# 11.7.3. Reducing wireup time

Open MPI’s run-time uses an out-of-band (OOB) communication subsystem to pass messages during the launch, initialization, and termination stages for the job. These messages allow `mpirun` to tell its daemons what processes to launch, and allow the daemons in turn to forward stdio to `mpirun`, update `mpirun` on process status, etc.

The OOB uses TCP sockets for its communication, with each daemon opening a socket back to `mpirun` upon startup. In a large cluster, this can mean thousands of connections being formed on the node where `mpirun` resides, and requires that `mpirun` actually process all these connection requests. `mpirun` defaults to processing connection requests sequentially — so on large clusters, a backlog can be created that can cause remote daemons to timeout waiting for a response.

Fortunately, Open MPI provides an alternative mechanism for processing connection requests that helps alleviate this problem. Setting the MCA parameter `oob_tcp_listen_mode` to `listen_thread` causes `mpirun` to startup a separate thread dedicated to responding to connection requests. Thus, remote daemons receive a quick response to their connection request, allowing `mpirun` to deal with the message as soon as possible.

Error

TODO This seems very out of date. We should have content about PMIx instant on.

This parameter can be included in the default MCA parameter file, placed in the user’s environment, or added to the `mpirun` command line. See this FAQ entry for more details on how to set MCA parameters.