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63 lines
2.9 KiB
63 lines
2.9 KiB
2 years ago
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# Gradient Clipping
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Author: Boxiang Wang, Haichen Huang, Yongbin Li
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**Prerequisite**
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- [Define Your Configuration](../basics/define_your_config.md)
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- [Use Engine and Trainer in Training](../basics/engine_trainer.md)
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**Example Code**
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- [ColossalAI-Examples Gradient Clipping](https://github.com/hpcaitech/ColossalAI-Examples/tree/main/features/gradient_clipping)
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**Related Paper**
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- [On the difficulty of training Recurrent Neural Networks](https://arxiv.org/abs/1211.5063)
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## Introduction
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In order to speed up training process and seek global optimum for better performance, more and more learning
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rate schedulers have been proposed. People turn to control learning rate to adjust descent pace during training,
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which makes gradient vector better to be uniformed in every step. In that case, the descent pace can be
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controlled as expected. As a result, gradient clipping, a technique which can normalize the gradient vector
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to circumscribe it in a uniformed length, becomes indispensable for those who desire their better
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performance of their models.
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You do not have to worry about implementing gradient clipping when using Colossal-AI, we support gradient
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clipping in a powerful and convenient way. All you need is just an additional command in your configuration
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file.
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## Why you should use gradient clipping provided by Colossal-AI
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The reason of why we do not recommend users to write gradient clipping by themselves is that naive gradient clipping
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may fail when applying tensor parallelism, pipeline parallelism or MoE.
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According to the illustration below, each GPU only owns a portion of parameters of the weight in a linear layer.
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To get correct norm of gradient vector of the weight of the linear layer, the norm of every gradient vector in each GPU
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should be summed together.
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More complicated thing is that the distribution of bias is different from the distribution of the weight.
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The communication group is different in the sum operation.
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(PS: This situation is an old version of 2D parallelism, the implementation in the code is not the same.
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But it is a good example about the difficulty to unify all communication in gradient clipping.)
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<figure style={{textAlign: "center"}}>
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<img src="https://s2.loli.net/2022/01/28/KXiJPHt3Dum82cA.png"/>
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<figcaption>Layout of parameters</figcaption>
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</figure>
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Do not worry about it, since Colossal-AI have handled it for you.
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### Usage
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To use gradient clipping, you can just simply add gradient clipping norm in your configuration file.
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```python
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clip_grad_norm = 1.0
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```
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### Hands-On Practice
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We provide a [runnable example](https://github.com/hpcaitech/ColossalAI-Examples/tree/main/features/gradient_clipping)
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to demonstrate gradient clipping. In this example, we set the gradient clipping vector norm to be 1.0. You can run the script using this command:
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```shell
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python -m torch.distributed.launch --nproc_per_node 1 --master_addr localhost --master_port 29500 train_with_engine.py
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```
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