What is the usage of reduction in openmp? -


i have piece of code parallelized.

int i,n; double area,pi,x; area=0.0; #pragma omp parallel private(x) reduction (+:area) for(i=0; i<n; i++){ x= (i+0.5)/n; area+= 4.0/(1.0+x*x); } pi = area/n;

it said reduction remove race condition happen if didn't use reduction. still i'm wondering need add lastprivate area since used outside parallel loop , not visible outside of it. else reduction cover well?

reduction takes care of making private copy of area each thread. once parallel region ends area reduced in 1 atomic operation. in other words area exposed aggregate of private areas of each thread.

thread 1 - private area = compute(x) thread 2 - private area = compute(y) thread 3 - private area = compute(z)  reduction step - public area = area<thread1> + area<thread2> + area<thread3> ...

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