MPI_Probe() for determining message size

Question

A common usage of MPI_Probe is in determining the size of an incoming message so that enough memory is allocated for the receive buffer. But this can also be done with a separate pair of MPI_Send-MPI_Recv calls, i.e. the sender process sends the message size to the receiver in a different message. Can it be assumed that MPI_Probe is in general the faster option? Why? We can perform some tests and compare the walltimes, but the results may be implementation-dependent.

Answer 1

1. For short messages the latency is more important than the size of the message, so probing for a small message is probably faster.
2. Probing makes it easier to deal with MPI_ANY_SOURCE as a sender: otherwise you'd have to first determine where the size msg comes from, and then do a specific receive from that source.

Instead of MPI_Probe, people often do MPI_Iprobe which tells you if there is a message at all. Yes, you can emulate that with multiple Irecvs, but why would you make your code so complicated?