[Omp] Does omp really work?

[Mircea Tonceanu]

    Hello everybody, 
   
   
  I am dealing with simulations for n-body systems and I was very happy learning about omp. Anyway, after trying to implement it in my programs I was forced to test it in  the simpliest program possible. Then I found out that something went wrong. I mean timing. Here you can see an extra simplified program I am concerned about: 
   
    #include <conio.h>
  #include <stdio.h>
  #include <windows.h>
  #include <omp.h>
   
   
  const long n = 5000000; // 5 million particles
  long arr[n];
  long arrb[n];
  DWORD time_init;
  DWORD time_final;
  DWORD time_elapsed;
   
  int main()
  {
  time_init = GetTickCount();
   
  
  #pragma omp parallel for
  for ( long k=1; k<n-1; k++)
  {
  arrb[k] = arr[k-1] + arr[k+1];
  }
  
   
  time_final = GetTickCount();
  time_elapsed = time_final - time_init;
  printf("Intial time:\t %u \n", time_init);
  printf("Final time:\t %u \n", time_final);
  printf("Elapsed time:\t %u \n", time_elapsed);
   
  _getch();
  return 0;
  }
   
   
  Conditions:
  - AMD x64 dual core 3800+;
  - 1 GB RAM;
  -  Win XP Pro X64;
  - the program was compiled for 32 bits;
  - with pragma enabled,  task manager shows 2 threads and without, 1 thread;
  - Visual Studio 2005 's compiler
   
   
  The results are:
   
  1. With pragma directive enabled:
   
  -  n = 5 million ->  time_elapsed = 47 (meaning 47* 15 milisec which is the resolution of my timer);
  -  n = 50 million ->  time_elapsed = 344;
  - n< 5 million -> time_elapsed = 16;
   
   
  2. With pragma directive disabled:
   
  -  n = 5 million ->  time_elapsed = 31 ;
  -  n = 50 million ->  time_elapsed = 344;
  - n< 5 million ->  time_elapsed = 0 (less then 15 ms).
   
   
  In all other cases, time_elapsed is allways greater in case of  #pragma omp enabled, which is not good. 
   
  Questions:
   
  1. Why?
  2. Why task manager shows 2 threads continously after the for loop has finished ?
  3. Am I doing wrong something, or I have a misunderstanding of the omp ?
  4. Am I stupid ? 
   
  Please help .
   
   
   
   
   
   
   
   




		
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