LLM Pretraining and Finetuning

This README walks through pretraining and finetuning a large language model using MosaicML's StreamingDataset format, Composer trainer, and MPT architecture. When used in concert on high-performance hardware such as A100 GPUs, these tools enable incredibly efficient and optimized LLM training.

Part 1: LLM Pretraining

Example model setup and training configurations are in ./yamls/pretraining. We include configurations for MPT models of various sizes.

Installation

If you haven't already, make sure to install the requirements.

Dataset preparation

To run pretraining, you'll need to make yourself a copy of a pretraining dataset and format it for efficient streaming. Check out the llm-foundry/data_prep folder for detailed instructions on how to convert your dataset to the MosaicML StreamingDataset format.

As a quickstart, we elaborate on how to prepare the C4 (Colossal, Cleaned, Common Crawl) dataset here.

We first convert the dataset from its native format (a collection of zipped JSONs) to MosaicML's StreamingDataset format, which is a collection of binary .mds files. Once in .mds format, we can store the dataset in a central location (filesystem, S3, GCS, etc.) and stream the data to any compute cluster, with any number of devices, and any number of CPU workers, and it all ~ just works ~ . You can read more about the benefits of using mosaicml-streaming here.

Converting C4 to StreamingDataset `.mds` format

To make yourself a copy of C4, use convert_dataset_hf.py like so:

Download the train_small and val_small splits and convert to StreamingDataset format. This will take 20-60 seconds depending on your internet bandwidth. You should see two folders once completed: ./my-copy-c4/train_small and ./my-copy-c4/val_small that are ~1.0GB total. Note that we are using the --concat_tokens option to pre tokenize our samples to be of the max sequence length without padding

python ../data_prep/convert_dataset_hf.py --dataset allenai/c4 --data_subset en --out_root ./my-copy-c4 --splits train_small val_small --concat_tokens 2048 --tokenizer EleutherAI/gpt-neox-20b --eos_text '<|endoftext|>'

Alternatively, you can download the full train and val splits if you really want to train the model (i.e. not just profile the model). This will take 1-to-many hours depending on bandwidth, number of CPUs, etc. The final folder ./my-copy-c4/train will be ~800GB so make sure you have space!

python ../data_prep/convert_dataset_hf.py --dataset allenai/c4 --data_subset en --out_root ./my-copy-c4 --splits train val --concat_tokens 2048 --tokenizer EleutherAI/gpt-neox-20b --eos_text '<|endoftext|>'

For any of the above commands, you can also choose to compress the .mds files. This is useful if your plan is to store these in object store after conversion.

python ../data_prep/convert_dataset_hf.py ... --compression zstd

Alternatively, feel free to substitute our dataloader with one of your own in train.py.

Test the Dataloader

To verify that the dataloader works, run a quick test on your val_small split like so:

# This will construct a `StreamingTextDataset` dataset from your `val` split,
# pass it into a PyTorch Dataloader, and iterate over it and print samples.
# Since we only provide a local path, no streaming/copying takes place.
python ../../llmfoundry/data/text_data.py --local_path ./my-copy-c4 --split val_small

# This will do the same thing, but stream data to {local} from {remote}.
# The remote path can be a filesystem or object store URI.
python ../../llmfoundry/data/text_data.py --local_path /tmp/cache-c4 --remote_path ./my-copy-c4  --split val_small # stream from filesystem, e.g. a slow NFS volume to fast local disk
# python ../data_prep/text_data.py --local_path /tmp/cache-c4 --remote_path s3://my-bucket/my-copy-c4  # stream from object store

How to start single and multi-node pretraining

Now that you've installed dependencies and built a local copy of the C4 dataset, let's start training!

Please remember to edit the data_local and (optionally) data_remote paths in your YAML. Our streaming dataloader always streams to data_local <- from <- data_remote, and if the remote path is missing, the files are expected to be present in data_local.

Also remember that if you only downloaded the train_small split, you need to make sure your train_loader uses that split. Just change split: train to split: train_small in your YAML. And similarly for val and val_small.

Single-Node training

We run the train.py script using our composer launcher, which generates N processes (1 per device).

If training on a single node, the composer launcher will autodetect the number of devices, so all you need to do is:

composer train.py yamls/pretrain/mpt-125m.yaml train_loader.dataset.split=train_small eval_loader.dataset.split=val_small

To train with high performance on multi-node clusters, the easiest way is with the MosaicML platform ;) Check out the mcli/ folder for examples!

If you want to implement this manually on your own cluster, then just provide a few variables to composer either directly via CLI, or via environment variables that can be read. Then launch the appropriate command on each node:

Multi-Node via CLI args

# Using 2 nodes each with 8 devices
# Total world size: 16
# IP Address for Node 0 = [0.0.0.0]

# Node 0
composer --world_size 16 --node_rank 0 --master_addr 0.0.0.0 --master_port 7501 train.py yamls/pretrain/mpt-125m.yaml

# Node 1
composer --world_size 16 --node_rank 1 --master_addr 0.0.0.0 --master_port 7501 train.py yamls/pretrain/mpt-125m.yaml

Multi-Node via environment variables

# Using 2 nodes with 8 devices each
# Total world size is 16
# IP Address for Node 0 = [0.0.0.0]

# Node 0
# export WORLD_SIZE=16
# export NODE_RANK=0
# export MASTER_ADDR=0.0.0.0
# export MASTER_PORT=7501
composer train.py yamls/pretrain/mpt-125m.yaml

# Node 1
# export WORLD_SIZE=16
# export NODE_RANK=1
# export MASTER_ADDR=0.0.0.0
# export MASTER_PORT=7501
composer train.py yamls/pretrain/mpt-125m.yaml

You should see logs being printed to your terminal like so. You can also easily enable other experiment trackers like Weights and Biases or CometML, by using Composer's logging integrations.

[batch=1/100]:
         Train LanguageCrossEntropy: 10.9736
         Train Perplexity: 58312.0586
         Train loss/train/total: 10.9736
[batch=2/100]:
         Train LanguageCrossEntropy: 10.9724
         Train Perplexity: 58243.8086
         Train loss/train/total: 10.9724
[batch=3/100]:
         Train LanguageCrossEntropy: 10.9745
         Train Perplexity: 58365.8047
         Train loss/train/total: 10.9745
[batch=4/100]:
         Train LanguageCrossEntropy: 10.6459
         Train Perplexity: 42018.5508
         Train loss/train/total: 10.6459

Part 2: LLM Finetuning

If you are unfamiliar with the LLM-Foundry in general, we recommend first going through the instructions for LLM Pretraining above before skipping to LLM Finetuning. This repository was designed to optimize pretraining, finetuning, and inference, and as such the structure and setup will make most sense when understood as a whole.

There are 3 different types of data sources you can use for finetuning:

A dataset from the HuggingFace Hub
A dataset stored on your local device
A local or remote dataset in the StreamingDataset .mds format

We'll cover these in broad detail below.

Example model finetuning YAML configurations can be found in ./yamls/finetune. We include configurations for MPT models of various sizes, as well as T5 and Dolly. Finetuning is enabled via the train_loader and eval_loader fields in your configuration YAML.

As in the above section for pretraining, we use the same train.py script to do finetuning.

For a minimal concrete example of finetuning a GPT2 model on a locally-stored ARC-Easy dataset, see ./finetune_example
For a minimal example of finetuning MPT-7B, we recommend starting with yamls/finetune/mpt-7b_dolly_sft.yaml

Before actually finetuning any models, we describe an important consideration: data formatting!

Data formatting

The finetuning dataloader requires training examples to be formatted as dictionaries with the following key-value structure:

formatted_example = {'prompt': <prompt_text>, 'response': <response_text>}

"prompt" refers to the text that you feed into the model, e.g. Tell me a few facts about dogs.
"response" refers to the text that the model is trained to produce in response to the prompt, e.g. Dogs are great pets. They love to play fetch. They are better than cats...

How to ensure that your data follows that format. Our tooling attempts to simplify any reformatting by making it easy to insert preprocessing functions into the data pipeline. Here's a (simplified) example of a preprocessing function in llmfoundry.data.finetuning.tasks:

@dataset_constructor.register('tatsu-lab/alpaca')
def alpaca_preprocessing_function(inp: Dict):
    """Split out prompt/response from text."""
    prompt, response = inp['text'].split('### Response:')
    prompt +=  '### Response:'
    return {'prompt': prompt, 'response': response}

Pre-defined preprocessing functions

As shown above, the preprocessing functions in llmfoundry.data.finetuning.tasks use the register() decorator to connect them to the HuggingFace datasets they apply to. To get a list of all the HuggingFace datasets that already have preprocessing functions registered for them, you can run:

python -c "from llmfoundry.data.finetuning.tasks import dataset_constructor; dataset_constructor.print_registered_tasks()"

Custom data preprocessing

If the dataset you want to use is in that list, or if it already has the "prompt"/"response" format, you're in luck! You can skip the rest of this section.

If not, you just need to write your own processing function. You can write the function wherever is convenient for you, as long as it importable.

Let's say you want to finetune on a HuggingFace dataset named mosaicml/doge-facts (which, sadly, is made up for this example), and it contains examples that look like this:

>>> import datasets
>>> dogefacts = datasets.load_dataset('mosaicml/doge-facts', split='train')
>>> dogefacts[0]
{'question': 'What doge is the best doge?', 'answer': 'All of them!'}
>>> dogefacts[1]
{'question': 'When was the first doge?', 'answer': 'The original doge meme is based on a photograph taken in 2010.'}

The only preprocessing required here is to map "question"-->"prompt" and "answer"-->"response".

def dogefacts_prep_fn(inp: Dict):
    return {'prompt': inp['question'], 'response': inp['answer']}

For this example, let's say we add this function to a file that we can import from. For example, with from my_data.formatting import dogefacts_prep_fn

Still have questions about custom data preprocessing? In the ./finetune_example/ directory, we demonstrate a more concrete example of training on a local dataset with custom preprocessing. Check out those resources for added information!

Usage

Now we'll cover the different ways you can use the finetuning utilities. This will mostly focus on how to configure your YAML, assuming you have already prepared any custom preprocessing functions as described above.

1) Using a dataset on the HuggingFace Hub

Let's say you want to finetune using a dataset available on the HuggingFace Hub. If the dataset has a pre-defined preprocessing function, e.g., tatsu-lab/alpaca, or if the dataset already has the "prompt"/"response" format, simply point the dataloader to that dataset.

train_loader:
    name: finetuning
    dataset:
        hf_name: tatsu-lab/alpaca
        split: train
        ...

If the dataset requires a custom preprocessing function, such as in the example described above, use preprocessing_fn to tell the dataloader where it should import the function from.

train_loader:
    name: finetuning
    dataset:
        hf_name: mosaicml/doge-facts
        preprocessing_fn: my_data.formatting:dogefacts_prep_fn
        split: train
        ...

2) Using a local dataset

Let's say you have your finetuning dataset stored in local jsonl files. Reference this in your YAML, such as the one in yamls/finetune/1b_local_data_sft.yaml

train_loader:
    name: finetuning
    dataset:
        hf_name: json # assuming data files are json formatted
        hf_kwargs:
            data_dir: /path/to/data/dir/
        preprocessing_fn: my.import.path:my_preprocessing_fn
        split: train
        ...

As before, if your local dataset already has the "prompt"/"response" format, you don't need to include preprocessing_fn since no preprocessing is needed.

3) Using a StreamingDataset (MDS) formatted dataset locally or in an object store

To enable streaming, you must first use the convert_finetuning_dataset.py script to convert a HuggingFace dataset into an MDS-formatted dataset (which you totally should -- they're amazing).

python ../data_prep/convert_finetuning_dataset.py \
    --dataset tatsu-lab/alpaca \
    --splits train \
    --out_root s3://my-bucket/my-copy-alpaca

Note Streaming datasets must follow the required "prompt"/"response" format, but you can preprocess during conversion by setting the --preprocessor argument.
python ../data_prep/convert_finetuning_dataset.py \
    --dataset mosaicml/doge-facts \
    --preprocessor my_data.formatting:dogefacts_prep_fn \
    --splits train \
    --out_root s3://my-bucket/my-copy-doge-facts

Once you have converted your HuggingFace dataset to a streaming dataset, just update your YAML like so:

train_loader:
    name: finetuning
    dataset:
        remote: s3://my-bucket/my-copy-doge-facts
        local: /tmp/mds-cache/
        split: train
        ...

Using Flash Attention

Flash Attention is an optimized implementation of the attention mechanism, first introduced by Dao et al.. LLM Foundry supports Flash Attention V2. To start, we recommend using one of our provided Docker images corresponding to the Flash Attention version you would like to use. Next, how you specify to use Flash Attention depends on which model you are using.

For MPT, you can specify Flash Attention in your YAML like so:

model:
    name: mpt_causal_lm
    ...
    attn_config:
        attn_impl: flash
        ...

If loading MPT from the HuggingFace Hub, you can specify Flash Attention in your YAML like so:

model:
    name: hf_causal_lm
    pretrained_model_name_or_path: mosaicml/mpt-7b
    ...
    config_overrides:
        attn_config:
            attn_impl: flash
        ...

For any HuggingFace model that supports Flash Attention (e.g. Llama and Mistral), you can specify Flash Attention in your YAML like so:

model:
    name: hf_causal_lm
    attn_implementation: "flash_attention_2"
    ...

HuggingFace models currently only support Flash Attention V2.

FAQ: How many GPUs do I need to train a LLM?

This is a complicated question in general, but if we assume that you are using FSDP with FULL_SHARD, activation checkpointing, and DecoupledLionW, then a good rule of thumb is:

Your total cluster memory in GB should be larger than 12 * N (# billions of params).

E.g. To train a GPT-13B model which has ~13 billion params, have at least 12 * 13 = 156 GB of total memory across your GPUs. You can accomplish this with 4xA100-40GB, or 2xA100-80GB, etc.

If you run into OOM errors when using small device counts, reduce device_train_microbatch_size until it succeeds.

Keep in mind: even though training will work in these minimalist settings, you will get much better throughput_per_device if you use a larger cluster or devices with higher memory capacity, because this will enable you to use larger microbatch sizes.

Check out our scripts/train/benchmarking folder for detailed throughput measurements of specific model sizes on specific cluster configs!

FAQ: Optimizing Performance

The YAMLs in this repo are relatively well tuned for medium-to-large NVIDIA A100-40GB clusters.

If you are running with a CUDA-compatible GPU and have installed the LLM requirements, we turn on by default a kernel fusion optimization for the Cross Entropy loss function at the end of the model. This should not affect your model convergence, but if you would like to disable this, you can set model.loss_fn=torch_crossentropy. To re-enable, set model.loss_fn=fused_crossentropy or omit it from your YAML.

On devices with more / less GPU memory, you may wish to edit the device_train_microbatch_size or fsdp_config values. In general, larger microbatch sizes and disabling activation_checkpointing lead to higher throughput.

Note that each YAML specifies a global_train_batch_size, which is an optimization choice, i.e. the math being performed, and a device_train_microbatch_size, which is a system choice, i.e. how to execute that math.

Given these two values, our code automatically adjusts the # of gradient accumulation steps based on the # of devices, so you should be able to run the exact same YAML on 8 or 16 or 256 GPUs and get the same training results (within numerics). This is nice because it means you can write device-count-agnostic training configs, and not worry about OOM-ing or accidentally changing the optimization math.

In previous blog posts (1, 2) we also demonstrated auto microbatching, which takes things a step further by letting Composer determine the device_train_microbatch_size on its own. This makes our configs not only device-count-agnostic, but hardware-agnostic too! You can try out this feature by setting device_train_microbatch_size: auto, but bear in mind that FSDP support is still in alpha mode and may not always work with auto microbatching (we are working on it!).

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