OpenMP Race Condition when finding Closest Pair - c++

Question

I'm doing an assignment to find the closest pair between two disjoint sets A and B. I'm using OpenMP to parallelize the recursion of the algorithm, but I am running into some data races. I am very new to OpenMP, so I think it has something to do with incorrect privating/sharing of variables. I have put the full algorithm below:
float OMPParticleSim::efficient_closest_pair(int n, vector<Particle> & p, vector<Particle> & q)
{
// brute force
if(n <= 3) {
float m = numeric_limits<float>::max();
for(int i = 0; i < n - 2; i++) {
for(int j = i + 1; j < n - 1; j++) {
if((set_A.find(p[i].id) != set_A.end() && set_A.find(p[j].id) != set_A.end()) || (set_B.find(p[i].id) != set_B.end() && set_B.find(p[j].id) != set_B.end())) {
continue;
}
float distsq = pow(p[i].x - p[j].x, 2) + pow(p[i].y - p[j].y, 2) + pow(p[i].z - p[j].z, 2);
pair<pair<Particle, Particle>, float> pa = make_pair(make_pair(p[i], p[j]), sqrt(distsq));
#pragma omp critical
insert(pa);
m = min(m, distsq);
}
}
return sqrt(m);
}
// copy first ceil(n/2) points of p to pl
vector<Particle> pl;
int ceiling = ceil(n/2);
for(int i = 0; i < ceiling; i++) {
pl.push_back(p[i]);
}
// copy first ceil(n/2) points of q to ql
vector<Particle> ql;
for(int i = 0; i < ceiling; i++) {
ql.push_back(q[i]);
}
// copy remaining floor(n/2) points of p to pr
vector<Particle> pr;
for(int i = ceiling; i < p.size(); i++) {
pr.push_back(p[i]);
}
// copy remaining floor(n/2) points of q to qr
vector<Particle> qr;
for(int i = ceiling; i < q.size(); i++) {
qr.push_back(p[i]);
}
float dl, dr, d;
#pragma omp task firstprivate(pl, ql, p, q, n) private(dl) shared(closest_pairs)
dl = efficient_closest_pair(ceil(n / 2), pl, ql);
#pragma omp task firstprivate(pl, ql, p, q, n) private(dr) shared(closest_pairs)
dr = efficient_closest_pair(ceil(n / 2), pr, qr);
#pragma omp taskwait
d = min(dl, dr);
float m = p[ceil(n / 2) - 1].x;
vector<Particle> s;
for(int i = 0; i < q.size(); i++) {
if(fabs(q[i].x - m) < d) {
s.push_back(Particle(q[i]));
}
}
int num = s.size();
float dminsq = d * d;
for (int i = 0; i < num - 2; i++) {
int k = i + 1;
while(k <= num - 1 && pow(s[k].y - s[i].y, 2) < dminsq) {
if((set_A.find(s[i].id) != set_A.end() && set_A.find(s[k].id) != set_A.end()) || (set_B.find(s[i].id) != set_B.end() && set_B.find(s[k].id) != set_B.end())) {
k++;
continue;
}
float dist = pow(s[k].x - s[i].x, 2) + pow(s[k].y - s[i].y, 2) + pow(s[k].z - s[i].z, 2);
pair<pair<Particle, Particle>, float> pa = make_pair(make_pair(s[i], s[k]), sqrt(dist));
#pragma omp critical
insert(pa);
dminsq = min(dist, dminsq);
k++;
}
}
return sqrt(dminsq);
}
The insert method looks like this:
void OMPParticleSim::insert(pair<pair<Particle, Particle>, float> & pair) {
if(closest_pairs.size() == 0) {
closest_pairs.push_back(pair);
return;
}
for(int i = 0; i < closest_pairs.size(); ++i) {
if(closest_pairs[i].second > pair.second) {
closest_pairs.insert(closest_pairs.begin() + i, 1, pair);
break;
}
}
if(closest_pairs.size() > k) {
closest_pairs.pop_back();
}
}
The start of the parallel region is here:
void OMPParticleSim::do_closest_pair(int num_threads) {
vector<Particle> p = set;
// presort on x
sort(p.begin(), p.end(), sortxomp);
vector<Particle> q = p;
// presort on y
sort(q.begin(), q.end(), sortyomp);
float cp;
#pragma omp parallel num_threads(num_threads)
{
#pragma omp single
{
cp = efficient_closest_pair(set.size(), p, q);
}
}
sort(closest_pairs.begin(), closest_pairs.end(), sortpairsomp);
}
All of the results are stored in a list closest_pairs and output to a file. The reason I know there are data races is because some of the Particle id's are negative (all of them start positive), and running the program multiple times results in different values being written to the file. Any help would be great!

Answer 1

The error was the dl and dr should have been shared between the tasks.