I got confused with my recent C++ MPI program's behavior. I have an MPI program in which I use clock() to measure the time spent between to MPI_Barrier, just like this:

MPI::COMM_WORLD.Barrier();
if if(rank == master) t2 = clock();

"code B";

I need to measure t2-t1 to see the time spent on the code A between these two MPI_Barriers. I notice that if I comment code B, the time seems much less the original time (almost half). How does it happen? What is a possible reason for it? I have no idea.

Thanks for your help.

Linbao