Introduction to MPI Programming for Fortran

Overview of the MPI API
- The MPI header file
  Every Fortran MPI program must include the MPI header file (which contains the MPI function type declarations)
- Every MPI function is a subroutine and has the following prototype:
  Every MPI subroutine returns an integer ERROR CODE as the last parameter of the subroutine
- If an MPI function is successful (no error), it will return the error code MPI_SUCCESS (it's a symbolic name for some constant that you don't need to remember by value....)

Structure of a MPI Program

Every MPI program has the following structure:

PROGRAM Main implicit none include "mpif.h" integer :: err_code CALL MPI_Init( err_code ); ..... ..... MPI_Finalize( err_code );

Most commonly used MPI functions

Here are a number of very frequently used MPI functions

Function Name Usage

MPI_Init(int err_code) Initialize the MPI system - must be the first statement in an MPI program

MPI_Finalize(int err_code) Terminates the MPI system

MPI_Comm_size(MPI_Comm comm,
     int *size,
     int err_code) Determines the size of the group associated with the communictor comm
The size is returned in the integer variable size

MPI_Comm_rank(MPI_Comm comm,
     int *rank,
     int err_code) Determines the rank of the calling process in the communicator comm
The rank is returned in the integer variable rank

MPI_Send(void *buff,
     int count,
     MPI_Datatype type,
     int dest,
     int tag,
     int comm,
     int err_code) Send a point-to-point message to process dest in the communication group comm
The message is stored at memory location buff and consists of count items of datatype type
The message will be tagged with the tag value tag
The MPI_Send() function will only return if the message sent has been received by the destination. (It's safe to reuse the buffer buff right away).

MPI_Recv(void *buff,
     int count,
     MPI_Datatype type,
     int source,
     int tag,
     int comm,
     MPI_Status *status
     int err_code) Receive a point-to-point message
The message MUST BE from the process source in the communication group comm AND the message MUST BE tagged with the tag value tag
The message received will be stored at memory location buff which will have space to store count items of datatype type
When the function exits, the exit status is stored in status.
Information about the received message is returned in a status variable, e.g.,:

The received message tag is status.MPI_TAG and
The rank of the sending process (of the message) is status.MPI_SOURCE.

In most cases, you know the structure of data received and then you can ignore the status value... If you pass NULL as status parameter, MPI will not return the status value.
The MPI_Recv() function will only return if the desired message (from source with tag tag) has been received - or exits with an error code...

Function Name	Usage
MPI_Init(int err_code)	Initialize the MPI system - must be the first statement in an MPI program
MPI_Finalize(int err_code)	Terminates the MPI system
MPI_Comm_size(MPI_Comm comm, int *size, int err_code)	Determines the size of the group associated with the communictor comm The size is returned in the integer variable size
MPI_Comm_rank(MPI_Comm comm, int *rank, int err_code)	Determines the rank of the calling process in the communicator comm The rank is returned in the integer variable rank
MPI_Send(void *buff, int count, MPI_Datatype type, int dest, int tag, int comm, int err_code)	Send a point-to-point message to process dest in the communication group comm The message is stored at memory location buff and consists of count items of datatype type The message will be tagged with the tag value tag The MPI_Send() function will only return if the message sent has been received by the destination. (It's safe to reuse the buffer buff right away).
MPI_Recv(void buff, int count, MPI_Datatype type, int source, int tag, int comm, MPI_Status status int err_code)	Receive a point-to-point message The message MUST BE from the process source in the communication group comm AND the message MUST BE tagged with the tag value tag The message received will be stored at memory location buff which will have space to store count items of datatype type When the function exits, the exit status is stored in status. Information about the received message is returned in a status variable, e.g.,: The received message tag is status.MPI_TAG and The rank of the sending process (of the message) is status.MPI_SOURCE. In most cases, you know the structure of data received and then you can ignore the status value... If you pass NULL as status parameter, MPI will not return the status value. The MPI_Recv() function will only return if the desired message (from source with tag tag) has been received - or exits with an error code...

Hello World MPI Program

Fact:
The "traditional" "hello world" printing program is not possible with MPI because other processes cannot print
We will therefore make processes send "hello" message to each other...

The following MPI Program will do the following:

The process 0 is the master
process 0 will send a "Hello X" message to the process X (X = 1, 2, ...)
Process X will reply back to process 0 with "Hello 0, Process is X present and accounted for".
Process 0 will print out Process X's reply

Example Program:

program Hello implicit none include 'mpif.h' integer :: mpierror, numprocs integer :: i, myid integer status(MPI_STATUS_SIZE) character*128 buff ! String... call MPI_Init(mpierror) call MPI_Comm_size(MPI_COMM_WORLD, numprocs, mpierror) call MPI_Comm_rank(MPI_COMM_WORLD, myid, mpierror) if ( myid == 0 ) then !! Master processor print *, "We have ", numprocs, " processor" do i = 1, numprocs-1 write ( buff , *) "Hello ", myid call MPI_Send(buff, 128, MPI_CHARACTER, i, 0, MPI_COMM_WORLD, & mpierror) end do do i = 1, numprocs-1 call MPI_Recv(buff, 128, MPI_CHARACTER, i, 0, MPI_COMM_WORLD, & status, mpierror) write (6, *) buff end do else !! Slave processors call MPI_Recv(buff, 128, MPI_CHARACTER, 0, 0, MPI_COMM_WORLD, & status, mpierror); write(buff, *) "Hello 0, Processor ", myid, & " is present and accounted for !" call MPI_Send(buff, 128, MPI_CHARACTER, 0, 0, MPI_COMM_WORLD, & mpierror); end if call MPI_Finalize(mpierror) end program

Example Program: (Demo above code)
- Prog file: click here
To compile the program:
To run the program: