OMP005 serialisation-in-innermost-loop

warning enabled by default

An OpenMP critical or atomic is inside the innermost loop of a parallel loop.

Why

critical lets one thread at a time through, atomic serialises the threads that update the same location: inside the innermost loop they run once per iteration and the parallel loop runs at the speed of its serial part, or slower. Accumulate in a private variable (or a reduction clause) and combine once per thread, outside the loop.

Reported

!$omp parallel do
do i = 1, n
  !$omp critical
  total = total + w(i)
  !$omp end critical
end do

Preferred

!$omp parallel do reduction(+:total)
do i = 1, n
  total = total + w(i)
end do

Suppressing

For one occurrence, with the reason on the same line (or alone on the line before the statement):

! fobis: ignore[OMP005] <why this occurrence is fine>

For the whole project, ignore = OMP005 in the fobos [lint] section.

References

Sections of the shipped knowledge packs behind this rule:

• openmp:ch16-synchronization#anti-patterns (fobis kb show openmp:ch16-synchronization#anti-patterns)
• openmp:cheatsheet#synchronization (fobis kb show openmp:cheatsheet#synchronization)