Reduction omp example
For example, if we would parallelize the previous for loops by simply adding #pragma omp parallel for we would introduce a data race, because multiple threads could try to update the shared variable at the same time.
This statements occur in the body liste des concours 2018 maroc of the for loop.The threads then perform the following computations Thread 1 sumloc_1 a0 a1 a2 Thread 2 sumloc_2 a3 a4 a5 Thread 3 sumloc_3 a6 a7 a8 In the end, when the treads join together, OpenMP reduces local copies to the shared reduction variable sum sumloc_1 sumloc_2.OpenMP creates a team of threads and then shares the iterations of the for loop between the threads.Fortran example do i 1, n sum sum a(i) enddo C/C Example for(i1; i n; i) sum sum ai; How reduction works: sum is the reduction variable cannot be declared shared threads would overwrite the value of sum cannot be declared private private variables don't.X expr.Implementation How does OpenMP parallelize a for loop declared with a reduction clause?The thread modifies only the local copy of this variable.In this case the operator o is not allowed.But a parallelization of the previous for loops is tricky.It shows many meanings of a reduction.Therefore, OpenMP has the special reduction clause which can express the reduction of a for loop.If the reduction operator o is one of the operators listed above and if the statement for the reduction variable x is one of the statements listed above, then we can express the reduction with the loop construct: #pragma omp for reduction(o: x) for (.).There is a shared variable (sum / product / reduction) which is modified in every iteration.We looked at the specifications of the clause and familiarized ourselves with its implementational details.But for loops which represent a reduction are quite common.More information about the specification of the reduction clause is available in the OpenMP specification on the page 201.Therefore, we can argue that the following example is a reduction sum a0 a1 a2 a3 a4 a5 a6 a7.The statements are x x o expr.I clicked on several entries from this page and I realized that a reduction simplifies something complex into something which is understandable and uncomplicated.
It simplifies the long expression a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 into something shorter sum.
An example of a reduction operation is a summation!
Each thread has its own local copy of the reduction variable.
When the threads join together, all the local copies of the reduction variable are combined to the global shared variable.