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Mpi, or message passing interface, is a standard library for message-passing that enables the development of portable message-passing programs using c or fortran. An overview of the mpi standard, its core routines, and their usage for starting and terminating the mpi library, querying information, and sending and receiving messages. It also includes an example mpi program.
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MPI defines a standard library for message-passingthat can be used to develop portable message-passing programs using either C or Fortran.
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The MPI standard defines both the syntax as well asthe semantics of a core set of library routines.
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Vendor implementations of MPI are available onalmost all commercial parallel computers.
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It is possible to write (min) fully-functional message-passing programs by using only the six routines.
The minimal set of MPI routines.
MPI_Init
Initializes MPI.
MPI_Finalize
Terminates MPI.
MPI_Comm_size
Determines the number of processes.
MPI_Comm_rank
Determines the label of calling process.
MPI_Send
Sends a message.
MPI_Recv
Receives a message.
A communicator defines a
communication
domain
to communicate with each other.
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Information about communication domains isstored in variables of type MPI_Comm.
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Communicators are used as arguments to allmessage transfer MPI routines.
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A process can belong to many different(possibly overlapping) communicationdomains.
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MPI defines a default communicator calledMPI_COMM_WORLD which includes all theprocesses.
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MPI Datatype
C Datatype
MPI_CHAR
signed char
MPI_SHORT
signed short int
MPI_INT
signed int
MPI_LONG
signed long int
MPI_UNSIGNED_CHAR
unsigned char
MPI_UNSIGNED_SHORT
unsigned short int
MPI_UNSIGNED
unsigned int
MPI_UNSIGNED_LONG
unsigned long int
MPI_FLOAT
float
MPI_DOUBLE
double
MPI_LONG_DOUBLE
long double
MPI_BYTEMPI_PACKED
MPI provides equivalent datatypes for all C datatypes. This is done for portability reasons.The datatype MPI_BYTE corresponds to a byte (8 bits) and MPI_PACKED correspondsto a collection of data items that has been created by packing non-contiguous data.
The basic functions for sending and receiving messages inMPI are the MPI_Send and MPI_Recv, respectively.
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The calling sequences of these routines are as follows: int MPI_Send (void *buf, int count, MPI_Datatype datatype,
int dest, int tag, MPI_Comm comm)
int MPI_Recv (void *buf, int count, MPI_Datatype datatype,
int source, int tag, MPI_Comm comm,
MPI_Status *status)
On the receiving end, the status variable can be usedto get information about the
MPI_Recv
operation.
The corresponding data structure contains:
typedef struct MPI_Status {
int MPI_SOURCE;int MPI_TAG;int MPI_ERROR; };
The MPI_Get_count function returns the precise countof data items received.
**int MPI_Get_count(MPI_Status status,MPI_Datatype datatype, int count)
(N > p , N mod p = 0)
