mpi a FORTRAN77 code which contains some examples of the use of the Message Passing Interface (MPI).
MPI allows a user to write a program in a familiar language, such as C, C++, FORTRAN, or Python, and carry out a computation in parallel on an arbitrary number of cooperating computers.
A remarkable feature of MPI is that the user writes a single program which runs on all the computers. However, because each computer is assigned a unique identifying number, it is possible for different actions to occur on different machines, even though they run the same program:
if ( I am processor A ) then
add a bunch of numbers
else if ( I am processor B ) then
multipy a matrix times a vector
end
Another feature of MPI is that the data stored on each computer is entirely separate from that stored on other computers. If one computer needs data from another, or wants to send a particular value to all the other computers, it must explicitly call the appropriate library routine requesting a data transfer. Depending on the library routine called, it may be necessary for both sender and receiver to be "on the line" at the same time (which means that one will probably have to wait for the other to show up), or it is possible for the sender to send the message to a buffer, for later delivery, allowing the sender to proceed immediately to further computation.
Here is a simple example of what a piece of the program would look like, in which the number X is presumed to have been computed by processor A and needed by processor B:
if ( I am processor A ) then
call MPI_Send ( X )
else if ( I am processor B ) then
call MPI_Recv ( X )
end
Often, an MPI program is written so that one computer supervises the work, creating data, issuing it to the worker computers, and gathering and printing the results at the end. Other models are also possible.
It should be clear that a program using MPI to execute in parallel will look much different from a corresponding sequential version. The user must divide the problem data among the different processes, rewrite the algorithm to divide up work among the processes, and add explicit calls to transfer values as needed from the process where a data item "lives" to a process that needs that value.
A FORTRAN77 program, subroutine or function that calls any MPI function, or uses an MPI-defined variable, must include the line
include "mpif.h"
You probably compile and link your program with a single command, as in
f77 myprog.f
mpif77 myprog.f
Some systems allow users to run an MPI program interactively. You do this with the mpirun command:
mpirun -np 4 a.out
The mpirun command may be a convenience for beginners, with very small jobs, but this is not the way to go once you have a large lengthy program to run! Also, what actually happens can vary from machine to machine. When you ask for 4 processors, for instance,
The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.
mpi examples are available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version.
COMMUNICATOR_MPI, a FORTRAN77 program which creates new communicators involving a subset of initial set of MPI processes in the default communicator MPI_COMM_WORLD.
HEAT_MPI, a FORTRAN77 program which solves the 1D time dependent heat equation using the finite difference method, with parallelization from MPI.
HELLO_MPI, a FORTRAN77 program which prints out "Hello, world!" using the MPI parallel programming environment.
MOAB, examples which illustrate the use of the MOAB job scheduler for a computer cluster.
MPI_STUBS, a FORTRAN77 library which allows a user to compile, load, and possibly run an MPI program on a serial machine.
MULTITASK_MPI, a FORTRAN77 program which demonstrates how to "multitask", that is, to execute several unrelated and distinct tasks simultaneously, using MPI for parallel execution.
POISSON_SERIAL, a FORTRAN77 program which computes an approximate solution to the Poisson equation in a rectangle, and is intended as the starting point for the creation of a parallel version.
PRIME_MPI, a FORTRAN77 program which counts the number of primes between 1 and N, using MPI for parallel execution.
PTHREADS, C programs which illustrate the use of the POSIX thread library to carry out parallel program execution.
QUAD_MPI, a FORTRAN77 program which approximates an integral using a quadrature rule, and carries out the computation in parallel using MPI.
RING_MPI, a FORTRAN77 program which uses the MPI parallel programming environment, and measures the time necessary to copy a set of data around a ring of processes.
SATISFY_MPI, a FORTRAN77 program which demonstrates, for a particular circuit, an exhaustive search for solutions of the circuit satisfiability problem, using MPI to carry out the calculation in parallel.
SEARCH_MPI, a FORTRAN77 program which searches integers between A and B for a value J such that F(J) = C, using MPI for parallel execution.
TASK_DIVISION, a FORTRAN77 library which implements a simple procedure for smoothly dividing T tasks among P processors; such a method can be useful in MPI and other parallel environments, particularly when T is not an exact multiple of P, and when the processors can be indexed starting from 0 or from 1.
WAVE_MPI, a FORTRAN77 program which uses finite differences and MPI to estimate a solution to the wave equation.