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Finetuning with LoRA

Low-rank adaption (LoRA) is a technique to approximate the update to the linear layers in a LLM with a low-rank matrix factorization. This significantly reduces the number of trainable parameters and speeds up training with little impact on the final performance of the model. We demonstrate this method by instruction-finetuning Lit-GPT StableLM 3B on the Alpaca dataset on a single RTX 3090 (24GB) GPU.

Preparation

The steps here only need to be done once:

  1. Follow the instructions in the README to install the dependencies.
  2. Download and convert the weights and save them in the ./checkpoints folder. Weights can be downloaded following these instructions:

  1. Download the data and generate the instruction tuning dataset:

    python scripts/prepare_alpaca.py

(See this blog article for how to prepare and use custom datasets.)

Running the finetuning

python finetune/lora.py

The finetuning requires at least one GPU with ~24 GB memory (RTX 3090).

This script will save checkpoints periodically to the folder out/.

Note: LoRA can be applied to not only query, key or value matrices, but also to projection, mlp and classification head. According to QLoRA paper (section 4): "LoRA on all linear transformer block layers are required to match full finetuning performance". By default LoRA is applied only to the query and value matrices. In order to apply LoRA to other weight matrices - change the variables in finetune/lora.py accordingly.

Test the model

You can test the finetuned model with your own instructions by running:

python generate/lora.py --prompt "Recommend a movie to watch on the weekend."

Output:

I would recommend the movie The Martian (2015). It is a sci-fi movie starring Matt Damon that follows the story of...

If your GPU supports bfloat16, you can additionally pass --precision bf16-true to bring the memory consumption down to ~11 GB for StableLM-3B.

Tune on your dataset

With only a few modifications, you can prepare and train on your own instruction dataset.

  1. Create a json file in which each row holds one instruction-response pair. A row has an entry for 'instruction', 'input', and 'output', where 'input' is optional an can be the empty string if the instruction doesn't require a context. Below is an example json file:

    [
    {
        "instruction": "Arrange the given numbers in ascending order.",
        "input": "2, 4, 0, 8, 3",
        "output": "0, 2, 3, 4, 8"
    },
    ...
]

  2. Make a copy of scripts/prepare_alpaca.py and name it what you want:

    cp scripts/prepare_alpaca.py scripts/prepare_mydata.py

  3. Modify scripts/prepare_mydata.py to read the json data file.

  4. Run the script to generate the preprocessed, tokenized train-val split:

    python scripts/prepare_mydata.py --destination_path data/mydata/

  5. Run finetune/lora.py by passing in the location of your data (and optionally other parameters):

    python finetune/lora.py --data_dir data/mydata/ --out_dir out/myexperiment

Troubleshooting

If you run into a CUDA error "Expected is_sm80 to be true, but got false", uncomment the line torch.backends.cuda.enable_flash_sdp(False) in the script below (see Lightning-AI/lit-llama#101).