What is the best way to parellize using openMP in c - c

Question

I am beginner in using OpenMP in C. I am trying to parallelize four nested loops. I've read that it's advisable to only parallelize the outer loop but it's taking a very long time.
What is the best way to parallelize the following
int nt=2500, nx=400; nz=200; nh=50;
#pragma omp parallel for
for(it=0; it<nt; it++)
for(ix=0; ix<nx; ix++)
for(iz=0; iz<nz; iz++)
for(ih=-nh; ih<=nh; ih++) {
if (ix+ih<nx && ix+ih>=0 && ix-ih<nx && ix-ih>=0 ) {
dR[it][ix+ih][iz] += ii[ih+nh][ix][iz]*us[it][ix-ih][iz];
dS[it][ix-ih][iz] += ii[ih+nh][ix][iz]*ur[it][ix+ih][iz];
}
}

Answer 1

As far as data races go, it is unsafe to parallelize loops in a way that causes the same memory location to be accessed by two different threads and at least one access is a write.
You never read and write to the same variable, so it should be safe to parallelize every loop. (Not necessarily more efficient though)
Your actual loops can also be rewritten.
Your if condition can be written logically as 0 <= ix+ih < nx && 0 <= ix-ih < nx, or in other words you only want to write between 0 and nx.
If we can show that the ranges of ix+ih and ix-ih is larger than 0 to nx we can eliminate the check and manually loop over those ranges.
Examining the loops we see that 0 < ix < nx and -nh < ih < nh which allows us to find the ranges of ix+ih and ix-ih.
ix+ih ranges from -nh to nx + nh and ix-ih ranges from -nh to nx+nh. Both of these ranges are larger than 0,nx as long as nh is positive and so we don't need to do a check at all. We can just loop from 0 to nx.
omp_set_nested(1);
#pragma omp parallel for
for(it=0; it<nt; it++) {
#pragma omp parallel for
for (iy = 0; iy < nx; iy++) {
#pragma omp parallel for
for(iz=0; iz<nz; iz++) {
dR[it][iy][iz] += ii[ih+nh][ix][iz] * us[it][ix-ih][iz] ;
dS[it][iy][iz] += ii[ih+nh][ix][iz] * ur[it][ix+ih][iz] ;
}
}
}