mpicc, mpif90, and mpirun

Overview

Teaching: 5 min
Exercises: 0 min

Questions

Objectives

What mpicc/ mpif90 do

• Just wrappers for the system C, Fortran compilers that have the various -I, -L clauses in there automaticaly
• –showme (an Open MPI option) shows which options are being used, e.g.:

$ mpicc --showme hello-world.c -o hello-world

gcc -I/usr/local/openmpi.gcc-1.2.9/include 
  -pthread hello.c -o hello-c 
  -L/usr/local/openmpi.gcc-1.2.9/lib
  -lmpi -lopen-rte -lopen-pal -ldl 
   -Wl,--export-dynamic -lnsl -lutil -lm -ldl

• Just runs gcc hello.world.c -o hello-world (or whatever the appropriate compiler is - gfortran, icc, pgf90, etc) with a number of options (-I, -L, -l) to make sure right libraries, headers are available.

What mpirun does

• Launches n processes, assigns each an MPI rank and starts the program
• For multinode run, has a list of nodes, ssh’s (or moral equivalent) to each node and launches the program

Number of Processes

• Number of processes to use is almost always equal to the number of processors
• But not necessarily.
• On your nodes, what happens when you run this?

$ mpirun -np 24 hello-world

mpirun runs any program

• mpirun will start that process rlaunching procedure for any program
• Sets variables somehow that mpi programs recognize so that they know which process they are

$ hostname  
$ mpirun -np 4 hostname  
$ ls
$ mpirun -np 4 ls

Key Points