2.2 Point to Point Communication

The fundamental basis of coordination between independent processes is point-to-point communication between processes through the communication links in the MPI.COMM_WORLD. The form of communication is called message passing, where one process sends data to another one, who in turn must receive it from the sender. This is illustrated as follows:

Point-to-point communication is a way that pair of processors transmits the data between one another, one processor sending, and the other receiving. MPI provides send() and receive() functions that allow point-to-point communication taking place in a communicator.

send(message, destination, tag)

  - message:  some type of data to be sent
  - destination:  rank of the receiving process
  - tag:    message tag is a way to identify the type of a message (optional)

recv(message, source, tag)

  - message: some type of data to be received
  - source: rank of the sending process
  - tag: message tag is a way to identify the type of a message

Send and Receive Hello World

Here is a simple example where strings are sent from a conductor to the workers.

from mpi4py import MPI def main(): comm = MPI.COMM_WORLD id = comm.Get_rank() numProcesses = comm.Get_size() if id == 0: hello_message = "Hello from conductor node!" print("Process {0} (conductor) sending messages to workers: {1}".format(\ id, hello_message)) for i in range(1,numProcesses): #send to each worker process comm.send(hello_message, dest=i) else: my_message = comm.recv(source=0) print("Process {0} (worker) received message from conductor: {1}".format(\ id, my_message)) ########## Run the main function main()

This MPI program illustrates the use of send() and recv() functions with the conductor-worker pattern from the previous section.

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