...

(For example, on one side of the intercommunicator sendbuf=1 byte, recvbuf=1 gigabyte; on the other side, sendbuf=1 gigabyte, recvbuf=1 byte)

For Allreduce, you cannot have 1 byte/ 1gigabyte at each side.

For intercommunicator collectives, one side gets the results from the other side. For allgather, it is specified that the communication can be of unequal amounts: MPI-3 pg 166, 9-15. I am assuming the same holds for allreduce, which just says "Both groups should provide count and datatype arguments that specify the same type signature." (188:23-24) Rajeev