c++ - openMP excessive synchronization -


i trying add openmp parallelization quite big project , found out openmp synchronization outside parallel blocks.

this synchronization done of variables, not used in parallel block , done continuously, not before entering block.

i made example proving this:

#include <cmath>  int main() {     double dummy1 = 1.234;      int const size = 1000000;     int const size1 = 2500;     int const size2 = 500;      for(unsigned int i=0; i<size; ++i){          //for (unsigned int j=0; j<size1; j++){         //  dummy1 = pow(dummy1/2 + 1, 1.5);         //}          #pragma omp parallel         (unsigned int j=0; j<size2; j++){             double dummy2 = 2.345;             dummy2 = pow(dummy2/2 + 1, 1.5);         }     } }

if run code (with cycle commented), runtimes 6.75s parallelization , 30.6s without. great.

but if uncomment cycle , run again, excessive synchronization kicks in , results 67.9s parallelization , 73s without. if increase size1 slower results parallelization without it.

is there way disable synchronization , force before second cycle? or other way how improve speed?

note outer neither first cycle in real example parallelizable. outer 1 in fact ode solver , first inner 1 updating of loads of inner values.

i using gcc (suse linux) 4.8.5

thanks answers.


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