c++ - Doing a section with one thread and a for-loop with multiple threads -

i using openmp , want spawn threads such 1 thread executes 1 piece of code , finishes, in parallel n threads running iterations of parallel-for loop.

execution should this:

section (one thread)       ||      section b (parallel-for, multiple threads)          |                   ||        | | | | | | | | | |          |                   ||        | | | | | | | | | |          |                   ||        | | | | | | | | | |          |                   ||        | | | | | | | | | |          |                   ||        | | | | | | | | | |          v                   ||        v v v v v v v v v v 

i cannot write parallel-for #pragma omp once because not want thread executes section execute for-loop.

i have tried this:

#pragma omp parallel sections {     #pragma omp section     {          // section     }      #pragma omp section     {          // section b;          #pragma omp parallel          (int = 0; < x; ++i)              something();     } } 

however, parallel-for executes 1 thread (i know because made body of loop print omp_get_thread_num() , same number, either 1 or 0 depending on thread of 2 executed second parallel section).

i have tried

#pragma omp sections {     #pragma omp section     {          // section     }      #pragma omp section     {          // section b;          #pragma omp parallel          (int = 0; < x; ++i)              something();     } } 

which allows for-loop execute multiple threads, makes sections non-parallel, , first section executed sequentially before second section.

what need combination of 2 approaches, each iteration of for-loop , first section run in parallel.

nested parallelism must explicitly set, disabled default in implementations. standing openmp 4.0 standard, must set omp_nested environment variable:

the omp_nested environment variable controls nested parallelism setting initial value of nest-var icv. value of environment variable must true or false. if environment variable set true, nested parallelism enabled; if set false, nested parallelism disabled. behavior of program implementation defined if value of omp_nested neither true nor false.

the following line should work bash:

 export omp_nested=true 

futhermore, noted @hristoiliev in comment below, it's want set omp_num_threads environment variable tune performance. quoting standard:

the value of environment variable must list of positive integer values. values of list set number of threads use parallel regions @ corresponding nested levels.

this means 1 should set value of omp_num_threads similar n,n-1 n number of cpu cores. instance:

export omp_num_threads=8,7 

for 8-core system (example copied comment below).