Multiprocessing runtime error freeze_support()

当linux上的代码在win10上运行时可能会出现Multiprocessing错误，

报错情况如下：

Traceback (most recent call last):
  File "<string>", line 1, in <module>
Traceback (most recent call last):
  File "main.py", line 19, in <module>
  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\spawn.py", line 105, in spawn_main
        exitcode = _main(fd)t.train()

  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\spawn.py", line 114, in _main
  File "D:\project\pytorch\EDSR-m\src\trainer.py", line 45, in train
    prepare(preparation_data)
for batch, (lr, hr, _, idx_scale) in enumerate(self.loader_train):  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\spawn.py", line 225, in prepare

      File "D:\project\pytorch\EDSR-m\src\dataloader.py", line 144, in __iter__
_fixup_main_from_path(data['init_main_from_path'])
return _MSDataLoaderIter(self)  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\spawn.py", line 277, in _fixup_main_from_path

  File "D:\project\pytorch\EDSR-m\src\dataloader.py", line 117, in __init__
        w.start()
run_name="__mp_main__")  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\process.py", line 105, in start

      File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\runpy.py", line 263, in run_path
self._popen = self._Popen(self)
pkg_name=pkg_name, script_name=fname)  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\context.py", line 223, in _Popen

      File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\runpy.py", line 96, in _run_module_code
return _default_context.get_context().Process._Popen(process_obj)
mod_name, mod_spec, pkg_name, script_name)  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\context.py", line 322, in _Popen

      File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\runpy.py", line 85, in _run_code
return Popen(process_obj)
exec(code, run_globals)  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\popen_spawn_win32.py", line 65, in __init__

      File "D:\project\pytorch\EDSR-m\src\main.py", line 19, in <module>
reduction.dump(process_obj, to_child)
t.train()  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\reduction.py", line 60, in dump

      File "D:\project\pytorch\EDSR-m\src\trainer.py", line 45, in train
ForkingPickler(file, protocol).dump(obj)
for batch, (lr, hr, _, idx_scale) in enumerate(self.loader_train):BrokenPipeError
:   File "D:\project\pytorch\EDSR-m\src\dataloader.py", line 144, in __iter__
[Errno 32] Broken pipe
return _MSDataLoaderIter(self)
  File "D:\project\pytorch\EDSR-m\src\dataloader.py", line 117, in __init__
    w.start()
  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\process.py", line 105, in start
    self._popen = self._Popen(self)
  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\context.py", line 223, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\context.py", line 322, in _Popen
    return Popen(process_obj)
  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\popen_spawn_win32.py", line 33, in __init__
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\spawn.py", line 143, in get_preparation_data
    _check_not_importing_main()
  File "C:\Users\YongboLiang\Anaconda3\envs\pytorchG\lib\multiprocessing\spawn.py", line 136, in _check_not_importing_main
    is not going to be frozen to produce an executable.''')
RuntimeError:
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.

此时需要将添加main函数，下面是原linux上的代码

import torch

import utility
import data
import model
import loss
from option import args
from trainer import Trainer

torch.manual_seed(args.seed)
checkpoint = utility.checkpoint(args)

if checkpoint.ok:
    loader = data.Data(args)
    model = model.Model(args, checkpoint)
    loss = loss.Loss(args, checkpoint) if not args.test_only else None
    t = Trainer(args, loader, model, loss, checkpoint)
    while not t.terminate():
        t.train()
        t.test()

    checkpoint.done()

将其修改为以下，就可以在win10上运行了

import torch

import utility
import data
import model
import loss
from option import args
from trainer import Trainer

torch.manual_seed(args.seed)
checkpoint = utility.checkpoint(args)

def main():    
    if checkpoint.ok:
        loader = data.Data(args)
        global model,loss
        model = model.Model(args, checkpoint)
        loss = loss.Loss(args, checkpoint) if not args.test_only else None
        t = Trainer(args, loader, model, loss, checkpoint)
        while not t.terminate():
            t.train()
            t.test()

        checkpoint.done()

if __name__ == '__main__':
    main()

参考pytorch doc上的解决方案：

Windows FAQ – PyTorch master documentation​ pytorch.org