Hello everyone,
So i stumbled up on a strange behavior while trying to implement a batch-level concurrent loader.  The problem arises when i use a transfom function within a function that is used by a process/threadpool.
I think the  snippetshould be self explanatory and also should allow you to reproduce this issue. I must say that it is quite mysterious to me:)
Ofcourse there are easy ways to get around the problem but, i thought that it is a peculiar case and might be interesting to know the root cause.
from torchvision.transforms import ToTensor
import PIL.Image
import cv2
import numpy as np
import time
from concurrent.futures import ThreadPoolExecutor, ProcessPoolExecutor



transform = ToTensor()
PIL.Image.fromarray(np.random.randn(1024, 1024, 3).astype('uint8')).save('im.jpg')
batch_of_images = ['im.jpg'] * 128


def load_image(path):
    image = cv2.cvtColor(cv2.imread(path), cv2.COLOR_BGR2RGB)
    return image
   
def load_image_w_transfom(path):
    image = cv2.cvtColor(cv2.imread(path), cv2.COLOR_BGR2RGB)
    return transform(image)
           
for func in [load_image, load_image_w_transfom]:
    t = time.time()
    with ThreadPoolExecutor(4) as tpe:
        res = list(tpe.map(func, batch_of_images))
    td = time.time()
    print("execution time w threads", str(func), td - t)
    
    t = time.time()
    for path in batch_of_images: func(path)
    td = time.time()
    print("execution time no threads", str(func), td - t)
        
execution time w threads <function load_image at 0x7fb51c02cef0> 0.6988275051116943
execution time no threads <function load_image at 0x7fb51c02cef0> 2.2862112522125244
execution time w threads <function load_image_w_transfom at 0x7fb51c02c3b0>11.793959856033325
execution time no threads <function load_image_w_transfom at 0x7fb51c02c3b0>5.095749616622925

torch v 1.3.0
torchvision v 0.4.1a0+d94043a