paralellizing QuickSort algorithm in Java, in run method I everytime create two seperate new threads for parallelizing processing of Array elements. But as it encounters join() statements for both created threads, threads never backs and halts on joins(), seems join() never releases them.

Below is the code.

class StartJoinQuickSort implements Runnable 
{
    private int m_Low, m_High;
    private int[] m_Array = null;

    private final static int NR_OF_VALUES = 10; // TOTAL_NO_VALUES
    private int PivotElement;   
    private static Random m_random = new Random( );


    public StartJoinQuickSort(int[] a_Array,int a_Low,int a_High)
    {
        this.m_Array = a_Array;
        this.m_Low = a_Low;
        this.m_High = a_High;
    }
    private void SwapArrayElements(int a_i,int a_j)
    {
        int temp = this.m_Array[a_i];
        this.m_Array[a_i] = this.m_Array[a_j];
        this.m_Array[a_j] = temp;

    }// end of SwapArrayElements

    private static int nextRandomFunctionValue(int aStart, int aEnd)
    {
        if ( aStart > aEnd )
        {
           throw new IllegalArgumentException("Start cannot exceed End.");
        }

        //get the range, casting to long to avoid overflow problems
        long range = (long)aEnd - (long)aStart + 1;

        // compute a fraction of the range, 0 <= frac < range

        long fraction = (long)(range * m_random.nextDouble());
        int randomNumber =  (int)(fraction + aStart);    
        return randomNumber;

    }// end of nextRandomFunctionValue

    private static int[] GetArrayWithRandomValues()
    {
        int[] ArrayToBePopulatedWithRandomValues  = new int[NR_OF_VALUES];
        for(int index =0; index<NR_OF_VALUES;index++)
        {
            int RandomValue = StartJoinQuickSort.nextRandomFunctionValue(0,NR_OF_VALUES);
            ArrayToBePopulatedWithRandomValues[index] =  RandomValue;

        }//end of for

        return ArrayToBePopulatedWithRandomValues;

    }//end of GetArrayWithRandomValues
    private int middleIndex(int left, int right) 
    {
        return left + (right - left) / 2;
    }
    public int Partition(int a_Start,int a_end)
    {
    //  System.out.println("Partition ..thId : " + Thread.currentThread().getId());
        int pivotIndex = 0;

        int i = a_Start;
        int j = a_end;

        try
        {
            pivotIndex = middleIndex(a_Start , a_end);
            this.PivotElement = this.m_Array[pivotIndex];

            do
            {
                while(this.m_Array[i] < PivotElement )
                    i++;

                if(j>0)
                {
                    try
                    {
                        while( this.m_Array[j] > PivotElement )
                            j--;
                    }
                    catch(Exception ex){System.out.println(" j : " + j);}

                }//end of if

                if(i<=j)
                {
                    SwapArrayElements(i,j);

                //  System.out.println("Swap .." + + Thread.currentThread().getId());

                    i++;
                    j--;

                }//end of if

            }while(i<=j);
        }
        catch(Exception except)
        {
            System.out.println("exception in Partition " + except);
        }
        return j;
    }

    public void run()  
    {

        //System.out.println("in run..");


        //System.out.println("after PARTITION");

        StartJoinQuickSort oStartQuickSort_1 = null;
        StartJoinQuickSort oStartQuickSort_2 = null;



        if(this.m_Low < this.m_High )
        {
            int Index = Partition(this.m_Low,this.m_High);



            Thread thPart_1 = new Thread ( new StartJoinQuickSort( this.m_Array,this.m_Low,Index ) );
            Thread thPart_2 = new Thread ( new StartJoinQuickSort( this.m_Array,Index + 1,this.m_High ) );

            thPart_1.start();   thPart_2.start();


        //}//end of if
        //if( Index + 1 < this.m_High)
        //{         
            try
            {           
                    thPart_1.join(); thPart_2.join();

            }catch (InterruptedException e) { e.printStackTrace();}         
        }
    }//end of run

Regards
Usman

Answer 1

Hmmm, it is never a good idea to implement a recursive algorithm in parallel like this. You will end up creating a huge number of threads (exponential at every level) and will eventually oversubscribe the system.

The best idea is to have a cutoff point, which let’s say is equal to the number of available cores. Then when the current level of recursion has a number of branches equal to the cutoff point switch to a sequential quicksort. Some very rough pseudocode of the flow:

parallel_quicksort(level, interval) {
    // compute subintervals interval1, interval2
    if(level < cutoff) { 
        spawn1: parallel_quicksort(level + 1, interval1);
        spawn2: parallel_quicksort(level + 1, interval2);

        join1();     
        join2();
    } else {
        quicksort(interval1);
        quicksort(interval2);
    }
}

Also have a look over this implementation to see if you’ve missed something: http://www.inf.fh-flensburg.de/lang/algorithmen/sortieren/quick/quicken.htm