Parallel sections in OpenMP using a loop
openmp, parallel-processing, sections
Solution
With explicit OpenMP tasks:
#pragma omp parallel
{
// Let only one thread create all tasks
#pragma omp single nowait
{
for (int i = 0; i < num_tasks; i++)
#pragma omp task
{
// Code for task with parameters, based on i
}
}
// Let the threads process all tasks
#pragma omp taskwait
// Further parallel processing ...
}
The code block that follows the OpenMP `task` directive is an explicit task. Explicit tasks are queued an executed later. The `taskwait` directive acts similar to `barrier`, but for tasks. Also see this answer to a similar question.
Tasks can create other tasks recursively. So explicit tasking can be used to process graphs and trees. But beware of the overhead - it is bigger than the overhead from most other constructs and is quite similar to the one that comes from loops with `schedule(dynamic)`. Also the variables from the outer scope, referenced inside the task are by default `firstprivate`.
Note that explicit tasks are feature, that was added in OpenMP 3.0. Compilers that conform to earlier OpenMP versions might not support the `task` directive. Almost all modern compilers do support OpenMP 3.0 or later with the notable exception of Microsoft Visual C++, which only supports OpenMP 2.0 (even in VS2012).
Problem
I wonder if there is any technique to create parallel sections in OpenMp using a for-loop. For example, instead of creating n different #pragma omp sections, I want to create them using an n-iteration for-loop with some changing parameters for each section. ``` #pragma omp parallel sections { #pragma omp section { /* Executes in thread 1 */ } #pragma omp section { /* Executes in thread 2 */ } #pragma omp section { /* Executes in thread n */ } } ```