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Combined Parallel Work-sharing Constructs
- OpenMP provides combined Parallel Worksharing directive that are merely shortcuts:
- PARALLEL DO/parallel for
- Parallel SECTIONS
- These directives behave same as an individual PARALLEL directive being immediately followed by a separate
work-sharing directive.
- Most of the rules and clauses that apply to both directives are in effect
#pragma omp parallel
#pragma omp for
for (...) |
#pragma omp parallel for
for (....) |
#pragma omp parallel
#pragma omp sections
{...} |
#pragma omp parallel sections
{.... } |
Synchronization
- Consider a simple example where two threads on two different processors are both trying to increment a variable x at the same time (assume x is initially 0):
THREAD 1
increment(x){
x = x+1;
}
THREAD 1
10 LOAD A, (x address)
20 ADD A, 1
30 STORE A, (x address) |
THREAD 2
increment(x){
x = x+1;
}
THREAD 2
10 LOAD A, (x address)
20 ADD A, 1
30 STORE A, (x address) |
- What are the possible outputs?
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