# Buggy code
def mpi_run(settings, nics, driver, env, stdout=None, stderr=None):
    """
    Runs mpirun.

    :param settings: Settings for running MPI.
                     Note: settings.num_proc and settings.hosts must not be None.
    :param nics: Interfaces to include by MPI.
    :param driver: The Spark driver service that tasks are connected to.
    :param env: Environment dictionary to use for running MPI.  Can be None.
    :param stdout: Stdout of the mpi process.
                   Only used when settings.run_func_mode is True.
    :param stderr: Stderr of the mpi process.
                   Only used when settings.run_func_mode is True.
    """
    env = (
        {} if env is None else copy.copy(env)
    )  # copy env so we do not leak env modifications

    # Pass secret key through the environment variables.
    env[secret.HOROVOD_SECRET_KEY] = codec.dumps_base64(settings.key)

    rsh_agent = (
        sys.executable,
        "-m",
        "horovod.spark.driver.mpirun_rsh",
        codec.dumps_base64(driver.addresses()),
        codec.dumps_base64(settings),
    )
    settings.extra_mpi_args = (
        '{extra_mpi_args} -x NCCL_DEBUG=INFO -mca plm_rsh_agent "{rsh_agent}"'.format(
            extra_mpi_args=settings.extra_mpi_args if settings.extra_mpi_args else "",
            rsh_agent=" ".join(rsh_agent),
        )
    )
    command = (
        sys.executable,
        "-m",
        "horovod.spark.task.mpirun_exec_fn",
        codec.dumps_base64(driver.addresses()),
        codec.dumps_base64(settings),
    )
    hr_mpi_run(settings, nics, env, command, stdout=stdout, stderr=stderr)


# Traceback (struct.error)
root@ip-172-31-37-52:/# horovodrun -np 2 -H localhost:1,172.31.35.37:1 -p 12345 --verbose ls
Filtering local host names.
Remote host found: 172.31.35.37
Checking ssh on all remote hosts.
SSH was successful into all the remote hosts.
Testing interfaces on all the hosts.
Launched horovod server.
Attempted to launch horovod task servers.
Waiting for the hosts to acknowledge.
----------------------------------------
----------------------------------------
Exception happened during processing of request from ('127.0.0.1', 37436)
Exception happened during processing of request from ('172.31.37.52', 52632)
----------------------------------------
Exception happened during processing of request from ('172.31.37.52', 52636)
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/socketserver.py", line 654, in process_request_thread
self.finish_request(request, client_address)
File "/opt/conda/lib/python3.6/socketserver.py", line 364, in finish_request
self.RequestHandlerClass(request, client_address, self)
File "/opt/conda/lib/python3.6/socketserver.py", line 724, in __init__
self.handle()
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 105, in handle
req = server._wire.read(self.rfile)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 79, in read
message_len = struct.unpack('i', rfile.read(4))[0]
Traceback (most recent call last):
struct.error: unpack requires a buffer of 4 bytes
----------------------------------------
File "/opt/conda/lib/python3.6/socketserver.py", line 654, in process_request_thread
self.finish_request(request, client_address)
File "/opt/conda/lib/python3.6/socketserver.py", line 364, in finish_request
self.RequestHandlerClass(request, client_address, self)
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/socketserver.py", line 724, in __init__
self.handle()
File "/opt/conda/lib/python3.6/socketserver.py", line 654, in process_request_thread
self.finish_request(request, client_address)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 105, in handle
req = server._wire.read(self.rfile)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 79, in read
message_len = struct.unpack('i', rfile.read(4))[0]
struct.error: unpack requires a buffer of 4 bytes
----------------------------------------
File "/opt/conda/lib/python3.6/socketserver.py", line 364, in finish_request
self.RequestHandlerClass(request, client_address, self)
File "/opt/conda/lib/python3.6/socketserver.py", line 724, in __init__
self.handle()
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 105, in handle
req = server._wire.read(self.rfile)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 79, in read
message_len = struct.unpack('i', rfile.read(4))[0]
struct.error: unpack requires a buffer of 4 bytes
----------------------------------------
----------------------------------------
----------------------------------------
Exception happened during processing of request from ('172.31.37.52', 55358)
----------------------------------------
Exception happened during processing of request from ('172.31.37.52', 52642)
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/socketserver.py", line 654, in process_request_thread
self.finish_request(request, client_address)
File "/opt/conda/lib/python3.6/socketserver.py", line 364, in finish_request
self.RequestHandlerClass(request, client_address, self)
Exception happened during processing of request from ('127.0.0.1', 37442)
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/socketserver.py", line 654, in process_request_thread
self.finish_request(request, client_address)
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/socketserver.py", line 654, in process_request_thread
self.finish_request(request, client_address)
File "/opt/conda/lib/python3.6/socketserver.py", line 364, in finish_request
self.RequestHandlerClass(request, client_address, self)
File "/opt/conda/lib/python3.6/socketserver.py", line 724, in __init__
self.handle()
File "/opt/conda/lib/python3.6/socketserver.py", line 724, in __init__
self.handle()
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 105, in handle
req = server._wire.read(self.rfile)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 79, in read
message_len = struct.unpack('i', rfile.read(4))[0]
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 105, in handle
req = server._wire.read(self.rfile)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 79, in read
message_len = struct.unpack('i', rfile.read(4))[0]
File "/opt/conda/lib/python3.6/socketserver.py", line 364, in finish_request
self.RequestHandlerClass(request, client_address, self)
File "/opt/conda/lib/python3.6/socketserver.py", line 724, in __init__
self.handle()
struct.error: unpack requires a buffer of 4 bytes
struct.error: unpack requires a buffer of 4 bytes
----------------------------------------
----------------------------------------
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 105, in handle
req = server._wire.read(self.rfile)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 79, in read
message_len = struct.unpack('i', rfile.read(4))[0]
struct.error: unpack requires a buffer of 4 bytes
----------------------------------------
Exception happened during processing of request from ('172.31.37.52', 55364)
----------------------------------------
Exception happened during processing of request from ('127.0.0.1', 37448)
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/socketserver.py", line 654, in process_request_thread
self.finish_request(request, client_address)
File "/opt/conda/lib/python3.6/socketserver.py", line 364, in finish_request
self.RequestHandlerClass(request, client_address, self)
File "/opt/conda/lib/python3.6/socketserver.py", line 724, in __init__
self.handle()
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 105, in handle
req = server._wire.read(self.rfile)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 79, in read
message_len = struct.unpack('i', rfile.read(4))[0]
struct.error: unpack requires a buffer of 4 bytes
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/socketserver.py", line 654, in process_request_thread
self.finish_request(request, client_address)
----------------------------------------
----------------------------------------
----------------------------------------
Exception happened during processing of request from ('172.31.37.52', 55368)
File "/opt/conda/lib/python3.6/socketserver.py", line 364, in finish_request
self.RequestHandlerClass(request, client_address, self)
File "/opt/conda/lib/python3.6/socketserver.py", line 724, in __init__
self.handle()
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 105, in handle
req = server._wire.read(self.rfile)
File "/opt/conda/lib/python3.6/socketserver.py", line 654, in process_request_thread
self.finish_request(request, client_address)
File "/opt/conda/lib/python3.6/socketserver.py", line 364, in finish_request
self.RequestHandlerClass(request, client_address, self)
File "/opt/conda/lib/python3.6/socketserver.py", line 724, in __init__
self.handle()
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 105, in handle
req = server._wire.read(self.rfile)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 79, in read
message_len = struct.unpack('i', rfile.read(4))[0]
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 79, in read
message_len = struct.unpack('i', rfile.read(4))[0]
struct.error: unpack requires a buffer of 4 bytes
----------------------------------------
struct.error: unpack requires a buffer of 4 bytes
----------------------------------------
----------------------------------------
Exception happened during processing of request from ('172.31.35.37', 39790)
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/socketserver.py", line 654, in process_request_thread
self.finish_request(request, client_address)
File "/opt/conda/lib/python3.6/socketserver.py", line 364, in finish_request
self.RequestHandlerClass(request, client_address, self)
File "/opt/conda/lib/python3.6/socketserver.py", line 724, in __init__
self.handle()
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 105, in handle
req = server._wire.read(self.rfile)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 79, in read
message_len = struct.unpack('i', rfile.read(4))[0]
struct.error: unpack requires a buffer of 4 bytes
----------------------------------------
----------------------------------------
Exception happened during processing of request from ('172.31.35.37', 39796)
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/socketserver.py", line 654, in process_request_thread
self.finish_request(request, client_address)
File "/opt/conda/lib/python3.6/socketserver.py", line 364, in finish_request
self.RequestHandlerClass(request, client_address, self)
File "/opt/conda/lib/python3.6/socketserver.py", line 724, in __init__
self.handle()
----------------------------------------
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 105, in handle
req = server._wire.read(self.rfile)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 79, in read
message_len = struct.unpack('i', rfile.read(4))[0]
struct.error: unpack requires a buffer of 4 bytes
----------------------------------------
Exception happened during processing of request from ('172.31.35.37', 39798)
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/socketserver.py", line 654, in process_request_thread
self.finish_request(request, client_address)
File "/opt/conda/lib/python3.6/socketserver.py", line 364, in finish_request
self.RequestHandlerClass(request, client_address, self)
File "/opt/conda/lib/python3.6/socketserver.py", line 724, in __init__
self.handle()
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 105, in handle
req = server._wire.read(self.rfile)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 79, in read
message_len = struct.unpack('i', rfile.read(4))[0]
struct.error: unpack requires a buffer of 4 bytes
----------------------------------------
Launching horovod task function was not successful:
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/runpy.py", line 193, in _run_module_as_main
"__main__", mod_spec)
File "/opt/conda/lib/python3.6/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/task_fn.py", line 67, in <module>
_task_fn(index, driver_addresses, settings)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/task_fn.py", line 27, in _task_fn
driver_addresses, settings.key, settings.verbose)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/driver/driver_service.py", line 44, in __init__
match_intf=match_intf)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/service/driver_service.py", line 159, in __init__
match_intf=match_intf)
File "/opt/conda/lib/python3.6/site-packages/horovod/run/common/util/network.py", line 172, in __init__
'Linux.'.format(service_name=service_name, addresses=addresses))
horovod.run.common.util.network.NoValidAddressesFound: Horovod was unable to connect to horovod driver service on any of the following addresses: {'lo': [('127.0.0.1', 4548)], 'ens3': [('172.31.37.52', 4548)], 'docker0': [('172.17.0.1', 4548)]}.

One possible cause of this problem is that horovod currently requires every host to have at least one routable network interface with the same name across all of the hosts. You can run "ifconfig -a" on every host and check for the common routable interface. To fix the problem, you can rename interfaces on Linux.

# Fixed code
def mpi_run(settings, nics, driver, env, stdout=None, stderr=None):
    """
    Runs mpirun.

    :param settings: Settings for running MPI.
                     Note: settings.num_proc and settings.hosts must not be None.
    :param nics: Interfaces to include by MPI.
    :param driver: The Spark driver service that tasks are connected to.
    :param env: Environment dictionary to use for running MPI.  Can be None.
    :param stdout: Stdout of the mpi process.
                   Only used when settings.run_func_mode is True.
    :param stderr: Stderr of the mpi process.
                   Only used when settings.run_func_mode is True.
    """
    env = (
        {} if env is None else copy.copy(env)
    )  # copy env so we do not leak env modifications

    # Pass secret key through the environment variables.
    env[secret.HOROVOD_SECRET_KEY] = codec.dumps_base64(settings.key)
    # we don't want the key to be serialized along with settings from here on
    settings.key = None

    rsh_agent = (
        sys.executable,
        "-m",
        "horovod.spark.driver.mpirun_rsh",
        codec.dumps_base64(driver.addresses()),
        codec.dumps_base64(settings),
    )
    settings.extra_mpi_args = (
        '{extra_mpi_args} -x NCCL_DEBUG=INFO -mca plm_rsh_agent "{rsh_agent}"'.format(
            extra_mpi_args=settings.extra_mpi_args if settings.extra_mpi_args else "",
            rsh_agent=" ".join(rsh_agent),
        )
    )
    command = (
        sys.executable,
        "-m",
        "horovod.spark.task.mpirun_exec_fn",
        codec.dumps_base64(driver.addresses()),
        codec.dumps_base64(settings),
    )
    hr_mpi_run(settings, nics, env, command, stdout=stdout, stderr=stderr)