Memory error when iterate over two dataloaders simultaneously in pytorch
I am trying to train my model using 2 dataloaders from 2 different datasets. 

I found how to set up this by using cycle() and zip() because my datasets are not the same length from here: How to iterate over two dataloaders simultaneously using pytorch?

  File "/home/Desktop/example/train.py", line 229, in train_2
    for i, (x1, x2) in enumerate(zip(cycle(train_loader_1), train_loader_2)):
  File "/home/.conda/envs/3dcnn/lib/python3.7/site-packages/torch/utils/data/dataloader.py", line 346, in __next__
    data = self.dataset_fetcher.fetch(index)  # may raise StopIteration
  File "/home/.conda/envs/3dcnn/lib/python3.7/site-packages/torch/utils/data/_utils/fetch.py", line 47, in fetch
    return self.collate_fn(data)
  File "/home/.conda/envs/3dcnn/lib/python3.7/site-packages/torch/utils/data/_utils/collate.py", line 80, in default_collate
    return [default_collate(samples) for samples in transposed]
  File "/home/.conda/envs/3dcnn/lib/python3.7/site-packages/torch/utils/data/_utils/collate.py", line 80, in &lt;listcomp&gt;
    return [default_collate(samples) for samples in transposed]
  File "/home/.conda/envs/3dcnn/lib/python3.7/site-packages/torch/utils/data/_utils/collate.py", line 56, in default_collate
    return torch.stack(batch, 0, out=out)
RuntimeError: [enforce fail at CPUAllocator.cpp:64] . DefaultCPUAllocator: can't allocate memory: you tried to allocate 154140672 bytes. Error code 12 (Cannot allocate memory)



I tried to solve that by setting num_workers=0, decreasing the batch size, using pinned_memory=False and shuffle=False...
But none of it worked... I am having 256GB of RAM and 4 NVIDIA TESLA V100 GPUs. 

I tried to run it just by not training in 2 dataloaders simultaneously but individually and it worked. However for my project I need this parallel training with 2 datasets...

Based on this discussion, instead of cycle() and zip() I avoid any errors by using:

  try:
     data, target = next(dataloader_iterator)
  except StopIteration:
     dataloader_iterator = iter(dataloader)
     data, target = next(dataloader_iterator)


kudos to @srossi93 from this pytorch post!
