- 1 RoBERTa: A Robustly Optimized BERT Pretraining Approach
- 1.1 Presented by
- 1.2 Introduction
- 1.3 Background
- 1.4 Training Procedure Analysis
- 1.5 RoBERTa
- 1.6 Results
- 1.7 Conclusion
- 1.8 The comparison at a glance
- 1.9 Critique
- 1.10 Source Code
- 1.11 Refrences
RoBERTa: A Robustly Optimized BERT Pretraining Approach
Self-training methods in the NLP domain(Natural Language Processing) like ELMo, GPT, BERT, XLM, and XLNet have shown significant improvements, but it remains challenging to determine which parts the methods have the most contribution. This paper proposed Roberta, which replicates BERT pretraining, and investigates the effects of hyperparameters tuning and training set size. In summary, what they did can be categorized as follows. (1) they modified some BERT design choices and training schemes. (2) they used a new set of datasets. These 2 modification categories help them improve performance on downstream tasks.
In this section, they tried to have an overview of BERT as they used this architecture. In short terms, BERT uses transformer architecture with 2 training objectives; they use masks language modeling (MLM) and next sentence prediction (NSP) as their objectives. The MLM objective randomly selects some of the tokens in the input sequence and replaces them with the special token [MASK]. Then they try to predict these tokens based on the surrounding information. NSP employs a binary classification loss for the prediction of whether the two sentences are adjacent to each other or not. It is noteworthy that, while selecting positive next sentences is trivial, generating negative ones is often much more difficult. Originally, they used a random next sentence as a negative. They used the Adam optimizer with some specific parameters to train the network. They used different types of datasets to train their networks. Finally, they did some experiments on the evaluation tasks such as GLUE, SQuAD, RACE, and showed their performance on those downstream tasks.
Training Procedure Analysis
In this section, they elaborate on which choices are important for successfully pretraining BERT.
Static vs. Dynamic Masking
First, they discussed static vs. dynamic masking. As I mentioned in the previous section, the masked language modeling objective in BERT pretraining masks a few tokens from each sequence at random and then predicts them. However, in the original implementation of BERT, the sequences are masked just once in the preprocessing. This implies that the same masking pattern is used for the same sequence in all the training steps. To extend this single static mask, the authors duplicated training data 10 times so that each sequence was masked in 10 different ways. The model was trained on those data for 40 epochs, with each sequence with the same mask being used 4 times.Unlike static masking, dynamic masking was tried, wherein a masking pattern is generated every time a sequence is fed to the model. The results show that dynamic masking has slightly better performance in comparison to the static one.
Input Representation and Next Sentence Prediction
The next thing they tried to investigate was the necessity of the next sentence prediction objective. They tried different settings to show it would help with eliminating the NSP loss in pretraining.
(1) Segment-Pair + NSP: Each input has a pair of segments (segments, not sentences) from either the original document or some different document chosen at random with a probability of 0.5, and then these are trained for a textual entailment or a Natural Language Inference (NLI) objective. The total combined length must be < 512 tokens (the maximum fixed sequence length for the BERT model). This is the input representation used in the BERT implementation.
(2) Sentence-Pair + NSP: This is the same as the segment-pair representation but with pairs of sentences instead of segments. However, the total length of sequences here would be a lot less than 512. Hence, a larger batch size is used so that the number of tokens processed per training step is similar to that in the segment-pair representation.
(3) Full-Sentences: In this setting, they didn't use any kind of NSP loss. Input sequences consist of full sentences from one or more documents. If one document ends, then sentences from the next document are taken and separated using an extra separator token until the sequence's length is at most 512.
(4) Doc-Sentences: Similar to the full sentences setting, they didn't use NSP loss in their loss again. This is the same as Full-Sentences, with the difference that the sequence doesn’t cross document boundaries, i.e. once the document is over, sentences from the next ones aren’t added to the sequence. Here, since the document lengths are varying, to solve this problem, they used some kind of padding to make all of the inputs the same length.
In the following table, you can see each setting's performance on each downstream task as you can see the best result achieved in the DOC-SENTENCES setting with removing the NSP loss. However, the authors chose to use FULL-SENTENCES for convenience sake, since the DOC-SENTENCES results in variable batch sizes.
Large Batch Sizes
The next thing they tried to investigate was the importance of the large batch size. They tried a different number of batch sizes and realized the 2k batch size has the best performance among the other ones. The below table shows their results for a different number of batch sizes. The result suggested the original BERT batch size was too small. The authors used 8k batch size in the remainder of their experiments.
In Roberta, they use byte-level Byte-Pair Encoding (BPE) for tokenization compared to BERT, which uses character level BPE. BPE is a hybrid between character and word level modeling based on sub-word units. The authors also used a vocabulary size of 50k rather than 30k at the original BERT implementation did, this increases the total parameters by approximately 15M to 20M for the BERT based and BERT large respectively. This change actually results in slight degradation of end-task performance in some cases, however, the authors preferred having the ability to universally encode text without introducing the "unknown" token.
They claim that if they apply all the aforementioned modifications to the BERT and pre-trained the model on a larger dataset, they can achieve higher performance on downstream tasks. They used different types of datasets for their pre-training; you can see a list of them below.
(1) BookCorpus + English Wikipedia (16GB): This is the data on which BERT is trained.
(2) CC-News (76GB): The authors have collected this data from the English portion of the CommonCrawl News Data. It contains 63M English news articles crawled between September 2016 and February 2019.
(3) OpenWebText (38GB): Open Source recreation of the WebText dataset used to train OpenAI GPT.
(4) Stories (31GB): A subset of CommonCrawl data filtered to match the story-like style of Winograd schemas.
In conjunction with the modifications of dynamic masking, full-sentences whiteout an NLP loss, large mini-batches, and a larger byte-level BPE.
RoBERTa has outperformed state-of-the-art algorithms in almost all GLUE tasks, including ensemble models. More than that, they compare the performance of the RoBERTa with other methods on the RACE and SQuAD evaluation and show their results in the below table.
In conclusion, they basically said the reasons why they make gains may be questionable, and if you decently pre-train BERT, you will achieve the same performances as RoBERTa.
The comparison at a glance
While the results are outstanding and appreciable (reasonably due to using more data and resources), the technical novelty contribution of the paper is marginally incremental as the architecture is largely unchanged from BERT.
The code for this paper is freely available at RoBERTa. The original repository for RoBERTa is in PyTorch. In case you are a TensorFlow user, you would be interested in PyTorch to TensorFlow repository as well.
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