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Reduction
How to handle this case?
Example |
double ave = 0.0, A [ MAX ]; int i;
for (i = 0; i < MAX; i++) {
ave + = A[i];
}
ave = ave/MAX; |
- We are combining values into a single accumulation variable (ave) there is a true dependence between loop iterations that can not be trivially removed
- This is a very common situation.it is called a “reduction”
- Support for reduction operations is included in most parallel programming environments.
Data Scope Attribute Clauses
“reduction” Clause |
Purpose: Reduction
Format: reduction (operation : list) |
- A local copy of each list variable is made and initialized depending on the “op” (e.g. 0 for “+”)
- Compiler finds standard reduction expressions containing “op” and uses them to update the local copy.
- Local copies are reduced into a single value and combined with the original global value
- Variables in the list must be named scalar variables. They can not be array or structure type variables.
- Reduction variables must be shared in the enclosing context.
- Note that the value of a reduction variable is undefined from the moment the first thread reaches the clause till the operation has completed
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