About Reduction
reduction is a key in OpenMP to apply multi-thread technique automatically during cumulative operation.
It assign private variables to each thread with a initial value, do the operation and accumulate them. The accepted operations and the initial values are listed as follow:
| operator | Private var initial value |
|---|---|
| + | 0 |
| - | 0 |
| * | 1 |
| & | ~0 |
| | | 0 |
| ^ | 0 |
| && | 1(True) |
| || | 0(False) |
Operators max and min are also accepted. max returns the largest representable number in the reduction list item type while min returns the least representable number in the reduction list item type.
Example
#include <stdio.h>
#include <omp.h>
int main() {
omp_set_num_threads(4);
int sum = 0;
#pragma omp parallel for reduction(+:sum)
for (int i = 0; i < 10; i++) {
sum += i;
int num = omp_get_thread_num();
printf("Current Sum: %d, Current i: %d, Current Thread: %d\n", sum, i, num);
}
printf("\nSum is : %d\n",sum);
}
How i in [0, 10) assigned to each thread:
Result:
