MPI. Frequently Asked Questions.

Q: Where can I download an free implementation of MPI?

A: There are a number of free implementations. The recommended one is MPICH2 by Argonne National Laboratory.

Q: Which languages is MPI compatible with?

A: It is recommended to use C/C++.

Q: What operating systems support MPICH2?

A: MPICH2 can be set up on Windows (32 and 64 bits) as well as UNIX.

Q: I have setup MPICH2 on Windows. What else do I need?

A: You need to use some C/C++ programming environment (IDE). It is recommended to use:

• MinGW (GNU compiler for C/C++ and linker for Windows)
• eclipse (open source IDE).

Q: How do I write a program in eclipse and how do I compile it?

A:

1. Create a new C project

   

2. Declare the project parameters (name, path, use of MinGW)
   

   

3. When the project appears in the left side, right click on it and select "Properties"
   

   

4. Add the include directory "C:\Program Files\MPICH2\include" in Include paths (-I)
   

   

5. Add the library directory "C:\Program Files\MPICH2\lib" in Library search path (-L)
   

   

6. Create connection with the MPI library C:\Program Files\MPICH2\lib\mpi.lib from the option "Miscellaneous" > "Other objects"
   

   

7. Create a new .c file and for your C source code. Give it any name, eg. myprog.c

   

8. After the file myprog.c write your C code in it

   

9. After you've written the code, build the application selecting the "hammer" icon (or from the menu "Project" > "Build Project")

   

10. We have now the executable file myprog.exe in folder C:\mpi\myprog\Debug\.

Q: I have created the executable myprog.exe. How do I run it?

A: There are a number of options:

• If you have acces to a computer cluster then the command, for example,
  mpiexec -n 8 myprog.exe
  generates 8 processes which run on machines chosen by MPI from the set of available machines.
  Alternatively, you can use the command
  mpiexec -hosts 3 machine1 3 machine2 4 machine3 1 myprog.exe
  generating a total of 8 processes executing on machines: machine1, machine2, and machine3. With this command we tell MPI that machine1 will execute 3 processes machine2 will execute 4 processes and machine3 will execute 1 process.
• If only one machine is available, we can emulate parallel execution. For example, using the command
  mpiexec -localonly 5 myprog.exe
  we generate a total of 5 processes which run locally and can exchange messages through network loopback. This is a very useful option for debugging programs, especially when a computer cluster is not readily available.

Note: if we just run
myprog.exe
then only 1 process will be generated, so there is no parallel execution. This is equivalent to the command
mpiexec -n 1 myprog.exe