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Fine-Tuning with LoRA or QLoRA

You can use use the mlx-lm package to fine-tune an LLM with low rank adaptation (LoRA) for a target task.1 The example also supports quantized LoRA (QLoRA).2 LoRA fine-tuning works with the following model families:

  • Mistral
  • Llama
  • Phi2
  • Mixtral
  • Qwen2
  • Gemma
  • OLMo
  • MiniCPM
  • InternLM2

Contents

Run

The main command is mlx_lm.lora. To see a full list of command-line options run:

mlx_lm.lora --help

Note, in the following the --model argument can be any compatible Hugging Face repo or a local path to a converted model.

You can also specify a YAML config with -c/--config. For more on the format see the example YAML. For example:

mlx_lm.lora --config /path/to/config.yaml

If command-line flags are also used, they will override the corresponding values in the config.

Fine-tune

To fine-tune a model use:

mlx_lm.lora \
    --model <path_to_model> \
    --train \
    --data <path_to_data> \
    --iters 600

To fine-tune the full model weights, add the --fine-tune-type full flag. Currently supported fine-tuning types are lora (default), dora, and full.

The --data argument must specify a path to a train.jsonl, valid.jsonl when using --train and a path to a test.jsonl when using --test. For more details on the data format see the section on Data.

For example, to fine-tune a Mistral 7B you can use --model mistralai/Mistral-7B-v0.1.

If --model points to a quantized model, then the training will use QLoRA, otherwise it will use regular LoRA.

By default, the adapter config and learned weights are saved in adapters/. You can specify the output location with --adapter-path.

You can resume fine-tuning with an existing adapter with --resume-adapter-file <path_to_adapters.safetensors>.

Evaluate

To compute test set perplexity use:

mlx_lm.lora \
    --model <path_to_model> \
    --adapter-path <path_to_adapters> \
    --data <path_to_data> \
    --test

Generate

For generation use mlx_lm.generate:

mlx_lm.generate \
    --model <path_to_model> \
    --adapter-path <path_to_adapters> \
    --prompt "<your_model_prompt>"

Fuse

You can generate a model fused with the low-rank adapters using the mlx_lm.fuse command. This command also allows you to optionally:

  • Upload the fused model to the Hugging Face Hub.
  • Export the fused model to GGUF. Note GGUF support is limited to Mistral, Mixtral, and Llama style models in fp16 precision.

To see supported options run:

mlx_lm.fuse --help

To generate the fused model run:

mlx_lm.fuse --model <path_to_model>

This will by default load the adapters from adapters/, and save the fused model in the path fused_model/. All of these are configurable.

To upload a fused model, supply the --upload-repo and --hf-path arguments to mlx_lm.fuse. The latter is the repo name of the original model, which is useful for the sake of attribution and model versioning.

For example, to fuse and upload a model derived from Mistral-7B-v0.1, run:

mlx_lm.fuse \
    --model mistralai/Mistral-7B-v0.1 \
    --upload-repo mlx-community/my-lora-mistral-7b \
    --hf-path mistralai/Mistral-7B-v0.1

To export a fused model to GGUF, run:

mlx_lm.fuse \
    --model mistralai/Mistral-7B-v0.1 \
    --export-gguf

This will save the GGUF model in fused_model/ggml-model-f16.gguf. You can specify the file name with --gguf-path.

Data

The LoRA command expects you to provide a dataset with --data. The MLX Examples GitHub repo has an example of the WikiSQL data in the correct format.

Datasets can be specified in *.jsonl files locally or loaded from Hugging Face.

Local Datasets

For fine-tuning (--train), the data loader expects a train.jsonl and a valid.jsonl to be in the data directory. For evaluation (--test), the data loader expects a test.jsonl in the data directory.

Currently, *.jsonl files support chat, tools, completions, and text data formats. Here are examples of these formats:

chat:

{"messages": [{"role": "system", "content": "You are a helpful assistant."}, {"role": "user", "content": "Hello."}, {"role": "assistant", "content": "How can I assistant you today."}]}

tools:

{"messages":[{"role":"user","content":"What is the weather in San Francisco?"},{"role":"assistant","tool_calls":[{"id":"call_id","type":"function","function":{"name":"get_current_weather","arguments":"{\"location\": \"San Francisco, USA\", \"format\": \"celsius\"}"}}]}],"tools":[{"type":"function","function":{"name":"get_current_weather","description":"Get the current weather","parameters":{"type":"object","properties":{"location":{"type":"string","description":"The city and country, eg. San Francisco, USA"},"format":{"type":"string","enum":["celsius","fahrenheit"]}},"required":["location","format"]}}}]}
View the expanded single data tool format 
{
    "messages": [
        { "role": "user", "content": "What is the weather in San Francisco?" },
        {
            "role": "assistant",
            "tool_calls": [
                {
                    "id": "call_id",
                    "type": "function",
                    "function": {
                        "name": "get_current_weather",
                        "arguments": "{\"location\": \"San Francisco, USA\", \"format\": \"celsius\"}"
                    }
                }
            ]
        }
    ],
    "tools": [
        {
            "type": "function",
            "function": {
                "name": "get_current_weather",
                "description": "Get the current weather",
                "parameters": {
                    "type": "object",
                    "properties": {
                        "location": {
                            "type": "string",
                            "description": "The city and country, eg. San Francisco, USA"
                        },
                        "format": { "type": "string", "enum": ["celsius", "fahrenheit"] }
                    },
                    "required": ["location", "format"]
                }
            }
        }
    ]
}

The format for the arguments field in a function varies for different models. Common formats include JSON strings and dictionaries. The example provided follows the format used by OpenAI and Mistral AI. A dictionary format is used in Hugging Face's chat templates. Refer to the documentation for the model you are fine-tuning for more details.

completions:

{"prompt": "What is the capital of France?", "completion": "Paris."}

For the completions data format, a different key can be used for the prompt and completion by specifying the following in the YAML config:

prompt_feature: "input"
completion_feature: "output"

Here, "input" is the expected key instead of the default "prompt", and "output" is the expected key instead of "completion".

text:

{"text": "This is an example for the model."}

Note, the format is automatically determined by the dataset. Note also, keys in each line not expected by the loader will be ignored.

Note

Each example in the datasets must be on a single line. Do not put more than one example per line and do not split an example across multiple lines.

Hugging Face Datasets

To use Hugging Face datasets, first install the datasets package:

pip install datasets

If the Hugging Face dataset is already in a supported format, you can specify it on the command line. For example, pass --data mlx-community/wikisql to train on the pre-formatted WikiwSQL data.

Otherwise, provide a mapping of keys in the dataset to the features MLX LM expects. Use a YAML config to specify the Hugging Face dataset arguments. For example:

hf_dataset:
  name: "billsum"
  prompt_feature: "text"
  completion_feature: "summary"

  • Use prompt_feature and completion_feature to specify keys for a completions dataset. Use text_feature to specify the key for a text dataset.

  • To specify the train, valid, or test splits, set the corresponding {train,valid,test}_split argument.

  • Arguments specified in config will be passed as keyword arguments to datasets.load_dataset.

In general, for the chat, tools and completions formats, Hugging Face chat templates are used. This applies the model's chat template by default. If the model does not have a chat template, then Hugging Face will use a default. For example, the final text in the chat example above with Hugging Face's default template becomes:

<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Hello.<|im_end|>
<|im_start|>assistant
How can I assistant you today.<|im_end|>

If you are unsure of the format to use, the chat or completions are good to start with. For custom requirements on the format of the dataset, use the text format to assemble the content yourself.

Memory Issues

Fine-tuning a large model with LoRA requires a machine with a decent amount of memory. Here are some tips to reduce memory use should you need to do so:

  1. Try quantization (QLoRA). You can use QLoRA by generating a quantized model with convert.py and the -q flag. See the Setup section for more details.

  2. Try using a smaller batch size with --batch-size. The default is 4 so setting this to 2 or 1 will reduce memory consumption. This may slow things down a little, but will also reduce the memory use.

  3. Reduce the number of layers to fine-tune with --num-layers. The default is 16, so you can try 8 or 4. This reduces the amount of memory needed for back propagation. It may also reduce the quality of the fine-tuned model if you are fine-tuning with a lot of data.

  4. Longer examples require more memory. If it makes sense for your data, one thing you can do is break your examples into smaller sequences when making the {train, valid, test}.jsonl files.

  5. Gradient checkpointing lets you trade-off memory use (less) for computation (more) by recomputing instead of storing intermediate values needed by the backward pass. You can use gradient checkpointing by passing the --grad-checkpoint flag. Gradient checkpointing will be more helpful for larger batch sizes or sequence lengths with smaller or quantized models.

For example, for a machine with 32 GB the following should run reasonably fast:

mlx_lm.lora \
    --model mistralai/Mistral-7B-v0.1 \
    --train \
    --batch-size 1 \
    --num-layers 4 \
    --data wikisql

The above command on an M1 Max with 32 GB runs at about 250 tokens-per-second, using the MLX Example wikisql data set.

Footnotes

  1. Refer to the arXiv paper for more details on LoRA. ↩

  2. Refer to the paper QLoRA: Efficient Finetuning of Quantized LLMs ↩