Lesson 12

Date: 4/16/2014
High Performance Computing (part I)
Linux for Engineering and IT applications


OpenMP


  • Shared memory is used by parallel threads.
  • The master process forks threads in the parallel region of the code.



  • Parallel regions are specified by the compiler directives in the code:

    C / C++ - General Code Structure 
    #include <omp.h>
main()  {
int var1, var2, var3;
Serial code 
      .
      .
Beginning of parallel section. Fork a team of threads. Specify variable scoping 

#pragma omp parallel private(var1, var2) shared(var3)
  {
  Parallel section executed by all threads 
        .
  All threads join master thread and disband 
  }  
Resume serial code 
}

  • GNU 4.2 and newer gcc and gfortran compilers include OpenMP support. 
    gcc -fopenmp testcode.c
export OMP_NUM_THREADS=4
./a.out
  • Usually, OMP_NUM_THREADS shouldn't exceed the total number of available cores in a system.
  • If OMP_NUM_THREADS is undefined, the run will utilize all available cores on the system.

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