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MPI consists of a library of routines with Fortran, C, and C++ bindings to facilitate the coding of large computer tasks in parallel in a distributed memory MIMD environment. In addition it provides a run-time environment to build and execute parallel codes. It comes with implementations on most popular platforms, including the home made beowulf type systems.
MPICH is an Open Source implementation of MPI in a platform independent way. It implements fully MPICH Version 3.0. The MPICH implementation of MPI is what we will use in this course.
MPI maintains internal data-structures related to the administrations of the parallel tasks and to allow internal communications between the tasks and the environment. The latter are referred to as handles.
By convention, the C MPI routines return an int and the Fortran routines return an integer, ierror, in the calls which contain the error status of the calls. Error detection and debugging of parallel codes are difficult in general, and MPI can only attempt to detect errors.
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All MPI routines are labeled MPI_Name, with the first letter of the name capitalized.
The C calls syntax is
int MPI_Name( <argument list> )
with an error code returned as an int upon execution. The header file is mpi.h. The MPI provided constants are all capitalized, i.e., MPI_COMM_WORLD.
Fortran calls follow the syntax
call MPI_Name ( <argument list>, ierror )
with the error code being an integer type. The header file is mpif.h. The MPI provided constants can be used with capitalized letter, i.e., MPI_COMM_WORLD. However, note that Fortran is case insensitive, and that consequently the use of capital letters is for clarity only.
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int MPI_Init( int *argc, char ***argv)
Initializes the executables on the nodes to the MPI handling daemons. It creates a communicator MPI_COMM_WORLD which encompasses all the initialized processes on the various nodes and give a unique identification to each process, the rank number. The rank and size of the communicator can be obtained via MPI query routines.
In addition, MPI_Init(), carries the command line
arguments, conventionaly written as
int *argc, char **argv,
to the MPI functions.
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Who Am I?
int MPI_Comm_size( MPI_Comm comm, int *size)
int MPI_Comm_rank( MPI_Comm comm, int *rank)
Returns respectively the size of the specified communicator and the rank (process number) of the current process within the specified communicator. The rank number goes between 0 and size-1.
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int MPI_Finalize()
Must be the last call to MPI in any MPI code. No other call to MPI routines can be executed after it. Note that a new communicator can not be invoked after MPI_finalize().
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Compiles a C code "name.c", produces the object under "name", and links it with the MPI library. The -o option is to specify the executable name, the default being "a.out". This command calls the default C compiler, usualy the GNU gcc compiler on linux based computers.
mpif77 <name.f> -o <name>Compiles a Fortran code "name.f", produces the object under "name.o", and links it with the MPI library. The -o option is to specify the executable name, the default being "a.out". This command calls the default Fortran compiler, usualy the GNU f77 compiler on linux based computers.
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Note that MPI will choose the nodes automatically, starting from the current one (launching node) and selecting from the list of available processors in the hosts file. If number-of-processes is larger than the number of nodes included in the communication ring, the executable will be launched more than once in some of the chosen nodes in a cyclic way.
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