Request::sendrecv((char *) rbuff + disps[send_idx] * extent, send_cnt,
dtype, dst, tag,
(char *) tmp_buf + disps[recv_idx] * extent, recv_cnt,
dtype, dst, tag, comm, &status);
// tmp_buf contains data received in this step.
Request::sendrecv((char *) rbuff + disps[send_idx] * extent, send_cnt,
dtype, dst, tag,
(char *) tmp_buf + disps[recv_idx] * extent, recv_cnt,
dtype, dst, tag, comm, &status);
// tmp_buf contains data received in this step.