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Point-to-Point Communication

Blocking Send and Receive Operations

Blocking Send

// buf: initial address of send buffer
// count: number of elements in send buffer
// datatype: datatype of each send buffer element
// dest: rank of destination
// tag: message tag
// comm: communicator
int MPI_Send(const void *buf, int count, MPI_Datatype datatype, int dest, int tag, MPI_Comm comm)

Blocking Receive

// source: rank of source or MPI_ANY_SOURCE
// tag: message tag or MPI_ANY_TAG
int MPI_Recv(void *buf, int count, MPI_Datatype datatype, int source, int tag, MPI_Comm comm, MPI_Status *status)

int MPI_Recv_c(void *buf, MPI_Count count, MPI_Datatype datatype, int source, int tag, MPI_Comm comm, MPI_Status *status)

Message Data

MPI datatype C/C++ datatype
MPI_C_BOOL _Bool
MPI_CXX_BOOL bool
MPI_SIGNED_CHAR signed char
MPI_SHORT signed short int
MPI_INT signed int
MPI_LONG signed long int
MPI_LONG_LONG_INT / MPI_LONG_LONG signed long long int
MPI_UNSIGNED_CHAR unsigned char
MPI_UNSIGNED_SHORT unsigned short int
MPI_UNSIGNED unsigned int
MPI_UNSIGNED_LONG unsigned long int
MPI_UNSIGNED_LONG_LONG unsigned long long int
MPI_INT8_T int8_t
MPI_INT16_T int16_t
MPI_INT32_T int32_t
MPI_INT64_T int64_t
MPI_UINT8_T uint8_t
MPI_UINT16_T uint16_t
MPI_UINT32_T uint32_t
MPI_UINT64_T uint64_t
MPI_FLOAT float
MPI_DOUBLE double
MPI_LONG_DOUBLE long double
MPI_C_COMPLEX / MPI_C_FLOAT_COMPLEX float _Complex
MPI_C_DOUBLE_COMPLEX double _Complex
MPI_C_LONG_DOUBLE_COMPLEX long double _Complex
MPI_CXX_FLOAT_COMPLEX std::complex<float>
MPI_CXX_DOUBLE_COMPLEX std::complex<double>
MPI_CXX_LONG_DOUBLE_COMPLEX std::complex<long double>
MPI_CHAR char
MPI_WCHAR wchar_t
MPI_BYTE
MPI_PACKED

Semantics of Point-to-Point Communication

Order

Messages are non-overtaking. If a sender sends two messages in succession to the same destination, and both match the same receive, then this operation cannot receive the second message if the first one is still pending. It guarantees that message-passing code is deterministic, if processes are single-threaded and the wildcard MPI_ANY_SOURCE is not used in receives.

Communication Modes

  • Standard mode (MPI_Send) Directly send or buffer and send, determined by the environment.
  • Buffered mode (MPI_Bsend)
  • Synchronous mode (MPI_Ssend)
  • Ready mode (MPI_Rsend) Send only when the receiver is ready.

Nonblocking Communication

// request: the handle of communication request
int MPI_Isend(const void *buf, int count, MPI_Datatype datatype, int dest, int tag, MPI_Comm comm, MPI_Request *request)

int MPI_Isend_c(const void *buf, MPI_Count count, MPI_Datatype datatype, int dest, int tag, MPI_Comm comm, MPI_Request *request)

int MPI_Irecv(void *buf, int count, MPI_Datatype datatype, int source, int tag, MPI_Comm comm, MPI_Request *request)

int MPI_Irecv_c(void *buf, MPI_Count count, MPI_Datatype datatype, int source, int tag, MPI_Comm comm, MPI_Request *request)
  • Isend 与 Bsend 的区别就是前者可以查询状态吗？

Communication Completion

int MPI_Wait(MPI_Request *request, MPI_Status *status)

// flag: true if operation completed
int MPI_Test(MPI_Request *request, int *flag, MPI_Status *status)