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Parallel Programming with MPI

Prof. Sivarama Dandamudi

School of Computer Science

Carleton University

Carleton University © S. Dandamudi 2

Introduction Problem

Lack of a standard for message–passing routines Big issue: Portability problems

MPI defines a core set of library routines (API) for message passing (more than 125 functions in total!!) Several commercial and public domain implementations

Cray, IBM, Intel MPI-CH from Argonne National Laboratory LAM from Ohio Supercomputer Center/Indiana University


Introduction (cont’d)

Some Additional Goals [Snir et al. 1996]

Allows efficient communicationAvoids memory-to-memory copyingAllows computation and communication overlap

Non-blocking communication

Allows implementation in a heterogeneous environment

Provides reliable communication interfaceUsers don’t have to worry about communication failures


MPI MPI is large but not complex

125 functionsBut….

Need only 6 functions to write a simple MPI program MPI_Init MPI_Finalize MPI_Comm_size MPI_Comm_rank MPI_Send Mpi_Recv


MPI (cont’d)

Before any other function is called, we must initialize

MPI_Init(&argc, &argc) To indicate end of MPI calls

MPI_Finalize()Cleans up the MPI stateShould be the last MPI function call


MPI (cont’d)

A typical program structureint main(int argc, char **argv) { MPI_Init(&argc, &argv); . . . /* main program */ . . . MPI_Finalize();}


MPI (cont’d)

MPI uses communicators to group processes that communicate with each other

Predefined communicator: MPI_COMM_WORLD

consists of all processes running when the program begins execution

Sufficient for simple programs


MPI (cont’d)

Process rankSimilar to mytid in PVM

MPI_Comm_rank(MPI_Comm comm,

int *rank)First argument: CommunicatorSecond argument: returns process rank


MPI (cont’d)

Number of processesMPI_Comm_size(MPI_Comm comm, int *size)

First argument: CommunicatorSecond argument: returns number of processesExample:

MPI_Comm_size(MPI_COMM_WORLD, &nprocs)


MPI (cont’d)

Sending a message (blocking version)MPI_Send( void* buf, int count,

MPI_Datatype datatype,

int dest, int tag, MPI_Comm comm )

Data types

MPI_CHAR, MPI_INT, MPI_LONG MPI_FLOAT, MPI_DOUBLE

Buffer description

Destination specification


MPI (cont’d)

Receiving a message (blocking version)MPI_Recv( void* buf, int count, MPI_Datatype datatype, int source, int tag, MPI_Comm comm,

MPI_Status *status )

Wildcard specification allowedMPI_ANY_SOURCEMPI_ANY_TAG


MPI (cont’d)

Receiving a messageStatus of the received message

Status gives two pieces of information directly Useful when wildcards are used

status.MPI_SOURCE Gives identity of the source

status.MPI_TAG Gives the tag information


MPI (cont’d)

Receiving a messageStatus also gives message size information indirectly

MPI_Get_count( MPI_Status *status,


int *count)

Takes status and the datatype as inputs and returns the number of elements via count


MPI (cont’d)

Non-blocking communicationPrefix send and recv by “I” (for immediate)

MPI_IsendMPI_Irecv

Need completion operations to see if the operation is completed

MPI_WaitMPI_Test


MPI (cont’d)

Sending a message (non-blocking version)

MPI_Isend( void* buf, int count,


int dest, int tag,

MPI_Comm comm,

MPI_Request *request )

Returns the request handle


MPI (cont’d)

Receiving a message (non-blocking version)

MPI_Irecv( void* buf, int count,


int source, int tag,

MPI_Comm comm,

MPI_Request *request )

Same arguments as Isend


MPI (cont’d)

How do we know when a non-blocking operation is done?

Use MPI_Test or MPI_Wait

Completion of a send indicates:Sender can access the send buffer

Completion of a receive indicatesReceive buffer contains the message


MPI (cont’d)

MPI_Test returns the statusDoes not wait for the operation to complete

MPI_Test( MPI_Request*request, int *flag,

MPI_Status *status )

Request handle

Operation status: true (if completed)

If flag = true, gives status


MPI (cont’d)

MPI_Wait waits until the operation is completed

MPI_Wait( MPI_Request *request, MPI_Status

*status )

Request handle

Gives status


MPI Collective Communication Collective communication

Several functions are provided to support collective communication

Some examples are given here:MPI_BarrierMPI_BcastMPI_ScatterMPI_GatherMPI_Reduce

Broadcast

Barrier synchronization

Global reduction


From Snir et al. 1996


MPI Collective Communication (cont’d)

MPI_Barrier blocks the caller until all group members have called it

MPI_Barrier( MPI_Comm comm ) The call returns at any process only after all group

members have entered the call



MPI_Bcast broadcasts a message from root to all processes of the group

MPI_Bcast( void* buf, int count,


int root,

MPI_Comm comm )



MPI_Scatter distributes data from the root process to all the others in the group

MPI_Scatter(void* send_buf, int send_count, MPI_Datatype send_type,

void* recv_buf, int

recv_count, MPI_Datatype recv_type,

int root, MPI_Comm comm )



MPI_Gather inverse of the scatter operation (gathers data and stores it in rank order)

MPI_Scatter(void* send_buf, int send_count, MPI_Datatype send_type,

void* recv_buf, int

recv_count, MPI_Datatype recv_type,




MPI_Reduce performs global reduction operations such as sum, max, min, AND, etc.

MPI_Reduce(void* send_buf,

void* recv_buf, int count, MPI_Datatype datatype,

MPI_Op operation,




Predefined reduce operations includeMPI_MAX maximumMPI_MIN minimumMPI_SUM sum MPI_PROD productMPI_LAND logical ANDMPI_BAND bitwise ANDMPI_LOR logical ORMPI_BOR bitwise ORMPI_LXOR logical XORMPI_BXOR bitwise XOR


FromSnir et al. 1996

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