Condenser

Code for Condenser family, Transformer architectures for dense retrieval pre-training. Details can be found in our papers, Condenser: a Pre-training Architecture for Dense Retrieval and Unsupervised Corpus Aware Language Model Pre-training for Dense Passage Retrieval .

Currently supports all models with BERT or RoBERTa architecture.

Resource

Pre-trained Models

Headless Condenser can be retrived from Huggingface Hub using the following identifier strings.

• Luyu/condenser: Condenser pre-trained on BookCorpus and Wikipedia
• Luyu/co-condenser-wiki: coCondenser pre-trained on Wikipedia
• Luyu/co-condenser-marco: coCondenser pre-trained on MS-MARCO collection

For example, to load Condenser weights,

from transformers import AutoModel
model = AutoModel.from_pretrained('Luyu/condenser')

You can also download models with head weights from our server. Note that head weights are not necessary if you just want to fine tune the model. On the other hand, these weights are critical if you'd like to do further pre-training, e.g. for domain transfer. Using a randomly initalized head will likely corrupt the rest of the model.

• condenser: link
• co-condenser-wiki: link
• co-condenser-marco: link

Fine-tuning

The saved model can be loaded directly using huggingface interface and fine-tuned,

from transformers import AutoModel
model = AutoModel.from_pretrained('path/to/train/output')

The head will then be automatically omitted in fine-tuninig.

• For reproducing open QA experiments on NQ/TriviaQA, you can use the DPR toolkit and set --pretrained_model_cfg to a Condenser checkpoint. If GPU memory is an issue running DPR, you can alternatively use our GC-DPR toolkit, which allows limited memory setup to train DPR without performance sacrifice.
• For supervised IR on MS-MARCO, you can use our Tevatron toolkit (an official version of our Dense prototype toolkit). We will also add open QA examples and pre-processing code to Dense soon.

Dependencies

The code uses the following packages,

pytorch
transformers
datasets
nltk

Condenser Pre-training

Pre-processing

We first tokenize all the training text before running pre-training. The pre-processor expects one-paragraph per-line format. It will then run for each line sentence tokenizer to construct the final training data instances based on passed in --max_len. The output is a json file. We recommend first break the full corpus into shards.

for s in shard1, shard2, shardN
do
 python helper/create_train.py \
  --tokenizer_name bert-base-uncased \
  --file $s \
  --save_to $JSON_SAVE_DIR \
  --max_len $MAX_LENGTH
done

Pre-training

The following code lauch training on 4 gpus and train Condenser warm starting from BERT (bert-base-uncased) .

python -m torch.distributed.launch --nproc_per_node 4 run_pre_training.py \
  --output_dir $OUTDIR \
  --model_name_or_path bert-base-uncased \
  --do_train \
  --save_steps 20000 \
  --per_device_train_batch_size $BATCH_SIZE \
  --gradient_accumulation_steps $ACCUMULATION_STEPS \
  --fp16 \
  --warmup_ratio 0.1 \
  --learning_rate 1e-4 \
  --num_train_epochs 8 \
  --overwrite_output_dir \
  --dataloader_num_workers 32 \
  --n_head_layers 2 \
  --skip_from 6 \
  --max_seq_length $MAX_LENGTH \
  --train_dir $JSON_SAVE_DIR \
  --weight_decay 0.01 \
  --late_mlm

coCondenser Pre-training

Pre-processing

First tokenize all the training text before running pre-training. The pre-processor expects one training document per line, with document broken into spans, e.g.

{'spans': List[str]}
...

We recommend breaking the full corpus into shards. Then run tokenization script,

for s in shard1, shard2, shardN
do
 python helper/create_train_co.py \
  --tokenizer_name bert-base-uncased \
  --file $s \
  --save_to $JSON_SAVE_DIR
done

Pre-training

Launch training with the following script. Our experiments in the paper warm start the coCondenser (both head and backbone) from a Condenser checkpoint.

python -m torch.distributed.launch --nproc_per_node $NPROC run_co_pre_training.py \
  --output_dir $OUTDIR \
  --model_name_or_path /path/to/pre-trained/condenser/model \
  --do_train \
  --save_steps 20000 \
  --model_type bert \
  --per_device_train_batch_size $BATCH_SIZE \
  --gradient_accumulation_steps 1 \
  --fp16 \
  --warmup_ratio 0.1 \
  --learning_rate 1e-4 \
  --num_train_epochs 8 \
  --dataloader_drop_last \
  --overwrite_output_dir \
  --dataloader_num_workers 32 \
  --n_head_layers 2 \
  --skip_from 6 \
  --max_seq_length $MAX_LENGTH \
  --train_dir $JSON_SAVE_DIR \
  --weight_decay 0.01 \
  --late_mlm

Having NPROC x BATCH_SIZE to be large is critical for effective contrastive pre-training. It is set to roughly 2048 in our experiments. Warning: gradient_accumulation_steps should be kept at 1 as accumulation cannot emulate large batch for contrative loss.

If total GPU memory is bottlnecking, you may consider using gradient cached update. Download and install GradCache package from its repo. Then set additional command line argument --cache_chunk_size to be the desired sub-batch size. More about grad cache can be found in its paper, Scaling Deep Contrastive Learning Batch Size under Memory Limited Setup.

@inproceedings{gao2021scaling,
     title={Scaling Deep Contrastive Learning Batch Size under Memory Limited Setup},
     author={Luyu Gao, Yunyi Zhang, Jiawei Han, Jamie Callan},
     booktitle ={Proceedings of the 6th Workshop on Representation Learning for NLP},
     year={2021},
}