Author: Guangyang Lu, Shenggui Li, Siqi Mai

Prerequisite:

## Introduction​

In Colossal-AI, a configuration file is required to specify the features the system will inject into the training process. In this tutorial, we will introduce you how to construct your configuration file and how this config file will be used. Using configuration file has several advantages:

1. You can store your feature configuration and training hyper-parameters in different configuration files
2. New features released in the future can be specified in the configuration without code change in the training script

In this tutorial, we will cover how to define your configuration file.

## Configuration Definition​

In a configuration file, there are two types of variables. One serves as feature specification and the other serves as hyper-parameters. All feature-related variables are reserved keywords. For example, if you want to use mixed precision training, you need to use the variable name fp16 in the config file and follow a pre-defined format.

### Feature Specification​

There is an array of features Colossal-AI provides to speed up training. Each feature is defined by a corresponding field in the config file. In this tutorial, we are not giving the config details for all the features, but rather we are providing an illustration of how to specify a feature. The details of each feature can be found in its respective tutorial.

To illustrate the use of config file, we use mixed precision training as an example here. In order to do so, you need to follow the steps below.

1. create a configuration file (e.g. config.py, the file name can be anything)

2. define the mixed precision configuration in the config file. For example, in order to use mixed precision training natively provided by PyTorch, you can just write these lines of code below into your config file.

from colossalai.amp import AMP_TYPE    fp16 = dict(  mode=AMP_TYPE.TORCH)
3. Tell Colossal-AI where your config file is when launch the distributed environment. For example, the config file is in the current directory.

import colossalaicolossalai.launch(config='./config.py', ...)

In this way, Colossal-AI knows what features you want to use and will inject this feature during colossalai.initialize.

### Global Hyper-parameters​

Besides feature specification, the config file can also serve as a place to define your training hyper-parameters. This comes handy when you want to perform multiple experiments, each experiment details can be put into a single config file to avoid confusion. These parameters will be stored in the global parallel context and can be accessed in the training script.

For example, you can specify the batch size in your config file.

BATCH_SIZE = 32

After launch, you are able to access your hyper-parameters through global parallel context.

import colossalaifrom colossalai.core import global_context as gpccolossalai.launch(config='./config.py', ...)# access your parameterprint(gpc.config.BATCH_SIZE)