MPI: Difference between revisions

While it is now possible to run this program under control of MPI, each process will still just output the original string which isn't very interesting. Let's begin by having each process output its rank and how many processes are running in total. This information is obtained at run-time by the use of the following functions.

<tt>MPI_Comm_size</tt> will report the number of processes running as part of this job by assigning it to the result parameter <tt>nproc</tt>.  Similarly, <tt>MPI_Comm_rank</tt> reports the rank of the calling process to the result parameter <tt>myrank</tt>. Ranks in MPI start counting from 0 rather than 1, so given N processes we expect the ranks to be 0..(N-1). The <tt>comm</tt> argument is a communicator, which is a set of processes capable of sending messages to one another. For the purpose of this tutorial we will always pass in the predefined value <tt>MPI_COMM_WORLD</tt>, which is simply all the processes started with the job (it is possible to define and use your own communicators, however that is beyond the scope of this tutorial and the reader is referred to the provided references for additional detail).

Let us incorporate these functions into our program, and have each process output its rank and size information. Note that since we are still having all processes perform the same function, there are no conditional blocks required in the code.

Compile and run this program on 2, 4 and 8 processors. Note that each running process produces output based on the values of its local variables, as one should expect. As you run the program on more processors, you will start to see that the order of the output from the different processes is not regular. The stdout of all running processes is simply concatenated together; you should make no assumptions about the order of output from different processes.