using same address for send and recv in MPI_Allreduce

Question

Is it safe to use the same input and output buffer for MPI_Allreduce in general? It seems that openmpi still works with that on Linux/Ubuntu.

int max_quantities = ...;
MPI_Allreduce(&max_quantities, &max_quantities, 1, MPI_DOUBLE, MPI_MAX, comm);

Answer 1

You should never "alias" buffers. Some MPI implementations explicitly give you an error about it. Instead use MPI_IN_PLACE :

MPI_Allreduce(MPI_IN_PLACE, &max_quantities, 1, MPI_DOUBLE, MPI_MAX, comm);

```