Speed Test: std::Vector vs Array
Mar/100
Most recently I have been working on a multithreaded ray tracer in order to port it to CUDA so I am not using any recursion. The first step is implementing the ray tracer without using any acceleration structure. So I simply test a ray with all the objects in a scene thus I needed to decide using whether an array or a vector.
As the current code strongly relies on the data structure I am using, I wanted to test the iteration speed of std::vector versus array.
Here is the code I used for testing.
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#include <iostream>
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#include <map>
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#include <vector>
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#include <array>
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#include <stack>
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#include <windows.h>
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#include <math.h>
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typedef DWORD TIME_TYPE;
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#define BEGIN_TIMING(repeat) TIME_TYPE dwStartTime = timeGetTime(); for(int __i = 0; __i < repeat; __i++){
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#define END_TIMING }; TIME_TYPE __ret = (timeGetTime() – dwStartTime); cout<< __FUNCTION__ << "\t\t: elapsed time =\t"<<__ret<<endl; return __ret;;
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const unsigned int g_uiStartTestSize = 1000000;
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const unsigned int g_uiStartRepeatCount = 1;
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unsigned int g_uiTestSize = 0;
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unsigned int g_uiRepeatCount = 0;
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unsigned int g_uiNumOfTestSteps = 10;
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using namespace std;
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using std::map;
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using std::vector;
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class ClassA
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{
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public:
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ClassA(){a=b=c=d=0;};
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~ClassA(){};
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double a;
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double b;
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double c;
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double d;
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};
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vector<ClassA*>* classAVector;
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ClassA** classAArray;
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TIME_TYPE testFillInArray()
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{
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BEGIN_TIMING(1)
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classAArray = new ClassA*[g_uiTestSize];
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for(int i =0; i < g_uiTestSize; i++)
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{
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classAArray[i] = new ClassA();
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}
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END_TIMING
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}
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TIME_TYPE testFillInVector()
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{
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BEGIN_TIMING(1)
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classAVector = new vector<ClassA*>();
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for(int i =0; i < g_uiTestSize; i++)
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{
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classAVector->push_back(new ClassA());
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}
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END_TIMING
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}
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TIME_TYPE testDestroyArray()
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{
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BEGIN_TIMING(1)
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for (int i = 0; i < g_uiTestSize; i++)
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{
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delete classAArray[i];
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}
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delete [] classAArray;
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END_TIMING
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}
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TIME_TYPE testDestroyVector()
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{
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BEGIN_TIMING(1)
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for (int i = 0; i < g_uiTestSize; i++)
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{
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delete (*classAVector)[i];
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}
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delete classAVector;
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END_TIMING
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}
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TIME_TYPE testIterateVector(void)
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{
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BEGIN_TIMING(g_uiRepeatCount)
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ClassA *pClassA;
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/*
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const int sz = classAVector->size();
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for (int i = 0; i < sz; i++)
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{
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pClassA = (*classAVector)[i];
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pClassA->a++;
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pClassA->b++;
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pClassA->c++;
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pClassA->d++;
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}*/
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vector<ClassA*>::iterator it = classAVector->begin();
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const vector<ClassA*>::iterator endIt = classAVector->end();
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while(it != endIt)
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{
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pClassA = (*it);
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pClassA->a++;
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pClassA->b++;
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pClassA->c++;
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pClassA->d++;
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it++;
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}
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END_TIMING
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}
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TIME_TYPE testIterateArray(void)
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{
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BEGIN_TIMING(g_uiRepeatCount)
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ClassA *pClassA;
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for (int i =0; i < g_uiTestSize; i++)
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{
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pClassA = classAArray[i];
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pClassA->a++;
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pClassA->b++;
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pClassA->c++;
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pClassA->d++;
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}
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END_TIMING
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}
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void verify()
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{
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cout<<"verifying"<<endl;
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for (int i=0; i < g_uiTestSize; i++)
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{
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if( (*classAArray[i]).a != classAVector->at(i)->a )
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{
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cout << "error!:" << i <<endl;
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}
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}
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}
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TIME_TYPE testAll()
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{
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BEGIN_TIMING(1)
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g_uiTestSize = g_uiStartTestSize;
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for (int i =1; i <= g_uiNumOfTestSteps; i++)
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{
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g_uiRepeatCount = g_uiStartRepeatCount * i;
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cout<< "repeat count =\t\t"<<g_uiRepeatCount<<endl;
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cout<< "test size =\t\t"<<g_uiTestSize<<endl;
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testFillInArray();
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testFillInVector();
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testIterateArray();
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testIterateVector();
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testDestroyArray();
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testDestroyVector();
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cout<<"———————————————"<<endl;
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}
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g_uiRepeatCount = g_uiStartRepeatCount;
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for (int i =1; i <= g_uiNumOfTestSteps; i++)
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{
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g_uiTestSize = g_uiStartTestSize * i;
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cout<< "repeat count =\t"<<g_uiRepeatCount<<endl;
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cout<< "test size =\t"<<g_uiTestSize<<endl;
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testFillInArray();
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testFillInVector();
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testIterateArray();
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testIterateVector();
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testDestroyArray();
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testDestroyVector();
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cout<<"———————————————"<<endl;
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}
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END_TIMING
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}
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int main(void)
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{
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testAll();
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return 0;
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}
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Results
Results are striking indeed. If you compiling the code in Release mode without any run-time error checking and disabled exection support iterating a Vector by using std::vector<>::iterator is almost as fast as using array.
However testing them on debug mode shows the difference, iteration time is terribly slow on std::vector ; about 60 times slower.
Conclusion
std::vector is almost as fast as using a static array on release mode (using Visual Studio 2008) but terribly slow on debug mode. So if your code is running slow on the debug mode, don’t give up std::vector and give a go on the release mode.
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Author: Hevi
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