Why MPI_Waitall return before MPI_Irecv get data needed?

Question

I'm solving Laplace equation with unstructured mesh partition using MPI. I plan to finish data sending and receiving from neighbor partition first and then do calculation for each processor. MPI_Waitall is used to wait for all MPI_Isend() and MPI_Irecv() finishing, but the problem is all processor pass the MPI_Waitall and stuck there when reading received buffer data because each processor actually didn't receive any data (flag of MPI_Testall returns 0). In my understanding, MPI_Irecv should have received data before MPI_Waitall returns.

double **sbuf = calloc(partition->ptn_nnbr[my_id], sizeof(double *));
double **rbuf = calloc(partition->ptn_nnbr[my_id], sizeof(double *));
for (i = 0; i < partition->ptn_nnbr[my_id]; i++)
{
    //rbuf[i] = calloc(partition->ptn_cnt[my_id][k1], sizeof(double));
    rbuf[i] = calloc(MAX_nnode, sizeof(double));
    sbuf[i] = calloc(MAX_nnode, sizeof(double));
}

nrm = 1;            // nrm = max(abs(r[i])), i = 1..n
iter = 0;
printf("Entering jacobi; iterations = %d, error norm = %e\n", iter, nrm);
while (nrm > TOL && iter<4 ){
    init_boundary_conditions_ptn(x_ptn, mesh, my_id, partition);

    iter++;     
    int req_idx= 0;     
    int idx = 0;
    MPI_Request *request = (MPI_Request *) calloc(2 * partition->ptn_nnbr[my_id], sizeof(MPI_Request));
    MPI_Status *status = calloc(2 * partition->ptn_nnbr[my_id], sizeof(MPI_Status));
    int *flag = calloc(2 * partition->ptn_nnbr[my_id], sizeof(int));
    for (k1 = 0; k1 < partition->nptn; k1++)
    {
        if (partition->ptn_list[my_id][k1] != NULL)
        {               
            for (i = 0; i < partition->ptn_cnt[k1][my_id]; i++)
            {
                sbuf[idx][i] = x_ptn->val[partition->ptn_list[k1][my_id][i] - partition->ptn[my_id] + 1];
            }
            MPI_Isend(sbuf[idx], partition->ptn_cnt[k1][my_id], MPI_DOUBLE, k1, TAG, MPI_COMM_WORLD, &request[req_idx]);
            //printf("isend done from nbr %d for partition %d \n", k1, my_id);
            req_idx++;              
            idx++;
        }
    }

    idx = 0;
    for (k1 = 0; k1 < partition->nptn; k1++)
    {
        if (partition->ptn_list[my_id][k1] != NULL)
        {
            MPI_Irecv(rbuf[idx], partition->ptn_cnt[my_id][k1], MPI_DOUBLE, k1, TAG, MPI_COMM_WORLD, &request[req_idx]);
            //printf("irecv done from nbr %d for partition %d \n", k1, my_id);
            req_idx++;

            idx++;
        }
    }

    printf("partition %d is waiting \n", my_id);
    MPI_Testall(2 * partition->ptn_nnbr[my_id],request,flag, status);
    for (i = 0; i < 2 * partition->ptn_nnbr[my_id]; i++)
    {
        printf("flag[%d] is %d from partition %d\n", i, flag[i], my_id);
    }

    MPI_Waitall(2 * partition->ptn_nnbr[my_id], request, status);
    printf("partition %d pass MPI_Wait \n", my_id);

    for (k1 = 0; k1 < partition->nptn; k1++)
    {
        if (partition->ptn_list[my_id][k1] != NULL)
        {
            MPI_Probe(k1, TAG, MPI_COMM_WORLD, status1);
            MPI_Get_count(status1, MPI_DOUBLE, &count);
            printf("count is %d from nbr %d \n", count, k1);
            for (i = 0; i < count; i++)
            {
                x->val[partition->ptn_list[my_id][k1][i]] = rbuf[idx][i];
            }
        }
    }    

    //printf("exchange complete from partition %d\n", my_id);

    jacobi_step_csr_matrix(A_ptn, x, b_ptn, y_ptn);     // y = inv(D)*(b + (D-A)*x), D = diag(A)

    copy_vector(y_ptn, x_ptn);
    MPI_Gatherv(x_ptn->val, x_ptn->n, MPI_DOUBLE, x->val, x_count, x_dis, MPI_DOUBLE,0, MPI_COMM_WORLD);
    if (my_id == 0)
    {
        init_boundary_conditions(x, mesh, partition->perm);
        matvec_csr_matrix(A, x, r);     // r = A*x
        sxapy(b, -1.0, r);          // r = b - r
        zero_boundary_conditions(r, mesh, partition->perm);
        nrm = norm_inf(r);
    }
    MPI_Bcast(&nrm, 1, MPI_DOUBLE, 0, MPI_COMM_WORLD);
    printf("nrm is %f from partition %d in iter %d \n", nrm, my_id, iter);
    free(request);
    free(status);

The output is:

Processor 0 start Jacobi 
MAx_node is 2 from partition 0 
Entering jacobi; iterations = 0, error norm = 1.000000e+00
Processor 2 start Jacobi 
MAx_node is 2 from partition 2 
Entering jacobi; iterations = 0, error norm = 1.000000e+00
Processor 3 start Jacobi 
MAx_node is 2 from partition 3 
Entering jacobi; iterations = 0, error norm = 1.000000e+00
Processor 1 start Jacobi 
MAx_node is 2 from partition 1 
Entering jacobi; iterations = 0, error norm = 1.000000e+00
partition 3 is waiting 
flag[0] is 0 from partition 3
flag[1] is 0 from partition 3
flag[2] is 0 from partition 3
flag[3] is 0 from partition 3
partition 3 pass MPI_Wait 
partition 0 is waiting 
flag[0] is 0 from partition 0
flag[1] is 0 from partition 0
flag[2] is 0 from partition 0
flag[3] is 0 from partition 0
partition 0 pass MPI_Wait 
partition 2 is waiting 
flag[0] is 0 from partition 2
flag[1] is 0 from partition 2
flag[2] is 0 from partition 2
flag[3] is 0 from partition 2
partition 2 pass MPI_Wait 
partition 1 is waiting 
flag[0] is 0 from partition 1
flag[1] is 0 from partition 1
flag[2] is 0 from partition 1
flag[3] is 0 from partition 1
partition 1 pass MPI_Wait

Hristo Iliev · Accepted Answer · 2015-11-16 11:53:16Z

It appears to me that your understanding of non-blocking communication in MPI is somewhat vague. First of all, you are using the wrong test call. MPI_Testall outputs a scalar completion flag, which indicates whether all requests have completed by the time the call to MPI_Testall was made. If you were to have used MPI_Testsome instead, you would have noticed that only some requests (or more likely none) would have completed. The MPI standard allows for progression of non-blocking operations to be postponed and only progressed in certain occasions. Completion is only guaranteed:

after a call to MPI_Wait{all|some|any} (which simply doesn't return before the requests complete);
after MPI_Test{all|some|any} returns a true completion flag. There is no guarantee that a single call to MPI_Test... will result in completion - the test functions are meant to be called repeatedly until the flag indicates the completion of the requests.

For performance reasons, most MPI libraries are single-threaded, that is there is no background thread that progresses the non-blocking calls, except on some specific architectures that implement progression in hardware. Therefore, periodic calls into the MPI library are needed in order for non-blocking communications to actually happen and your expectation that all non-blocking requests should have completed by the time you call MPI_Testall is simply wrong.

Also, your program gets stuck in MPI_Probe. It is a blocking call that has to be called before receiving a message, not after. The message has already been received by MPI_Irecv and the probe call is waiting for another message that never arrives. Do not call MPI_Probe. Pass the relevant element of the status array to MPI_Get_count instead.

As a final note, you are passing 2 * partition->ptn_nnbr[my_id] as the number of requests. Make sure that this value actually matches the value accumulated in req_idx, otherwise your program will crash. Inactive requests must be set to MPI_REQUEST_NULL and neither Open MPI nor MPICH uses NULL (as set in your case by the call to calloc(3)) for inactive requests. You should pass req_idx as the number of requests instead.

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