Performance Optimisation and Productivity
Replacing critical section with reduction
Pattern addressed: OpenMP critical sectionThe OpenMP standard provides a critical section construct, which only allows one thread to execute the block of code within the construct. This feature allows blocks of code to be protected from race conditions, for example with write accesses into a shared array or incrementing a shared counter. However, usage of this construct, especially within parallel loops, can severely reduce performance. This is due to serialisation of the execution causing threads to “queue” to enter the critical region, as well as introducing large lock-management overheads required to manage the critical region. (more...)
Required condition: When the critical section corresponds to a recurrent operation
This best practice recommends that if the critical block is performing a reduction operation, this be replaced by the OpenMP reduction clause which has a much lower overhead than a critical section.
Consider the following pseudo-code:
sum = 0.0;
#pragma omp parallel for
for ( int i = 0; i < Ni; i++ ) {
// work on array[:]
#pragma omp critical
sum += array[i];
}
This could be re-written to:
#pragma omp parallel for reduction(+:sum)
for ( int i = 0; i < Ni; i++ ) {
// work on array [:]
sum += array[i];
}
Implemented in program(s): OpenMP Critical (master) · OpenMP Critical (critical section replaced by a reduction) ·
