OpenMP: Include atomic section into parrallel region declaration - c++

Question

I have a parallel region where I monitor the progress. This means I use the the variable iteration to calculate the current state of the loop (percentage: 0 - 100 until loop is finished).
For this I increment with an atomic operation. Is there a way to make the code shorter, maybe by including iteration++ into the #pragma omp parallel for clause?
int iteration = 0;
#pragma omp parallel for
for (int64_t ip = 0; ip < num_voxels; ip++)
{
// calc stuff
#pragma omp atomic
iteration++;
// output stuff
// if thread == 0:
// Progress(iteration / num_voxels * 100);
}

Answer 1

I don't think it's possible to increment iteration elsewhere than inside the loop body. For instance, this is not allowed:
std::atomic<int> iteration{0};
#pragma omp parallel for
for (int64_t ip = 0; ip < num_voxels; ip++, iteration++) { ...
since OpenMP requires so-called Canonical Loop Form where the increment expression may not update multiple variables (see Section 2.6 of OpenMP 4.5 Spcification).
Also I would strongly advise against incrementing iteration within each loop, since it would be very inefficient (atomic memory operations = memory fences and cache contention).
I would prefer, e.g.:
int64_t iteration = 0;
int64_t local_iteration = 0;
#pragma omp parallel for firstprivate(local_iteration)
for (int64_t ip = 0; ip < num_voxels; ip++) {
{
... // calc stuff
if (++local_iteration % 1024 == 0) { // modulo using bitwise AND
#pragma omp atomic
iteration += 1024;
}
// output stuff
// if thread == 0:
// Progress(iteration / num_voxels * 100);
}
And, output only if progress in percents changes. This might be also tricky, since you need to read iteration atomically and you likely don't want to do that in each iteration. A possible solution, which also saves a lot of cycles regarding "expensive" integer division:
int64_t iteration = 0;
int64_t local_iteration = 0;
int64_t last_progress = 0;
#pragma omp parallel for firstprivate(local_iteration)
for (int64_t ip = 0; ip < num_voxels; ip++) {
{
... // calc stuff
if (++local_iteration % 1024 == 0) { // modulo using bitwise AND
#pragma omp atomic
iteration += 1024;
// output stuff:
if (omp_get_thread_num() == 0) {
int64_t progress;
#pragma omp atomic read
progress = iteration;
progress = progress / num_voxels * 100;
if (progress != last_prgoress) {
Progress(progress);
last_progress = progress;
}
}
}
}