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A Simple Parallel Loop

Syntax
example_a11c n_thread repeat size

Purpose
Runs a timing test of Example A.1.1.1c of the OpenMP 2.5 standard document.

n_thread
If the argument n_thread is equal to automatic, dynamic thread adjustment is used. Otherwise, n_thread must be a positive number specifying the number of OpenMP threads to use.

repeat
If the argument repeat is equal to automatic, the number of times to repeat the calculation of the number of zeros in total interval is automatically determined. In this case, the rate of execution of the total solution is reported.

If the argument repeat is not equal to automatic, it must be a positive integer. In this case repeat determination of the number of times the calculation of the zeros in the total interval is repeated. The rate of execution is not reported (it is assumed that the program execution time is being calculated some other way).

size
The argument size is the length of the arrays in the example code.

Example Source

# ifdef _OPENMP
# include <omp.h>
# endif

# include <cmath>
# include <cstring>
# include <cstdlib>

// see http://www.coin-or.org/CppAD/Doc/cppad_vector.htm
# include <cppad/vector.hpp>

// see http://www.coin-or.org/CppAD/Doc/speed_test.htm
# include <cppad/speed_test.hpp>

// Beginning of Example A.1.1.1c of OpenMP 2.5 standard document ---------
void a1(int n, float *a, float *b)
{    int i;
# ifdef _OPENMP
# pragma omp parallel for
# endif
     for(i = 1; i < n; i++) /* i is private by default */
          b[i] = (a[i] + a[i-1]) / 2.0;
}
// End of Example A.1.1.1c of OpenMP 2.5 standard document ---------------
          
// routine that is called to repeat the example a number of times
void test(size_t size, size_t repeat)
{    // setup
     size_t i;
     float *a = new float[size];
     float *b = new float[size];
     for(i = 0; i < size; i++)
          a[i] = float(i);
     int n = int(size);
     // run test
     for(i = 0; i < repeat; i++)
          a1(n, a, b);
     // tear down
     delete [] a;
     delete [] b;
     return;
}

// main program
int main(int argc, char *argv[])
{
     using std::cout;
     using std::endl;

     // get command line arguments -----------------------------------
     char *usage = "example_a11c n_thread repeat size";
     if( argc != 4 )
     {    std::cerr << usage << endl;
          exit(1);
     }
     argv++;
     // n_thread 
     int n_thread;
     if( std::strcmp(*argv, "automatic") == 0 )
          n_thread = 0;
     else n_thread = std::atoi(*argv);
     argv++;
     // repeat 
     size_t repeat;
     if( std::strcmp(*argv, "automatic") == 0 )
          repeat = 0;
     else
     {    assert( std::atoi(*argv) > 0 );
          repeat = std::atoi(*argv);
     }
     argv++;
     // size 
     assert( std::atoi(*argv) > 1 );
     size_t size = std::atoi(*argv++);
     // ---------------------------------------------------------------

     // minimum time for test (repeat until this much time)
     double time_min = 1.;
# ifdef _OPENMP
     if( n_thread > 0 )
     {    omp_set_dynamic(0);            // off dynamic thread adjust
          omp_set_num_threads(n_thread); // set the number of threads 
     }
     // now determine the maximum number of threads
     n_thread = omp_get_max_threads();
     assert( n_thread > 0 );
     
     // inform the user of the maximum number of threads
     cout << "OpenMP: version = "         << _OPENMP;
     cout << ", max number of threads = " << n_thread << endl;
# else
     cout << "_OPENMP is not defined, ";
     cout << "running in single tread mode" << endl;
     n_thread = 1;
# endif
     // Correctness check (store result in ok)
     size_t i;
     float *a = new float[size];
     float *b = new float[size];
     for(i = 0; i < size; i++)
          a[i] = float(i);
     int n = size;
     a1(n, a, b);
     bool ok = true;
     for(i = 1; i < size ; i++)
          ok &= std::fabs( 2. * b[i] - a[i] - a[i-1] ) <= 1e-6; 
     delete [] a;
     delete [] b;

     if( repeat > 0 )
     {    // user specified the number of times to repeat the test
          test(size, repeat);
     }
     else
     {    // automatic determination of number of times to repeat test

          // speed test uses a SimpleVector with size_t elements
          CppAD::vector<size_t> size_vec(1);
          size_vec[0] = size;
          CppAD::vector<size_t> rate_vec =
               CppAD::speed_test(test, size_vec, time_min);

          // report results
          cout << "size             = " << size_vec[0] << endl;
          cout << "repeats per sec  = " << rate_vec[0] << endl;
     }
     if( ok )
          cout << "Correctness Test Passed" << endl;
     else cout << "Correctness Test Failed" << endl;

     return static_cast<int>( ! ok );
}

Input File: openmp/example_a11c.cpp