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What is LMBERT?

LMBERT is BERT pre-training masked language model without next sentence prediction. This code is a adjustment of Google's original BERT code where we simply comment the next sentence prediction parts from the data preparation script (create_pretraining_data.py) and also edit the objective function to remove next sentence prediction in the running script (run_pretraining.py).

Pre-training with LMBERT

This code is to do "masked LM" on an arbitrary text corpus. For convenience, we copy the below from Google's GitHub.

Here's how to run the data generation. The input is a plain text file, with one sentence per line. (It is important that these be actual sentences for the "next sentence prediction" task). Documents are delimited by empty lines. The output is a set of tf.train.Examples serialized into TFRecord file format.

You can perform sentence segmentation with an off-the-shelf NLP toolkit such as spaCy. The create_pretraining_data.py script will concatenate segments until they reach the maximum sequence length to minimize computational waste from padding (see the script for more details). However, you may want to intentionally add a slight amount of noise to your input data (e.g., randomly truncate 2% of input segments) to make it more robust to non-sentential input during fine-tuning.

This script stores all of the examples for the entire input file in memory, so for large data files you should shard the input file and call the script multiple times. (You can pass in a file glob to run_pretraining.py, e.g., tf_examples.tf_record*.)

The max_predictions_per_seq is the maximum number of masked LM predictions per sequence. You should set this to around max_seq_length * masked_lm_prob (the script doesn't do that automatically because the exact value needs to be passed to both scripts).

python create_pretraining_data.py \
  --input_file=./sample_text.txt \
  --output_file=/tmp/tf_examples.tfrecord \
  --vocab_file=$BERT_BASE_DIR/vocab.txt \
  --do_lower_case=True \
  --max_seq_length=128 \
  --max_predictions_per_seq=20 \
  --masked_lm_prob=0.15 \
  --random_seed=12345 \
  --dupe_factor=5

Here's how to run the pre-training. Do not include init_checkpoint if you are pre-training from scratch. The model configuration (including vocab size) is specified in bert_config_file. This demo code only pre-trains for a small number of steps (20), but in practice you will probably want to set num_train_steps to 10000 steps or more. The max_seq_length and max_predictions_per_seq parameters passed to run_pretraining.py must be the same as create_pretraining_data.py.

python run_pretraining.py \
  --input_file=/tmp/tf_examples.tfrecord \
  --output_dir=/tmp/pretraining_output \
  --do_train=True \
  --do_eval=True \
  --bert_config_file=$BERT_BASE_DIR/bert_config.json \
  --init_checkpoint=$BERT_BASE_DIR/bert_model.ckpt \
  --train_batch_size=32 \
  --max_seq_length=128 \
  --max_predictions_per_seq=20 \
  --num_train_steps=20 \
  --num_warmup_steps=10 \
  --learning_rate=2e-5

This will produce an output like this:

***** Eval results *****
  global_step = 20
  loss = 0.0979674
  masked_lm_accuracy = 0.985479
  masked_lm_loss = 0.0979328
  next_sentence_accuracy = 1.0
  next_sentence_loss = 3.45724e-05

Note that since our sample_text.txt file is very small, this example training will overfit that data in only a few steps and produce unrealistically high accuracy numbers.

Pre-training data

We will not be able to release the pre-processed datasets used in the paper. For Wikipedia, the recommended pre-processing is to download the latest dump, extract the text with WikiExtractor.py, and then apply any necessary cleanup to convert it into plain text.

Unfortunately the researchers who collected the BookCorpus no longer have it available for public download. The Project Guttenberg Dataset is a somewhat smaller (200M word) collection of older books that are public domain.

Common Crawl is another very large collection of text, but you will likely have to do substantial pre-processing and cleanup to extract a usable corpus for pre-training BERT.

Learning a new WordPiece vocabulary

This repository does not include code for learning a new WordPiece vocabulary. The reason is that the code used in the paper was implemented in C++ with dependencies on Google's internal libraries. For English, it is almost always better to just start with our vocabulary and pre-trained models. For learning vocabularies of other languages, there are a number of open source options available. However, keep in mind that these are not compatible with our tokenization.py library:

• Google's SentencePiece library

• tensor2tensor's WordPiece generation script

• Rico Sennrich's Byte Pair Encoding library

Using BERT in Colab

If you want to use BERT with Colab, you can get started with the notebook "BERT FineTuning with Cloud TPUs". At the time of this writing (October 31st, 2018), Colab users can access a Cloud TPU completely for free. Note: One per user, availability limited, requires a Google Cloud Platform account with storage (although storage may be purchased with free credit for signing up with GCP), and this capability may not longer be available in the future. Click on the BERT Colab that was just linked for more information.