[mpich-discuss] MPI_Comm_split and end with Finalize

[K. N. Ramachandran]

Hello all,

I have recently begun working on a project that uses MPICH-3.2 and I am
trying to resolve an issue where a server process busy waits at
MPI_Finalize.

We are trying to create a server process that accepts incoming connections
from a known number of clients (say, N clients), forms a new communicator
amongst everyone (server and clients) and then splits itself from the group
and terminates, so that the clients now only work with each other.

For very problem specific reasons, we cannot do
'mpiexec -np N (other args)'

So we have a server that publishes a service name to a nameserver and
clients lookup the name to join the server. The server and client processes
are started with separate calls to mpiexec, one to start the server and the
rest N calls to start the clients.

The server process busy-waits at the MPI_Finalize call, after it splits
from the communicator and only finishes when all other clients reach their
MPI_Finalize too.

Consider a simplified case of only one server and one client. The
simplified pseudocode is:

*Server process:*
MPI_Init();
MPI_Open_port(...);
MPI_Publish_name(...); //publish service name to nameserver

MPI_accept(...); // accept incoming connections and store into intercomm
MPI_Intercomm_merge(...);  // merge new client into intra-comm

// now split the server from the client
MPI_Comm_rank(intra comm, rank); // rank=0
MPI_Comm_split(intra comm, (rank==0), rank, lone comm);

MPI_Finalize(); // busy-waits here till client's sleep duration

*Client process:* (simplified - assuming only one client is trying to
connect)
MPI_Init();
MPI_Lookup_name(..);
MPI_Connect(...)

// merge
MPI_Intercomm_merge(...); // merge with server

// get rank and split
MPI_Comm_rank(intra comm, rank);  // rank=1
MPI_Comm_split(intra comm, rank==0, rank, lone comm);

sleep(10); // sleep for 10 seconds - causes server to busy wait at
MPI_Finalize for sleep duration

MPI_Finalize(); // server and client finish here

So my questions are:

1) Is busy-wait at MPI_Finalize the expected behaviour?

2) How to truly "disconnect" the server, so that it can end immediately at
MPI_Finalize()? I had tried MPI_Comm_disconnect (also MPI_Comm_free) on
both the server and client, but that didn't help.

3)  We don't want to see the server process consuming one core at 100%
while it waits at MPI_Finalize. Are other alternatives apart from making
the server process sleep, wakeup and keep polling a client, and then
finally call MPI_Finalize?

Thank you for any inputs that you can give here.


Regards,
K.N.Ramachandran
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