omp_set_nested

 

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Enables nested parallelism.

void omp_set_nested(  
   int val  
);  

where,

val
If nonzero, enables nested parallelism. If zero, disables nested parallelism.

OMP nested parallelism can be turned on with omp_set_nested, or by setting the OMP_NESTED environment variable.

The setting for omp_set_nested will override the setting of the OMP_NESTED environment variable.

When enabled, the environment variable can break an otherwise operational program because the number of threads increases exponentially when nesting parallel regions. For example a function that recurses 6 times with the number of OMP threads set to 4 requires 4,096 (4 to the power of 6) threads In general, the performance of your application will degrade if the number of thread exceeds the number of processors. One exception to this would be I/O bound applications.

Use omp_get_nested to display the current setting of omp_set_nested.

For more information, see 3.1.9 omp_set_nested Function.

// omp_set_nested.cpp  
// compile with: /openmp  
#include <stdio.h>  
#include <omp.h>  
  
int main( )   
{  
    omp_set_nested(1);  
    omp_set_num_threads(4);  
    printf_s("%d\n", omp_get_nested( ));  
    #pragma omp parallel  
        #pragma omp master  
        {  
            printf_s("%d\n", omp_get_nested( ));  
        }  
}  

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