minimum example of strange speed change

example.py

import torch
import time
import numpy as np

time1 = time.time()
## double this tensor's second dimension(20->40), you will almost double the time
for _ in range(100):
  tensor = torch.randn(5, 20, 128, 128)
  array = np.zeros((128, 128), dtype=np.uint8)
  tensor[1, 2, :, :] = torch.from_numpy(array).float()
 
time2 = time.time()

print "time past: %.4f" %((time2-time1)/100)