Reduction with OpenMP
openmp
Solution
Yes, you can - the reduction clause is applicable to the whole parallel region as well as to individual `for` worksharing constructs. This allows for e.g. reduction over computations done in different parallel sections (the preferred way to restructure the code):
#pragma omp parallel sections private(val) reduction(+:sum)
{
#pragma omp section
{
bla bla code
sum += val;
}
#pragma omp section
{
bla bla code
sum += val;
}
}
You can also use the OpenMP `for` worksharing construct to automatically distribute the loop iterations among the threads in the team instead of reimplementing it using sections:
#pragma omp parallel for private(val) reduction(+:sum)
for (row = 0; row < Rows; row++)
{
bla bla code
sum += val;
}
Note that reduction variables are private and their intermediate values (i.e. the value they hold before the reduction at the end of the `parallel` region) are only partial and not very useful. For example the following serial loop cannot be (easily?) transformed to a parallel one with reduction operation:
for (row = 0; row < Rows; row++)
{
bla bla code
sum += val;
if (sum > threshold)
yada yada code
}
Here the `yada yada code` should be executed in each iteration once the accumulated value of `sum` has passed the value of `threshold`. When the loop is run in parallel, the private values of `sum` might never reach `threshold`, even if their sum does.
Problem
I am trying to compute mean of a 2d matrix using openmp. This 2d matrix is actually an image. I am doing the thread-wise division of data. For example, if I have `N` threads than I process Rows/`N` number of rows with `thread0`, and so on. My question is: Can I use the openmp reduction clause with "`#pragma omp parallel`"? ``` #pragma omp parallel reduction( + : sum ) { if( thread == 0 ) bla bla code sum = sum + val; else if( thread == 1 ) bla bla code sum = sum + val; } ```