LAMB Optimizer (TensorFlow)

This is a simple implementation of LAMB Optimizer, which appeared in the paper "Large Batch Optimization for Deep Learning: Training BERT in 76 minutes".

The older name of the paper was "Reducing BERT Pre-Training Time from 3 Days to 76 Minutes"

Update: official implementation of LAMB optimizer is now available: https://github.com/tensorflow/addons/blob/master/tensorflow_addons/optimizers/lamb.py

Notes

• This is NOT an official implementation.
• LAMB optimizer changes slightly from arXiv v1 ~ v3.
• We implement v3 version (which is the latest version on June, 2019.).
• Some uncertain parts are clarified by consulting original authors (such as scaling function).

Algorithm

LAMB optimizer is originally designed for large batch learning in neural networks, but could also used in small batch size as indicated by authors.

Usage

The implementation is based on BERT repository, which uses AdamWeightDecayOptimizer (appears in optimization.py) for pre-training and fine-tuning.

• Just use LAMBOptimizer as a regular optimizer in TensorFlow, similar to Adam or AdamWeightDecayOptimizer.
• Find LAMB optimizer in optimization.py.
• There is nothing special to tune other than initial learning_rate.

Results on MNIST

• I don't have TPU Pod to test its scalability on BERT with large batch 😂, but tested on MNIST for verify its effectiveness.
• All optimizers use an initial learning rate of 0.001 (default settings), and did NOT scale to the batch size (may bring another gain, but leave it for you to test).
• All the experiments are done on NVIDIA TESLA T4.

Here are the numbers on several three classical neural networks (MLP, CNN, Bi-RNN, Bi-GRU, Bi-LSTM) with different optimizers (Adam, AdamW, LAMB).

I only list results of batch={64, 128, 1024, 16384}. For full results, please see FULL_RESULTS.md.

Batch=64

Optimizer	MLP	CNN	Bi-RNN	Bi-GRU	Bi-LSTM	Note
Adam	97.03	98.93	96.24	98.92	99.04	Just ordinary Adam
AdamW	97.11	99.01	96.50	99.11	99.04	Used in BERT
LAMB	98.27	99.33	97.73	98.83	98.94	New optimizer for large batch

Batch=128

Optimizer	MLP	CNN	Bi-RNN	Bi-GRU	Bi-LSTM	Note
Adam	96.38	98.76	97.73	99.08	99.09	Just ordinary Adam
AdamW	96.57	98.72	98.05	98.96	99.00	Used in BERT
LAMB	97.90	99.20	98.04	98.87	98.76	New optimizer for large batch

Batch=1024

Optimizer	MLP	CNN	Bi-RNN	Bi-GRU	Bi-LSTM	Note
Adam	93.05	97.92	98.10	98.94	98.67	Just ordinary Adam
AdamW	93.67	98.00	98.19	98.86	98.82	Used in BERT
LAMB	97.68	98.82	98.27	98.61	98.47	New optimizer for large batch

Batch=16384

Optimizer	MLP	CNN	Bi-RNN	Bi-GRU	Bi-LSTM	Note
Adam	88.46	95.06	95.98	97.81	97.74	Just ordinary Adam
AdamW	91.46	96.57	96.34	98.45	98.39	Used in BERT
LAMB	93.23	97.89	93.76	87.60	80.36	New optimizer for large batch

Several Conclusions

Note: The conclusions are only made by the results above.

• LAMB consistently outperforms Adam and AdamW in most of the times, and shows consistent results among different batch sizes.
• LAMB shows big advantage than Adam and AdamW on large batch, showing its excellent scalability.
• LAMB failed to outperform than Adam and AdamW on complex RNN-based models, despite batch size.

Reproducibility

Check mnist_tensorflow.ipynb for details.

Note: You know the GPU/TPU won't get exactly the same results even we use fixed random seed.

References

• Large Batch Optimization for Deep Learning: Training BERT in 76 minutes. https://arxiv.org/abs/1904.00962v3
• BERT: Pre-training of Deep Bidirectional Transformers for Language Understanding. https://arxiv.org/abs/1810.04805

Issues

For help or issues, please submit a GitHub issue.