tag, tmp_buf, recv_cnt, dtype, dst, tag, comm, &status);
if(op!=MPI_OP_NULL) op->apply( tmp_buf, (char *) rbuff + recv_idx * extent, &recv_cnt,
tag, tmp_buf, recv_cnt, dtype, dst, tag, comm, &status);
if(op!=MPI_OP_NULL) op->apply( tmp_buf, (char *) rbuff + recv_idx * extent, &recv_cnt,