Parallel Processing Collision Pairs - c++

Question

I am writing some code for parallel processing of collisions, the expected result would be to have an acceleration for each thread, but I'm not getting any acceleration on the data processing because I have a critical section inside parallel_reduce() and I believe its serializing too much the access to the objects. This is how the code looks:
do {
totalVel = 0.;
#pragma omp parallel for
for (unsigned long i = 0; i < bodyContact.size(); i++) {
totalVel += bodyContact.at(i).bodyA()->parallel_reduce();
totalVel += bodyContact.at(i).bodyB()->parallel_reduce();
}
} while (totalVel >= 0.00001);
Is there any way to gain more speed by making it parallel or the serialization of access is too much?
Observations:
bodyA() and bodyB() are objects that repeat themselves a lot inside the bodyContact container.
For now parallel_reduce() only does one multiplication (the critical section), but will get more complex.
double parallel_reduce(){
#pragma omp critical
this->vel_ *= 0.99;
return vel_.length();
}
Actual timings:
serial, 25.635
parallel, 123.559

Answer 1

There is always cost of using OpenMP constructs, so avoid using a parallel inside a loop, following the implementation it could launch at each time new threads, instead of rewaking the previous launched threads.
In fact, if bodyContact.size() is small and the do {} while in number of step is big and parallel_reduce is very quick is very hard to have scalability with just a few OpenMP pragma.
#pragma omp parallel shared(totalVel) shared(bodyContact)
{
do {
totalVel = 0.;
#pragma omp for reduce(+:totalVel)
for (unsigned long i = 0; i < bodyContact.size(); i++) {
totalVel += bodyContact.at(i).bodyA()->parallel_reduce();
totalVel += bodyContact.at(i).bodyB()->parallel_reduce();
}
} while (totalVel >= 0.00001);
}

Answer 2

The above is likely not only slower, but very likely wrong; all the threads are trying to update the same totalVel. Tonnes of race conditions, but also contention, cache invalidation, etc.
Assuming the parallel_reduce() stuff is ok, you'd like something more like
do {
totalVel = 0.;
#pragma omp parallel for default(none) shared(bodyContact) reduction(+:totalVel)
for (unsigned long i = 0; i < bodyContact.size(); i++) {
totalVel += bodyContact.at(i).bodyA()->parallel_reduce();
totalVel += bodyContact.at(i).bodyB()->parallel_reduce();
}
} while (totalVel >= 0.00001);
which will do the reduction on totalVel correctly.