multithreading - C++ trying to improve performance of pthread program -

i need improving speed of multithread program in c++ using pthreads.

std::vector<double> solve_progon(std::vector<std::vector<double> > a, std::vector <double> b) {   // solving }  std::vector<double> solve(std::vector<double> left, std::vector<double> mid, std::vector<double> right, int j) {     //solving }  void * calc(void *thread) {     long t = (long) thread;     int start_index = t * (x_size / threads);      int end_index = (t != threads - 1)?(t + 1) * (x_size / threads) - 1: x_size - 1;     std::vector<std::vector<double> > local, next;     std::vector<double> zeros;     (int = 0; < y_size; i++) {         zeros.push_back(0);     }     double cur_time = 0;     while (cur_time < t) {         (int = start_index; <= end_index; ++) {                 next.push_back(solve(phi[i - 1], phi[i], phi[i + 1], - start_index));         }         cur_time += dt;         pthread_barrier_wait(&bar);         (int = start_index; <=end_index; i++) {             phi[i] = next[i - start_index];         }         next.clear();         pthread_barrier_wait(&syn);     }     pthread_exit(null); }  int main(int argc, char **argv) {     //some init     pthread_barrier_init(&bar, null, threads);     pthread_barrier_init(&syn, null, threads);     pthread_t *threads = new pthread_t[threads];     unsigned long long start = clock_time();     (long = 0; < threads; i++) {         if (pthread_create(&threads[i], null, calc, (void *)i) != 0) {             std::cout << "can't create thread " << << std::endl;         }     }     (int = 0; < threads; i++) {         pthread_join(threads[i], null);     }     std::cout << "it takes " << (double)(clock_time() - start) / 1e9 << std::endl;       return 0; } 

full version @ https://github.com/minaevmike/fedlab_pthread/blob/master/main.cpp f.e. if have 4 thread calculation time 118.288 sec. if 1 101.993. how can improve speed. thank you.