Maximum Number of Tasks can be launched in one MPI application

Question

In the C binding, MPI_Comm_size returns an int type value. the maximum value of int in C is 32767. Does it mean at most 32K processes can be created in a communicator? But this seems too small since I know there are applications that can run on millions of cores. What I have missed?

Answer 1

From the POSIX man page of limits.h:

   {INT_MAX}
          Maximum value of an int.
          Minimum Acceptable Value: 2 147 483 647

The minimum of the maximum int is like thousands of millions (2^31 - 1).