python进程池multiprocessing.Pool运行错误：

错误：

Traceback (most recent call last):
File "", line 1, in
File "C:\Program Files\Anaconda3\lib\multiprocessing\spawn.py", line 106, in spawn_main
exitcode = _main(fd)
File "C:\Program Files\Anaconda3\lib\multiprocessing\spawn.py", line 115, in _main
prepare(preparation_data)
File "C:\Program Files\Anaconda3\lib\multiprocessing\spawn.py", line 226, in prepare
_fixup_main_from_path(data['init_main_from_path'])
File "C:\Program Files\Anaconda3\lib\multiprocessing\spawn.py", line 278, in _fixup_main_from_path
run_name="mp_main")
File "C:\Program Files\Anaconda3\lib\runpy.py", line 254, in run_path
pkg_name=pkg_name, script_name=fname)
File "C:\Program Files\Anaconda3\lib\runpy.py", line 96, in _run_module_code
mod_name, mod_spec, pkg_name, script_name)
File "C:\Program Files\Anaconda3\lib\runpy.py", line 85, in _run_code
exec(code, run_globals)
File "D:\pycharm\python_accelerate\pkg_acc.py", line 121, in
pool = multiprocessing.Pool(processes=3)
File "C:\Program Files\Anaconda3\lib\multiprocessing\context.py", line 118, in Pool
context=self.get_context())
File "C:\Program Files\Anaconda3\lib\multiprocessing\pool.py", line 168, in init
self._repopulate_pool()
File "C:\Program Files\Anaconda3\lib\multiprocessing\pool.py", line 233, in _repopulate_pool
w.start()
File "C:\Program Files\Anaconda3\lib\multiprocessing\process.py", line 105, in start
self._popen = self._Popen(self)
File "C:\Program Files\Anaconda3\lib\multiprocessing\context.py", line 313, in _Popen
return Popen(process_obj)
File "C:\Program Files\Anaconda3\lib\multiprocessing\popen_spawn_win32.py", line 34, in init
prep_data = spawn.get_preparation_data(process_obj._name)
File "C:\Program Files\Anaconda3\lib\multiprocessing\spawn.py", line 144, in get_preparation_data
_check_not_importing_main()
File "C:\Program Files\Anaconda3\lib\multiprocessing\spawn.py", line 137, 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 likely indicates that you aren't employing fork to initiate your child processes and you have neglected to utilize the correct idiom within the main module:

if name == 'main':
freeze_support()
...

The freeze_support() statement may not be needed if the program will not require freezing for the creation of an .exe file.

代码：

 def muchjob(x):

    
     time.sleep(2)
    
     return x**2
    
  
    
 tic = time.time()
    
 import time
    
  
    
 ans = [muchjob(i) for i in range(4)]
    
 print('ans', ans)
    
 toc = time.time()
    
 print('used {:.10}s'.format(toc - tic))
    
 print('*' * 20)
    
  
    
 tic = time.time()
    
 import multiprocessing
    
 pool = multiprocessing.Pool(processes=3)
    
 res = []
    
 for i in range(4):
    
     res.append(pool.apply_async(muchjob, (i,)))
    
 pool.close()
    
 pool.join()
    
 ans = [r.get() for r in res]
    
 print('ans', ans)
    
 toc = time.time()
    
 print('used {:.10}s'.format(toc-tic))

这是由于mutiprocessing要写在main下，修改如下：

 def muchjob(x):

    
     time.sleep(2)
    
     return x**2
    
  
    
 if __name__ == '__main__':
    
     tic = time.time()
    
     import time
    
     
    
     ans = [muchjob(i) for i in range(4)]
    
     print('ans', ans)
    
     toc = time.time()
    
     print('used {:.10}s'.format(toc - tic))
    
     print('*' * 20)
    
  
    
     tic = time.time()
    
     import multiprocessing
    
     pool = multiprocessing.Pool(processes=3)
    
     res = []
    
     for i in range(4):
    
     res.append(pool.apply_async(muchjob, (i,)))
    
     pool.close()
    
     pool.join()
    
     ans = [r.get() for r in res]
    
     print('ans', ans)
    
     toc = time.time()
    
     print('used {:.10}s'.format(toc-tic))

reference：

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该方法表现出显著效果。
此问题询问在程序流程有限时是否可以省略Freeze支持行。
Freeze_support行是一种优化措施，旨在防止解释器冻结程序流程以便执行垃圾回收。
如果程序流程有限，则此行变得多余而不必存在。
否则必须包含该行。