Code likes below.  It will block at creating x1. If you remove line print("finish create\n", x), it works fine.
This really bothers me.
Thanks if anyone helps
import torch
import torch.multiprocessing as multiprocessing

def run():
  print("run")
  device=torch.device("cpu")
  x1 = torch.rand(720, 1280, dtype=torch.float32, device=device)
  print('a', x1)
  x2 = x1.to(dtype=torch.float64)
  print('b', x2.dtype)

x = torch.rand(1080, 1920,device=torch.device("cpu"))
print("finish create\n", x)

thread = multiprocessing.Process(target=run)
thread.start()
print("wait")
thread.join()
print("finish")