BERT with Flash-Attention

Installing dependencies

To run the model on GPU, you need to install Flash Attention. You may either install from pypi (which may not work with fused-dense), or from source. To install from source, clone the GitHub repository:

git clone git@github.com:Dao-AILab/flash-attention.git

The code provided here should work with commit 43950dd. Change to the cloned repo and install:

cd flash-attention && python setup.py install

This will compile the flash-attention kernel, which will take some time.

If you would like to use fused MLPs (e.g. to use activation checkpointing), you may install fused-dense also from source:

cd csrc/fused_dense_lib && python setup.py install

Configuration

The config adds some new parameters:

• use_flash_attn: If True, always use flash attention. If None, use flash attention when GPU is available. If False, never use flash attention (works on CPU).
• window_size: Size (left and right) of the local attention window. If (-1, -1), use global attention
• dense_seq_output: If true, we only need to pass the hidden states for the masked out token (around 15%) to the classifier heads. I set this to true for pretraining.
• fused_mlp: Whether to use fused-dense. Useful to reduce VRAM in combination with activation checkpointing
• mlp_checkpoint_lvl: One of {0, 1, 2}. Increasing this increases the amount of activation checkpointing within the MLP. Keep this at 0 for pretraining and use gradient accumulation instead. For embedding training, increase this as much as needed.
• last_layer_subset: If true, we only need the compute the last layer for a subset of tokens. I left this to false.
• use_qk_norm: Whether or not to use QK-normalization
• num_loras: Number of LoRAs to use when initializing a BertLoRA model. Has no effect on other models.