Overview

Point to point is sending a message from one process to another

Most message passing libraries define one system behavior

MPI defines four communication modes

• synchronous mode ("safest")
• ready mode (lowest system overhead)
• buffered mode (decouples sender from receiver)
• standard mode (compromise)

Communication mode is selected with send routine

Calls are also blocking or nonblocking.

• Blocking stops the program until the message buffer is safe to use
• Non-blocking separates communication from computation

Syntax review for a blocking send and receive:

• C:
  int MPI_Send(void *buf, int count, MPI_Datatype datatype, int dest, int tag, MPI_Comm comm)

  Fortran:

  MPI_SEND(buf, count, datatype, dest, tag, comm, ierror)

  The parameters:

  buf is the beginning of the buffer containing the data to be sent. For Fortran, this is often the name of an array in your program. For C, it is an address.

  count is the number of elements to be sent (not bytes)

  datatype is the type of data

  dest is the rank of the process which is the destination for the message

  tag is an arbitrary number which can be used to distinguish among messages

  comm is the communicator

  ierror is a return error code

• C:
  int MPI_Recv(void *buf, int count, MPI_Datatype datatype, int source, int tag, MPI_Comm comm, MPI_Status *status)

  Fortran :

  MPI_RECV(buf, count, datatype, source, tag, comm, status, ierror)

  The parameters:

  buf is the beginning of the buffer where the incoming data are to be stored. For Fortran, this is often the name of an array in your program. For C, it is an address.

  count is the number of elements (not bytes) in your receive buffer

  datatype is the type of data

  source is the rank of the process from which data will be accepted (This can be a wildcard, by specifying the parameter MPI_ANY_SOURCE.)

  tag is an arbitrary number which can be used to distinguish among messages (This can be a wildcard, by specifying the parameter MPI_ANY_TAG.)

  comm is the communicator

  status is an array or structure of information that is returned. For example, if you specify a wildcard for source or tag, status will tell you the actual rank or tag for the message received

  ierror is a return error code

2.1 Communication Modes

Behavior depends on message size

Number of Tasks	Eager Limit (bytes) = threshold
1 - 16	4096
17 - 32	2048
33 - 64	1024
65 - 128	512

Message size less than threshold

Send and Receive can be combined into one call

MPI_SENDRECV