问题
I have a program where there is a master/slave setup, and I have some functions implemented for the master which sends different kinds of data to the slaves. Some functions send to individual slaves, but some broadcast information to all the slaves via MPI_Bcast.
I want to have only one receive function in the slaves, so I want to know if I can probe for a message and know if it was broadcasted or sent as a normal blocking message, since there are different method to receive what was broadcasted and what was sent normally.
回答1:
No, you can't decide whether to call Bcast or Recv on the basis of a probe call.
A MPI_Bcast call is a collective operation -- all MPI tasks must participate. As a result, these are not like point to point communication; they make use of the fact that all processes are involved to make higher-order optimizations.
Because the collective operations imply so much synchronization, it just doesn't make sense to allow other tasks to check to see whether they should start participating in a collective; it's something which has to be built into the logic of a program.
The root process' role in a broadcast is not like a Send; it can't, in general, just call MPI_Bcast and then proceed. The implementation will almost certainly block until some other number of processes have participated in the broadcast; and
The other process' role in a broadcast is not like receiving a message; in general it will be both receiving and sending information. So participating in a broadcast is different from making a simple Recv call.
So Probe won't work; the documentation for MPI_Probe is fairly clear that it returns information about what would happen upon the next MPI_Recv, and Recv is a different operation than Bcast.
You may be able to get some of what you want in MPI 3.0, which is being finalized now, which allows for nonblocking collectives -- eg, MPI_Ibcast. In that case you could start the Broadcast and call MPI_Test to check on the status of the request. However, even here, everyone would need to call the MPI_Ibcast first; this just allows easier interleaving of collective and point-to-point communication.
来源：https://stackoverflow.com/questions/11014085/probe-for-mpi-bcast-or-mpi-send