MPI_send and MPI_receive function getting stuck - c

Question

I have a code of page_rank where I have to compute page rank in parallel. My code hangs where I wrote the following MPI_send and MPI_receive function. What could be the problem?
int **sendto_list = (int **)malloc(comm_size*sizeof(int*));
for(i=0; i < comm_size; i++) {
sendto_list[i] = (int *)malloc(18*sizeof(int));
sendto_list[i][0] = 16;
sendto_list[i][1] = 0;
}
int temp_data = 1;
for(i=0; i < comm_size; i++) {
if(request_list[i][1] > 0) {
for(k=0; k < request_list[i][1]; ) {
for(j=0; j < 200; j++) {
if( k >= request_list[i][1] )
break;
sendrecv_buffer_int[j] = request_list[i][k+2];
k++;
}
// Request appropriate process for pagerank.
if(i!= my_rank)
MPI_Send(&temp_data, 1, MPI_INT, i, TAG_PR_REQ, MPI_COMM_WORLD);
}
}
if( i != my_rank )
MPI_Send(&temp_data, 1, MPI_INT, i, TAG_PR_DONE, MPI_COMM_WORLD);
}
int expected_requests = 0, done = 0,temp,s;
s=0;
while( (done == 0) && (comm_size > 1) ) {
if(expected_requests == (comm_size - 1))
break;
int count;
// Receive pagerank requests or messages with TAG_PR_DONE(can be from any process).
MPI_Recv(&temp, 1, MPI_INT, MPI_ANY_SOURCE ,MPI_ANY_TAG, MPI_COMM_WORLD, &status);
MPI_Get_count(&status, MPI_INT, &count);
switch(status.MPI_TAG) {
case TAG_PR_REQ:{
for(i = 0 ; i < count; i++)
insert_into_adj_list(&sendto_list[status.MPI_SOURCE], sendrecv_buffer_int[i], num_nodes);
break;
}
case TAG_PR_DONE:{
expected_requests++;
break;
}
default:
break;
}
}

Answer 1

A cursory glance over your code looks as if your issue is because your MPI_Send() calls are blocking and thus nothing is receiving and freeing them up.
If (request_list[i][1] > 0) and (i!= my_rank) evaluate to true you try to perform 2 MPI_Send() operations to process rank i but you only have 1 matching MPI_Recv() operation in each process, i.e. process rank i.
You may want to try changing
if(request_list[i][1] > 0) {
...
}
if( i != my_rank )
MPI_Send(&temp_data, 1, MPI_INT, i, TAG_PR_DONE, MPI_COMM_WORLD);
to
if(request_list[i][1] > 0) {
...
} else if( i != my_rank ) {
MPI_Send(&temp_data, 1, MPI_INT, i, TAG_PR_DONE, MPI_COMM_WORLD);
}
Note the addition of else turning if into else if. This ensures only 1 MPI_Send() operation per process. It does not look like the two MPI_Send() operations should be executed if the above conditions are true.
Alternatively if you need you could look into MPI_Isend() and MPI_Irecv(). Although I don't think they will solve your problem entirely in this case. I still think you need the else if clause.
I'd also like to point out in C there is no need to cast the return of malloc(). This topic has been covered extensively on StackOverflow so I won't dwell on it too long.
You should also check the result of malloc() is a valid pointer. It returns NULL if an error occured.