Multiple threads taking more time than single process [duplicate] - c++

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C: using clock() to measure time in multi-threaded programs
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I am implementing pattern matching algorithm, by moving template gradient info over entire target's gradient image , that too at each rotation (-60 to 60). I have already saved the template info for each rotation ,i.e. 121 templates are already preprocessed and saved.
But the issue is, this is consuming lot of time (approx 110ms), so decided to split the matching at set of rotations (-60 to -30 , -30 to 0, 0 to 30 and 30 to 60) into 4 threads, but threading is taking more time that single process (approx 115ms to 120ms).
Snippet of code is...
#define MAXTARGETNUM 64
MatchResultA totalResultsTemp[MAXTARGETNUM];
void CShapeMatch::match(ShapeInfo *ShapeInfoVec, search_region SearchRegion, float MinScore, float Greediness, int width,int height, int16_t *pBufGradX ,int16_t *pBufGradY,float *pBufMag, bool corr)
{
MatchResultA resultsPerDeg[MAXTARGETNUM];
....
....
int startX = SearchRegion.StartX;
int startY = SearchRegion.StartY;
int endX = SearchRegion.EndX;
int endY = SearchRegion.EndY;
float AngleStep = SearchRegion.AngleStep;
float AngleStart = SearchRegion.AngleStart;
float AngleStop = SearchRegion.AngleStop;
int startIndex = (int)(ShapeInfoVec[0].AngleNum/2) + ShapeInfoVec[0].AngleNum%2+(int)AngleStart/AngleStep;
int stopIndex = (int)(ShapeInfoVec[0].AngleNum/2) + ShapeInfoVec[0].AngleNum%2+(int)AngleStop/AngleStep;
for (int k = startIndex; k < stopIndex ; k++){
....
for(int j = startY; j < endY; j++){
for(int i = startX; i < endX; i++){
for(int m = 0; m < ShapeInfoVec[k].NoOfCordinates; m++)
{
curX = i + (ShapeInfoVec[k].Coordinates + m)->x; // template X coordinate
curY = j + (ShapeInfoVec[k].Coordinates + m)->y ; // template Y coordinate
iTx = *(ShapeInfoVec[k].EdgeDerivativeX + m); // template X derivative
iTy = *(ShapeInfoVec[k].EdgeDerivativeY + m); // template Y derivative
iTm = *(ShapeInfoVec[k].EdgeMagnitude + m); // template gradients magnitude
if(curX < 0 ||curY < 0||curX > width-1 ||curY > height-1)
continue;
offSet = curY*width + curX;
iSx = *(pBufGradX + offSet); // get corresponding X derivative from source image
iSy = *(pBufGradY + offSet); // get corresponding Y derivative from source image
iSm = *(pBufMag + offSet);
if (PartialScore > MinScore)
{
float Angle = ShapeInfoVec[k].Angel;
bool hasFlag = false;
for(int n = 0; n < resultsNumPerDegree; n++)
{
if(abs(resultsPerDeg[n].CenterLocX - i) < 5 && abs(resultsPerDeg[n].CenterLocY - j) < 5)
{
hasFlag = true;
if(resultsPerDeg[n].ResultScore < PartialScore)
{
resultsPerDeg[n].Angel = Angle;
resultsPerDeg[n].CenterLocX = i;
resultsPerDeg[n].CenterLocY = j;
resultsPerDeg[n].ResultScore = PartialScore;
break;
}
}
}
if(!hasFlag)
{
resultsPerDeg[resultsNumPerDegree].Angel = Angle;
resultsPerDeg[resultsNumPerDegree].CenterLocX = i;
resultsPerDeg[resultsNumPerDegree].CenterLocY = j;
resultsPerDeg[resultsNumPerDegree].ResultScore = PartialScore;
resultsNumPerDegree ++;
}
minScoreTemp = minScoreTemp < PartialScore ? PartialScore : minScoreTemp;
}
}
}
for(int i = 0; i < resultsNumPerDegree; i++)
{
mtx.lock();
totalResultsTemp[totalResultsNum] = resultsPerDeg[i];
totalResultsNum++;
mtx.unlock();
}
n++;
}
void CallerFunction(){
int16_t *pBufGradX = (int16_t *) malloc(bufferSize * sizeof(int16_t));
int16_t *pBufGradY = (int16_t *) malloc(bufferSize * sizeof(int16_t));
float *pBufMag = (float *) malloc(bufferSize * sizeof(float));
clock_t start = clock();
float temp_stop = SearchRegion->AngleStop;
SearchRegion->AngleStop = -30;
thread t1(&CShapeMatch::match, this, ShapeInfoVec, *SearchRegion, MinScore, Greediness, width, height, pBufGradX ,pBufGradY,pBufMag, corr);
SearchRegion->AngleStart = -30;
SearchRegion->AngleStop=0;
thread t2(&CShapeMatch::match, this, ShapeInfoVec, *SearchRegion, MinScore, Greediness, width, height, pBufGradX ,pBufGradY,pBufMag, corr);
SearchRegion->AngleStart = 0;
SearchRegion->AngleStop=30;
thread t3(&CShapeMatch::match, this, ShapeInfoVec, *SearchRegion, MinScore, Greediness,width, height, pBufGradX ,pBufGradY,pBufMag, corr);
SearchRegion->AngleStart = 30;
SearchRegion->AngleStop=temp_stop;
thread t4(&CShapeMatch::match, this, ShapeInfoVec, *SearchRegion, MinScore, Greediness,width, height, pBufGradX ,pBufGradY,pBufMag, corr);
t1.join();
t2.join();
t3.join();
t4.join();
clock_t end = clock();
cout << 1000*(double)(end-start)/CLOCKS_PER_SEC << endl;
}
As we can see there are plenty of heap access but they just are read-only. Only totalResultTemp and totalResultNum are shared global resource on which write are performed.
My PC configuration is,
i5-7200U CPU # 2.50GHz 4 cores
4 Gig RAM
Ubuntu 18

Answer 1

for(int i = 0; i < resultsNumPerDegree; i++)
{
mtx.lock();
totalResultsTemp[totalResultsNum] = resultsPerDeg[i];
totalResultsNum++;
mtx.unlock();
}
You writing into static array, and mutexes are really time consuming. Instead of creating locks try to use std::atomic_int, or in my opinion even better, just pass to function exact place where to store result, so problem with sync is not your problem anymore

Answer 2

POSIX Threads in c/c++ are not concurrent since the time assigned by the operative system to each parent process must be split into the number of threads it has. Thus, your algorithm is executing only core. To leverage multicore technology, you must use OpenMP. This interface library let you split your algorithm in different physic cores. This is a good OpenMP tutorial