Java似乎比C ++更快地执行裸机算法。为什么？ [英] Java seems to be executing bare-bones algorithms faster than C++. Why?
问题描述
使用两个相同的合并算法，我测试了C ++（使用Visual Studios C ++ 2010 express）和Java（使用NetBeans 7.0）的执行速度。我猜想C ++执行将至少稍快，但测试显示，C ++执行比Java执行慢4-10倍。我相信我已经为C ++设置了所有的速度优化，我发布为发布，而不是作为调试。为什么会出现这种速度差异？
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?
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 ++：
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");
}
观察：
对于10000个元素，C ++执行大约是Java执行时间的4倍。
对于100000个元素，比率大约为7：1。
对于10000000个元素，比率约为10：1。
对于超过10000000，Java执行完成，但C ++执行停止，我必须手动杀死进程。
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.
推荐答案
我认为您运行程序的方式可能有误。当你在Visual C ++ Express 中打F5时，程序正在调试器下运行，并且会慢一点。在Visual C ++ 2010的其他版本（例如我使用的Ultimate），尝试点击CTRL + F5（即启动不调试）或尝试运行可执行文件本身（在Express），你看到的差异。
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.
我在我的机器上运行你的程序只有一个修改（添加 delete [] left; delete [] right; 泄漏;否则它将用完32位模式的内存！）。我有一个i7 950.公平地说，我也传递相同的数组到Arrays.sort（）在Java和std ::排序在C + +。我使用的数组大小为10,000,000。
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.
以下是结果（以秒为单位的时间）：
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
因此，C ++代码要快得多，甚至在Java和C ++中高度调优的标准库往往对C ++显示出轻微的优势。
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++.
编辑：我刚刚在你的原始测试中实现，你在调试模式下运行C ++代码。你应该切换到释放模式，并运行它在调试器之外（如我在我的帖子中解释），以获得一个公平的结果。
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.
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