Dataloader yields copied batches

Hello everyone! I have a small val dataset (~386 entries), so with a batch_size=256 the dataloader only does two iterations. With num_workers <= 1 that is. As I found out, setting num_workers to a larger number yields copies of batches: for num_workers=2, the dataset is yielded twice; for num_workers=4 the dataset is yielded 4 times (totalling to 8 batches), and so on. The batches are not empty, so since I stumbled across this on a prediction step, I got a dataset several times bigger than the original one. This took a while for me to figure out, it’s not indicated in the docs, and it seems that such behavior is unexpected.

That would be interesting behavior indeed but I cannot reproduce it on my end:

$ cat
import torch

class ToyDataset(
    def __getitem__(self, idx):
        data = torch.tensor(idx)
        return data

    def __len__(self):
        return 386

dataset = ToyDataset()
loader =, batch_size=256, num_workers=2)
for batch in loader:
$ python3

Thank you very much for answering! I could not reproduce it as well; the problem seems to have fixed itself. And yet I distinctly remember changing the num_workers argument and seeing the len(dm.dataloader()) change accordingly. Sorry I couldn’t be of more help.