Java seems to be executing bare-bones algorithms faster than C++. Why?

Question

Introduction:

Using two identical mergesort algorithms, I tested the execution speed of C++ (using Visual Studios C++ 2010 express) vs Java (using NetBeans 7.0). I conjectured that the C++ execution would be at least slightly faster, but testing revealed that the C++ execution was 4 - 10 times slower than the Java execution. I believe that I have set all the speed optimisations for C++, and I am publishing as a release rather than as a debug. Why is this speed discrepancy occurring?

Code:

Java:

public class PerformanceTest1
{
 /**
  * Sorts the array using a merge sort algorithm
  * @param array The array to be sorted
  * @return The sorted array
  */
 public static void sort(double[] array)
 {
      if(array.length > 1)
      {
           int centre;
           double[] left;
           double[] right;
           int arrayPointer = 0;
           int leftPointer = 0;
           int rightPointer = 0;

           centre = (int)Math.floor((array.length) / 2.0);

           left = new double[centre];
           right = new double[array.length - centre];

           System.arraycopy(array,0,left,0,left.length);
           System.arraycopy(array,centre,right,0,right.length);

           sort(left);
           sort(right);

           while((leftPointer < left.length) && (rightPointer < right.length))
           {
                if(left[leftPointer] <= right[rightPointer])
                {
                     array[arrayPointer] = left[leftPointer];
                     leftPointer += 1;
                }
                else
                {
                     array[arrayPointer] = right[rightPointer];
                     rightPointer += 1;
                }
                arrayPointer += 1;
           }
           if(leftPointer < left.length)
           {
                System.arraycopy(left,leftPointer,array,arrayPointer,array.length - arrayPointer);
           }
           else if(rightPointer < right.length)
           {
                System.arraycopy(right,rightPointer,array,arrayPointer,array.length - arrayPointer);
           }
      }
 }

 public static void main(String args[])
 {
      //Number of elements to sort
      int arraySize = 1000000;

      //Create the variables for timing
      double start;
      double end;
      double duration;

      //Build array
      double[] data = new double[arraySize];
      for(int i = 0;i < data.length;i += 1)
      {
           data[i] = Math.round(Math.random() * 10000);
      }

      //Run performance test
      start = System.nanoTime();
      sort(data);
      end = System.nanoTime();

      //Output performance results
      duration = (end - start) / 1E9;
      System.out.println("Duration: " + duration);
 }
}

C++:

#include <iostream>
#include <windows.h>
using namespace std;

//Mergesort
void sort1(double *data,int size)
{
if(size > 1)
{
    int centre;
    double *left;
    int leftSize;
    double *right;
    int rightSize;
    int dataPointer = 0;
    int leftPointer = 0;
    int rightPointer = 0;

    centre = (int)floor((size) / 2.0);
    leftSize = centre;
    left = new double[leftSize];
    for(int i = 0;i < leftSize;i += 1)
    {
        left[i] = data[i];
    }
    rightSize = size - leftSize;
    right = new double[rightSize];
    for(int i = leftSize;i < size;i += 1)
    {
        right[i - leftSize] = data[i];
    }

    sort1(left,leftSize);
    sort1(right,rightSize);

    while((leftPointer < leftSize) && (rightPointer < rightSize))
    {
        if(left[leftPointer] <= right[rightPointer])
        {
            data[dataPointer] = left[leftPointer];
            leftPointer += 1;
        }
        else
        {
            data[dataPointer] = right[rightPointer];
            rightPointer += 1;
        }
        dataPointer += 1;
    }
    if(leftPointer < leftSize)
    {
        for(int i = dataPointer;i < size;i += 1)
        {
            data[i] = left[leftPointer++];
        }
    }
    else if(rightPointer < rightSize)
    {
        for(int i = dataPointer;i < size;i += 1)
        {
            data[i] = right[rightPointer++];
        }
    }
            delete left;
            delete right;
}
}

void main()
{
//Number of elements to sort
int arraySize = 1000000;

//Create the variables for timing
LARGE_INTEGER start; //Starting time
LARGE_INTEGER end; //Ending time
LARGE_INTEGER freq; //Rate of time update
double duration; //end - start
QueryPerformanceFrequency(&freq); //Determinine the frequency of the performance counter (high precision system timer)

//Build array
double *temp2 = new double[arraySize];
QueryPerformanceCounter(&start);
srand((int)start.QuadPart);
for(int i = 0;i < arraySize;i += 1)
{
    double randVal = rand() % 10000;
    temp2[i] = randVal;
}

//Run performance test
QueryPerformanceCounter(&start);
sort1(temp2,arraySize);
QueryPerformanceCounter(&end);
    delete temp2;

//Output performance test results
duration = (double)(end.QuadPart - start.QuadPart) / (double)(freq.QuadPart);
cout << "Duration: " << duration << endl;

//Dramatic pause
system("pause");
}

Observations:

For 10000 elements, the C++ execution takes roughly 4 times the amount of time as the Java execution. For 100000 elements, the ratio is about 7:1. For 10000000 elements, the ratio is about 10:1. For over 10000000, the Java execution completes, but the C++ execution stalls, and I have to manually kill the process.

Answer 1

I think there might be a mistake in the way you ran the program. When you hit F5 in Visual C++ Express, the program is running under debugger and it will be a LOT slower. In other versions of Visual C++ 2010 (e.g. Ultimate that I use), try hitting CTRL+F5 (i.e. Start without Debugging) or try running the executable file itself (in the Express) and you see the difference.

I run your program with only one modification on my machine (added delete[] left; delete[] right; to get rid of memory leak; otherwise it would ran out of memory in 32 bits mode!). I have an i7 950. To be fair, I also passed the same array to the Arrays.sort() in Java and to the std::sort in C++. I used an array size of 10,000,000.

Here are the results (time in seconds):

Java code:            7.13
Java Arrays.sort:     0.93

32 bits
C++ code:             3.57
C++ std::sort         0.81

64 bits
C++ code:             2.77
C++ std::sort         0.76

So the C++ code is much faster and even the standard library, which is highly tuned for in both Java and C++, tends to show slight advantage for C++.

Edit: I just realized in your original test, you run the C++ code in the debug mode. You should switch to the Release mode AND run it outside the debugger (as I explained in my post) to get a fair result.

Answer 2

I don't program C++ professionally (or even unprofessionally:) but I notice that you are allocating a double on the heap (double *temp2 = new double[arraySize];). This is expensive compared to Java initialisation but more importantly, it constitutes a memory leak since you never delete it, this could explain why your C++ implementation stalls, it's basically run out of memory.