Sequential is faster than Multi threaded - OpenMp - C++ - c++

Question

I am using C++ & OpenMP to parallelize an algorithm to find the convex hull.
But I am not able to get the expected speedup. In fact, the sequential algorithm is faster.
The input & output set of points are stored in arrays.
Could you please look into the code and let me know the corrections?
Point *points = new Point[inp_size]; // contains the input
int th_id;
omp_set_num_threads(nthreads);
clock_t t1,t2;
t1=clock();
#pragma omp parallel private(th_id)
{
th_id = omp_get_thread_num();
///////////// …. Only Function called ….///////////////////////////////////
findParallelUCHWOUP(points,th_id+1, nthreads, inp_size);
}
t2=clock();
float diff ((float)t2-(float)t1);
float seconds = diff / CLOCKS_PER_SEC;
std::cout << "Time Elapsed in seconds:" << seconds << '\n';
///////////////////////////////////////////////////////////////
int findParallelUCHWOUP(Point iv[],int id, int thread_num, int inp_size){
int numElems = inp_size/thread_num;
int first = (id-1) * numElems;;
int last;
if(id == thread_num){
last = inp_size-1;
}
else{
last = id*numElems-1;
}
output[first]=iv[first];
std::stack<int> s;
s.push(first);
int i=first+1;
while(i<last){
if ( crossProduct(iv, i, first, last) > 0){
s.push(i);
i++;
break;
}else{
i++;
}
}
if(i==last){
s.push(last);
return 0;
}
for(;i<=last;i++){
if ( crossProduct(iv, i, first, last) >= 0){
while ( s.size()>1 && crossProduct(iv, s.top(), second(s), i) <= 0){
s.pop();
}
s.push(i);
}
}
int count=s.size();
sizes[id-1] = count;
while(!s.empty()){
output[first+count-1]=iv[s.top()];
s.pop();
count--;
}
return 0;
}
///////////tested on these machines/////
Sequential Time:0.016466
Using two threads:0.022979
Using four threads:0.035213
Using 8 threads: 0.03315
Machine used: Mac Book Pro
Processor: 2.5 GHz Intel Core i5(at least 4 logical cores)
Memory: 4GB 1600 MHz
Compiler: Mac OSX Compiler

Answer 1

The problem is the way you count time. Actually, you could write something like:
diff / (float) (CLOCKS_PER_SEC * nthreads)
And this is only an approximation (and not always true).
CLOCKS_PER_SEC stands for sum of clocks of all cores...
You'd better use OpenMP special functions...