Lowering batch-size eventually results in PyTorch increasing amount of memory reserved

Question

I'm trying to train a model meant for long text classification. I'm using this repo: https://github.com/franbvalero/BERT-long-sentence-classification/tree/optimizer_lstm But I modified it to work with a different dataset (i.e. I changed the inputs, not anything directly related to the training). I'm also using a different BERT model trained for another language.

The problem I'm facing is that going lower in bath-size increases the amount PyTorch reserves. Using 64 as batch-size results in the following error message:

RuntimeError: CUDA out of memory. Tried to allocate 20.95 GiB (GPU 0; 8.00 GiB total capacity; 700.89 MiB already allocated; 5.68 GiB free; 756.00 MiB reserved in total by PyTorch)

Definitely too high for the capacity available to me; I try 32 instead.

RuntimeError: CUDA out of memory. Tried to allocate 10.47 GiB (GPU 0; 8.00 GiB total capacity; 602.89 MiB already allocated; 5.77 GiB free; 658.00 MiB reserved in total by PyTorch)

Next 16:

RuntimeError: CUDA out of memory. Tried to allocate 5.24 GiB (GPU 0; 8.00 GiB total capacity; 5.78 GiB already allocated; 596.27 MiB free; 5.83 GiB reserved in total by PyTorch)

At this point the reserved total by PyTorch has sky-rocketed and this trend follows all the way down to a batch-size of 1 where it will try allocate just a few mb but PyTorch will reserve >6 gb.

RuntimeError: CUDA out of memory. Tried to allocate 336.00 MiB (GPU 0; 8.00 GiB total capacity; 6.20 GiB already allocated; 138.27 MiB free; 6.26 GiB reserved in total by PyTorch)

I'm a little bit confused as to why this happens. Lowering the batch-size does reduce the memory required in terms of the allocation that is attempted, but it also increases the amount that PyTorch reserves.