if( (op==MPI_OP_NULL || op->is_commutative()) && total_message_size > 524288) {
return reduce_scatter__mpich_pair(sbuf, rbuf, rcounts, dtype, op, comm);
} else if ((op != MPI_OP_NULL && not op->is_commutative())) {
if( (op==MPI_OP_NULL || op->is_commutative()) && total_message_size > 524288) {
return reduce_scatter__mpich_pair(sbuf, rbuf, rcounts, dtype, op, comm);
} else if ((op != MPI_OP_NULL && not op->is_commutative())) {