MPI: Difference between revisions

The MPI standard defines <tt>MPI_Send</tt> and <tt>MPI_Recv</tt> to be '''blocking calls'''. The correct way to interpret this is that <tt>MPI_Send</tt> will not return until it is safe for the calling module to modify the contents of the provided message buffer.  Similarly, <tt>MPI_Recv</tt> will not return until the entire contents of the message are available in the provided message buffer.

It should be obvious that the availability of buffering in the MPI library is irrelevant to receive operations. As soon as the data is received, it will be made available and <tt>MPI_Recv</tt> will return; until then it will be blocked and there is nothing to buffer. <tt>MPI_Send</tt> on the other hand need not block if there is buffering present in the library. Once the message is copied out of the buffer for delivery, it is safe for the user to modify the original data, so the call can return. This is why our parallel "Hello, world!" program doesn't deadlock as we have implemented it, even though all processes call <tt>MPI_Send</tt> first. Since the buffering is not required by the MPI standard and the correctness of our program relies on it, we refer to such a program as '''''unsafe''''' MPI.