Point to point communication

This is the simplest form of message passing, where one process sends message to another. Although, this can be of two types, namely synchronous and asynchronous.

_images/p2p.png

The important thing to note here is that the communication between two process takes place within a communicator. The source process sends message to the destination process. While processes are identified by their ranks in the communicator.

from mpi4py import MPI
import numpy
comm = MPI.COMM_WORLD
rank = comm.Get_rank()
if rank == 0:
    data = {'a': 7, 'b': 5.5}
    comm.send(data, dest=1)
elif rank == 1:
    data = comm.recv(source=0)
    print('On process 1, data is ',data)

In this example, a dictionary object data is transmitted from rank 0 to rank 1, performed by the comm.send command.

comm.send(obj, dest, tag)

Sends obj to dest. With a tag (piggyback information) transfered with the data.

Parameters:

  • obj – Any python object that can be serialized with the pickle method.

  • dest – int

  • tag – int

Return type:

None

comm.recv(source, tag)

Recievs obj from source.

Parameters:

  • source – int

  • tag – int

Return type:

Any

Any such object such as a string, integer or a Numpy array can be send this way. It should also be noted that send is compatible with recv and same is observed between Send and Recv.

For a successful point-to-point communication,

  1. Sender must specify a valid destination rank.

  2. Reciever must specify a valid source rank.

  3. Communicator must be same.

  4. Sending and recieving data type should match.

  5. Sending and recieving tags must also match [1].

Footnotes

Read about other MPI functions here.