CODEWITHSUNDEEP
cmpi
user5224720
user5224720

Reputation:

MPI hang on persistance calling

I am trying to implement some form of persistent calling. Somehow the following code keeps hanging - I guessed I must have introduced a deadlock but can't really wrap my head around it...

MPI_Request r[4];
[...]
MPI_Send_init(&Arr[1][1], 1, MPI_DOUBLE, 1, A, MPI_COMM_WORLD, &r[0]);
MPI_Recv_init(&Arr[1][0], 1, MPI_DOUBLE, 0, A, MPI_COMM_WORLD, &r[1]);

MPI_Send_init(&Arr[2][1], 1, MPI_DOUBLE, 0, B, MPI_COMM_WORLD, &r[2]);
MPI_Recv_init(&Arr[2][0], 1, MPI_DOUBLE, 1, B, MPI_COMM_WORLD, &r[3]);
[...]
MPI_Startall(4, r);
MPI_Waitall(4, r, MPI_STATUSES_IGNORE);

I think this is perfect material for deadlock - what would be the remedy here if I want to init these send/receive message and just invoke the processes later all with Startall and Waitall?

EDIT: So if I do

MPI_Start(&r[0]);
MPI_Wait(&r[0], &status):

Then it does not hang. Invoking something like:

for (int k=0; k<1; k++)
  {
    MPI_Start(&r[k]);
    MPI_Wait(&r[k], &status);
  }

fail and hang. if that helps

Upvotes: 1

Views: 119

Answers (2)

Gilles Gouaillardet
Gilles Gouaillardet

Reputation: 8380

your tags do not match.

for example, rank 0 receives from itself with tag A but it sends to itself with tag B

Upvotes: 1

Patrick
Patrick

Reputation: 184

I have to admit I'm not familiar with the concept of MPI requests and the MPI_Send/Recv_init. However, I could reproduce the deadlock with simple sends and receives. This is the code (it has a deadlock):

double someVal = 3.5;
const int MY_FIRST_TAG = 42;
MPI_Send(&someVal, 1, MPI_DOUBLE, 1, MY_FIRST_TAG, MPI_COMM_WORLD);
MPI_Recv(&someVal, 1, MPI_DOUBLE, 0, MY_FIRST_TAG, MPI_COMM_WORLD, MPI_STATUS_IGNORE);

Even if you run it with only two processes, the problem is the following: Both, process 0 and 1 send a message to process 1. Then both processes want to receive a message from process 0. Process 1 can because process zero actually sent a message to process 1. But nobody sent a message to process 0. Consequently, this process will wait there forever.

How to fix: You need to specify that only process 0 sends to process 1 and only process 1 is supposed to receive from process 0. You can simply do it with:

int rank;
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
if (rank == 0)
    MPI_Send(&someVal, 1, MPI_DOUBLE, 1, MY_FIRST_TAG, MPI_COMM_WORLD);
else // Assumption: Only two processes
    MPI_Recv(&someVal, 1, MPI_DOUBLE, 0, MY_FIRST_TAG, MPI_COMM_WORLD, MPI_STATUS_IGNORE);

I'm not 100% sure how to translate this to the concept of requests and MPI_Send/Recv_init but maybe this helps you nonetheless.

Upvotes: 0

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