Hi @mars,
I also meet the problem after I set  protocol=4. Could you share your code to work around the problem? Thank you!
 Traceback (most recent call last):
   File "<string>", line 1, in <module>
   File "/opt/conda/lib/python3.7/multiprocessing/spawn.py", line 105, in spawn_main
     exitcode = _main(fd)
   File "/opt/conda/lib/python3.7/multiprocessing/spawn.py", line 115, in _main
     self = reduction.pickle.load(from_parent)
 _pickle.UnpicklingError: pickle data was truncated
 Traceback (most recent call last):
   File "<string>", line 1, in <module>
   File "/opt/conda/lib/python3.7/multiprocessing/spawn.py", line 105, in spawn_main
     exitcode = _main(fd)
   File "/opt/conda/lib/python3.7/multiprocessing/spawn.py", line 115, in _main
     self = reduction.pickle.load(from_parent)
 _pickle.UnpicklingError: pickle data was truncated
 Traceback (most recent call last):
   File "<string>", line 1, in <module>
   File "/opt/conda/lib/python3.7/multiprocessing/spawn.py", line 105, in spawn_main
     exitcode = _main(fd)
   File "/opt/conda/lib/python3.7/multiprocessing/spawn.py", line 115, in _main
     self = reduction.pickle.load(from_parent)
 _pickle.UnpicklingError: pickle data was truncated
 Traceback (most recent call last):
   File "<string>", line 1, in <module>
   File "/opt/conda/lib/python3.7/multiprocessing/spawn.py", line 105, in spawn_main
     exitcode = _main(fd)
   File "/opt/conda/lib/python3.7/multiprocessing/spawn.py", line 115, in _main
     self = reduction.pickle.load(from_parent)
 _pickle.UnpicklingError: pickle data was truncated
 Traceback (most recent call last):
   File "/opt/conda/bin/fairseq-train", line 11, in <module>
     load_entry_point('fairseq', 'console_scripts', 'fairseq-train')()
   File "/code/fairseq/fairseq_cli/train.py", line 370, in cli_main
     nprocs=args.distributed_world_size,
   File "/opt/conda/lib/python3.7/site-packages/torch/multiprocessing/spawn.py", line 171, in spawn
     while not spawn_context.join():
   File "/opt/conda/lib/python3.7/site-packages/torch/multiprocessing/spawn.py", line 107, in join
     (error_index, name)
 Exception: process 5 terminated with signal SIGKILL