Actual dIfference between 2 ways of equal parallelism using omp threads - c

Question

I am trying to parallelize my program using OMP threads .
What I am doing is the following and it works perfectly :
#pragma omp parallel num_threads(threadnum) \
default(none) shared(scoreBoard, nDiag, qlength, dlength) private(nEle, i, si, sj, ai, aj, max)
{
for (i = 1; i < nDiag; ++i)
{
if (i <= qlength && i <= dlength) nEle = i;
else if(i <= findmax(qlength, dlength)) nEle = findmin(qlength, dlength);
else nEle = 2*findmin(qlength, dlength) - i + abs(qlength - dlength);
calcfirstele(%si, %sj);
#pragma omp for
for (j = 1; j <= nEle; ++j)
{
ai = si - j + 1;
aj = sj + j - 1
max = searchmax(ai,aj);
scoreBoard[ai][aj] = max;
}
}
}
But isn't it equal to :
for (i = 1; i < nDiag; ++i)
{
if (i <= qlength && i <= dlength) nEle = i;
else if(i <= findmax(qlength, dlength)) nEle = findmin(qlength, dlength);
else nEle = 2*findmin(qlength, dlength) - i + abs(qlength - dlength);
calcfirstele(%si, %sj);
#pragma omp parallel num_threads(threadnum) \
default(none) shared(scoreBoard) private(nEle, i, si, sj, ai, aj, max)
#pragma omp for
for (j = 1; j <= nEle; ++j)
{
ai = si - j + 1;
aj = sj + j - 1
max = searchmax(ai,aj);
scoreBoard[ai][aj] = max;
}
}
Why when i use the second one my program is making more time than the serial one , whereas in the first case it works lot faster than the serial ? Can't understand the difference between them

Answer 1

Your second code is wrong and has an undefined behavior.
The reason for that is that by declaring nEle, si and sj private, you create some local (per-thread) versions of these variables, without giving them any value. Therefore, nEle notably, which is the upper bound of you for loop, can have whatever value, likely increasing quite dramatically the length of your computation.
In order to fix your code, the snippet you gave should look like this (with a few simplifications, not tested obviously):
for (int i = 1; i < nDiag; ++i) {
if (i <= qlength && i <= dlength)
nEle = i;
else if(i <= findmax(qlength, dlength))
nEle = findmin(qlength, dlength);
else
nEle = 2*findmin(qlength, dlength) - i + abs(qlength - dlength);
calcfirstele(%si, %sj); // not sure what this suppose to mean...
#pragma omp parallel for num_threads(threadnum) private(ai, aj, max)
for (int j = 1; j <= nEle; ++j) {
ai = si - j + 1;
aj = sj + j - 1
max = searchmax(ai,aj);
scoreBoard[ai][aj] = max;
}
}