[mpich-discuss] mpi_comm_connect hangs in client/server mode runs in multiple process

[Shuwei Zhao]

Hi,

I was trying to study the usage of mpi client-server model based on the
example you provided on
https://www.mcs.anl.gov/research/projects/mpi/mpi-standard/mpi-report-2.0/node106.htm.
After writing simple separate client and server source file based on the
samples on your website. I found that:
1) when client and server both runs in single process like mpiexec -n 1
server, mpiexec -n 1 client.
it works fine, connection between server and client can successfully be
built.
2) when client runs in single process but server runs in multiple processes
(2 for example) but only rank 0 in server responsible to call
mpi_comm_accept, rank 1 of server is just sleeping. Usage is like mpiexec
-n 1 client, mpiexec -n 2 server. In this case, server blocks in
mpi_comm_accept, which is expected. However, client blocks in
mpi_comm_connect which is not expected.
3) when client runs in multiple processes and server runs in multiple
processes, only rank 0 of client responsible for mpi_comm_connect and only
rank 0 of server responsible for mpi_comm_accept. In this case, same
observation in 2).

May I ask is the hanging of mpi_comm_connect in 2) & 3) expected? Is there
a way to build connection successfully when mpi client/server runs under
multi-process?

Thanks a lot,
Shuwei
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