When is the reduction needed?

Question

I\'ve written this code which reads a Matrix and it basically sums the values of the matrix... But my question would be, since I\'ve tried doing the pragma in different ways

Answer 1

The code you posted is not correct without the reduction clause.

sum += matrixA[i][j];

Will cause a classic race condition when executed by multiple threads in parallel. Sum is a shared variable, but sum += ... is not an atomic operation.

(sum is initially 0, all matrix elements 1)
Thread 1                     |  Thread 2
-----------------------------------------------------------
tmp = sum + matrix[0][0] = 1 |
                             | tmp = sum + matrix[1][0] = 1
sum = tmp = 1                |
                             | sum = tmp = 1 (instead of 2)

The reduction fixes exactly this. With reduction, the loop will work on an implicit thread-local copy of the sum variable. At the end of the region, the original sum variable will be set to the sum of all thread-local copies (in a correct way without race-conditions).

Another solution would be to mark the sum += ... as atomic operation or critical section. That, however has a significant performance penalty.