Persistent communication registers a pattern once with MPI_Send_init and MPI_Recv_init, then reuses it with MPI_Start/MPI_Wait. Amortises setup cost for repeated communication.

MPI_Request reqs[2];
MPI_Send_init(send_buf, N, MPI_DOUBLE, right, 0, MPI_COMM_WORLD, &reqs[0]);
MPI_Recv_init(recv_buf, N, MPI_DOUBLE, left,  0, MPI_COMM_WORLD, &reqs[1]);
 
for (int iter = 0; iter < MAX_ITER; iter++) {
    pack_halo(send_buf);
    MPI_Startall(2, reqs);
    compute_interior();
    MPI_Waitall(2, reqs, MPI_STATUSES_IGNORE);
    use_halo(recv_buf);
}
 
MPI_Request_free(&reqs[0]);
MPI_Request_free(&reqs[1]);

The implementation can cache internal state and skip repeated argument validation, which reduces per-iteration overhead for small messages. On high-latency networks the benefit is modest since latency dominates; for many small messages in a tight loop it adds up. Persistent requests follow the same non-blocking semantics as MPI_Isend/MPI_Irecv: the buffer must not be modified between MPI_Start and MPI_Wait.