c++ - Basic OpenMP Parallel Program Not Scaling As Expected - 


#include <iostream> #include <vector> #include <stdexcept> #include <sstream> #include <omp.h>  std::vector<int> col_sums(const std::vector<std::vector<short>>& data) {     unsigned int height = data.size(), width = data[0].size();     std::vector<int> totalsums(width, 0), threadsums(width, 0);      #pragma omp parallel firstprivate(threadsums)     {         #pragma omp parallel         (unsigned int = 0; < height; i++) {             threadsums.data()[0:width] += data[i].data()[0:width];         }         #pragma omp critical         {             totalsums.data()[0:width] += threadsums.data()[0:width];         }     }     return totalsums; }  int main(int argc, char** argv) {     if (argc < 3) {         std::cout << "run program \"executable <rows> <columns>\n";     } else {         std::stringstream args;         args << argv[1] << " " << argv[2];         int rows, columns;         args >> rows >> columns;         std::vector<std::vector<short>> data(rows, std::vector<short>(columns));         std::vector<int> columnsums = col_sums(data);     } }
  • export omp_num_threads=4
  • icpc -ofast -fopenmp -g dummy.cpp -o dummy
  • /usr/bin/time -v ./dummy 115000 20000
  • cpu% = 225% (should 380%+)

i'm experienced openmp , cilkplus, barrier scaling here eludes me, , rudimentary program. know has obvious, feel i've erased data hazards , control hazards. i'm totally stumped.


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