[DataLoader] num_of_worker>0, error occurs

I use this code for dataset, DataLoader.
my env is window 10, anaconda3, python3.7, spyder
class Dataset(object):
def init(self, fname ,img_transform=None, mask_transform = None, edge_weight= False):
#nothing special here, just internalizing the constructor parameters
self.fname=fname
self.edge_weight = edge_weight

    self.img_transform=img_transform
    self.mask_transform = mask_transform
    
    self.tables=tables.open_file(self.fname)
    self.numpixels=self.tables.root.numpixels[:]
    self.nitems=self.tables.root.img.shape[0]
    self.tables.close()
    
    self.img = None
    self.mask = None
    
def __getitem__(self, index):
    #opening should be done in __init__ but seems to be
    #an issue with multithreading so doing here
    with tables.open_file(self.fname,'r') as db:
        self.img=db.root.img
        self.mask=db.root.mask
   
        #get the requested image and mask from the pytable
        img = self.img[index,:,:,:]
        mask = self.mask[index,:,:]
    
    #the original Unet paper assignes increased weights to the edges of the annotated objects
    #their method is more sophistocated, but this one is faster, we simply dilate the mask and 
    #highlight all the pixels which were "added"
    if(self.edge_weight):
        weight = scipy.ndimage.morphology.binary_dilation(mask==1, iterations =2) & ~mask
    else: #otherwise the edge weight is all ones and thus has no affect
        weight = np.ones(mask.shape,dtype=mask.dtype)
    
    mask = mask[:,:,None].repeat(3,axis=2) #in order to use the transformations given by torchvision
    weight = weight[:,:,None].repeat(3,axis=2) #inputs need to be 3D, so here we convert from 1d to 3d by repetition
    
    img_new = img
    mask_new = mask
    weight_new = weight
    
    seed = random.randrange(sys.maxsize) #get a random seed so that we can reproducibly do the transofrmations
    if self.img_transform is not None:
        random.seed(seed) # apply this seed to img transforms
        img_new = self.img_transform(img)

    if self.mask_transform is not None:
        random.seed(seed)
        mask_new = self.mask_transform(mask)
        mask_new = np.asarray(mask_new)[:,:,0].squeeze()
        
        random.seed(seed)
        weight_new = self.mask_transform(weight)
        weight_new = np.asarray(weight_new)[:,:,0].squeeze()

    return img_new, mask_new, weight_new
def __len__(self):
    return self.nitems

========================================

and i use this code

for ii , (X, y, y_weight) in enumerate(dataLoader[phase]): #for each of the batches
X = X.to(device) # [Nbatch, 3, H, W]
y_weight = y_weight.type(‘torch.FloatTensor’).to(device)
y = y.type(‘torch.LongTensor’).to(device) # [Nbatch, H, W] with class indices (0, 1)

error occur

File “”, line 1, in
debugfile(‘C:/Users/mbmhm/Desktop/unet/train_unet.py’, wdir=‘C:/Users/mbmhm/Desktop/unet’)

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\site-packages\spyder_kernels\customize\spydercustomize.py”, line 856, in debugfile
debugger.run(“runfile(%r, args=%r, wdir=%r)” % (filename, args, wdir))

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\bdb.py”, line 585, in run
exec(cmd, globals, locals)

File “”, line 1, in

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\site-packages\spyder_kernels\customize\spydercustomize.py”, line 827, in runfile
execfile(filename, namespace)

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\site-packages\spyder_kernels\customize\spydercustomize.py”, line 110, in execfile
exec(compile(f.read(), filename, ‘exec’), namespace)

File “c:/users/mbmhm/desktop/unet/train_unet.py”, line 267, in
for ii , (X, y, y_weight) in enumerate(dataLoader[phase]): #for each of the batches

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\site-packages\torch\utils\data\dataloader.py”, line 193, in iter
return _DataLoaderIter(self)

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\site-packages\torch\utils\data\dataloader.py”, line 469, in init
w.start()

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\multiprocessing\process.py”, line 112, in start
self._popen = self._Popen(self)

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\multiprocessing\context.py”, line 223, in _Popen
return _default_context.get_context().Process._Popen(process_obj)

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\multiprocessing\context.py”, line 322, in _Popen
return Popen(process_obj)

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\multiprocessing\popen_spawn_win32.py”, line 89, in init
reduction.dump(process_obj, to_child)

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\multiprocessing\reduction.py”, line 60, in dump
ForkingPickler(file, protocol).dump(obj)

File “stringsource”, line 2, in tables.hdf5extension.Array.reduce_cython

TypeError: self.dims,self.dims_chunk,self.maxdims cannot be converted to a Python object for pickling

=========================

then i tried this code

x,y,w in dataLoader[‘train’]:
print(x.shape, y.shape, w.shape)

File “”, line 1, in
debugfile(‘C:/Users/mbmhm/Desktop/unet/train_unet.py’, wdir=‘C:/Users/mbmhm/Desktop/unet’)

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\site-packages\spyder_kernels\customize\spydercustomize.py”, line 856, in debugfile
debugger.run(“runfile(%r, args=%r, wdir=%r)” % (filename, args, wdir))

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\bdb.py”, line 585, in run
exec(cmd, globals, locals)

File “”, line 1, in

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\site-packages\spyder_kernels\customize\spydercustomize.py”, line 827, in runfile
execfile(filename, namespace)

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\site-packages\spyder_kernels\customize\spydercustomize.py”, line 110, in execfile
exec(compile(f.read(), filename, ‘exec’), namespace)

File “c:/users/mbmhm/desktop/unet/train_unet.py”, line 200, in
for w, y, z in dataLoader[‘train’]:

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\site-packages\torch\utils\data\dataloader.py”, line 576, in next
idx, batch = self._get_batch()

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\site-packages\torch\utils\data\dataloader.py”, line 543, in _get_batch
success, data = self._try_get_batch()

File “C:\Users\mbmhm\ansel\Anaconda3\envs\moongpu\lib\site-packages\torch\utils\data\dataloader.py”, line 519, in _try_get_batch
raise RuntimeError(‘DataLoader worker (pid(s) {}) exited unexpectedly’.format(pids_str))

RuntimeError: DataLoader worker (pid(s) 7744, 4584) exited unexpectedly

=====================

what can i do to solve this problem.

pla give me a answer.

It looks like you are running into some issues using multiprocessing and a HDF5 file.
Have a look at this topic.