c++ - OpenMP unpredictable overhead -

i have simple openmp code runs 4 empty threads , reports time:

#include "omp.h" #include <sys/time.h> #include <cstdio> double start_time;  void process_thread() { }  int main(){     struct timeval tv;     gettimeofday(&tv, null);     start_time = 1000*((double)tv.tv_sec + 1e-6*(double)tv.tv_usec);     printf("beginning time: %.3fms   ",start_time-start_time);      #pragma omp parallel schedule(static,1)     for(int i=0;i<4;i++)         process_thread();      gettimeofday(&tv, null);     double cur_time = 1000*((double)tv.tv_sec + 1e-6*(double)tv.tv_usec);     printf("finishing    time:  %.3fms\n\n",cur_time-start_time); } 

without openmp running time small , consistent:

beginning time: 0.000ms   finishing time: 0.050ms beginning time: 0.000ms   finishing time: 0.050ms beginning time: 0.000ms   finishing time: 0.049ms beginning time: 0.000ms   finishing time: 0.049ms beginning time: 0.000ms   finishing time: 0.050ms beginning time: 0.000ms   finishing time: 0.049ms beginning time: 0.000ms   finishing time: 0.051ms beginning time: 0.000ms   finishing time: 0.063ms beginning time: 0.000ms   finishing time: 0.050ms beginning time: 0.000ms   finishing time: 0.051ms 

however, when use openmp takes longer , time becomes inconsistent:

beginning time: 0.000ms   finishing time: 0.497ms beginning time: 0.000ms   finishing time: 1.001ms beginning time: 0.000ms   finishing time: 0.484ms beginning time: 0.000ms   finishing time: 16.728ms beginning time: 0.000ms   finishing time: 4.205ms beginning time: 0.000ms   finishing time: 6.041ms beginning time: 0.000ms   finishing time: 3.754ms beginning time: 0.000ms   finishing time: 4.126ms beginning time: 0.000ms   finishing time: 0.230ms beginning time: 0.000ms   finishing time: 0.183ms beginning time: 0.000ms   finishing time: 2.438ms beginning time: 0.000ms   finishing time: 0.248ms 

i under impression openmp overheads in range of 150ms seems way more.

i use xeon e5 1650 6 cores ubuntu 12.04. use gcc 4.6.3. have tried other openmp configurations static works best case.

what responsible fluctuation? fault of compiler, openmp runtime, os, or else? how can improve it?