ColossalAI/docs/amp.md

Mixed precision training

In Colossal-AI, we have incorporated different implementations of mixed precision training:

1. torch.cuda.amp
2. apex.amp
3. naive amp

The first two rely on the original implementation of PyTorch (version 1.6 and above) and Nvidia Apex. The last mehtod is simialr to Apex O2 level.

Among these methods, apex.amp is not compatible with tensor parallelism. This is because that tensors are split across devices in tensor parallelism, thus, it is required to communicate among different processes to check if inf or nan occurs in the whole model weights. We modified the torch amp implementation so that it is compatible with tensor parallelism now.

To use mixed precision training, you can easily specify the fp16 field in the config file to be True. Currently, PyTorch and Apex amp cannot be guaranteed to work with tensor and pipeline parallelism. We recommend you to use torch amp as it generally gives better accuracy than naive amp.

The AMP module is designed to be completely modular and can be used independently from other colossalai modules. If you wish to only use amp in your code base without colossalai.initialize, you can use colossalai.amp.convert_to_amp.

from colossalai.amp import AMP_TYPE

# exmaple of using torch amp
model, optimizer, criterion = colossalai.amp.convert_to_amp(model, 
                                                            optimizer, 
                                                            criterion,
                                                            AMP_TYPE.TORCH)

PyTorch AMP

PyTorch provides mixed precision training in version 1.6 and above. It provides an easy way to cast data to fp16 format while keeping some operations such as reductions in fp32. You can configure the gradient scaler in the config file.

from colossalai.amp import AMP_TYPE

fp16=dict(
    mode=AMP_TYPE.TORCH,
    # below are default values for grad scaler
    init_scale=2.**16,
    growth_factor=2.0,
    backoff_factor=0.5,
    growth_interval=2000,
    enabled=True
)

Apex AMP

For this mode, we rely on the Apex implementation for mixed precision training. We support this plugin because it allows for finer control on the granularity of mixed precision. For example, O2 level (optimization level 2) will keep batch normalization in fp32.

The following code block shows a config file for Apex AMP.

from colossalai.amp import AMP_TYPE

fp16 = dict(
    mode=AMP_TYPE.APEX,
    # below are the default values
    enabled=True, 
    opt_level='O1', 
    cast_model_type=None, 
    patch_torch_functions=None, 
    keep_batchnorm_fp32=None, 
    master_weights=None, 
    loss_scale=None, 
    cast_model_outputs=None,
    num_losses=1, 
    verbosity=1, 
    min_loss_scale=None, 
    max_loss_scale=16777216.0
)

Tensor Parallel AMP

We leveraged the Megatron-LM implementation to achieve mixed precision training while maintaining compatibility with complex tensor and pipeline parallelism.

The following conde block show a config file for this mode.

from colossalai.amp import AMP_TYPE

fp16 = dict(
    mode=AMP_TYPE.NAIVE,
    # below are the default values
    clip_grad=0,
    log_num_zeros_in_grad=False,
    initial_scale=2 ** 32,
    min_scale=1,
    growth_factor=2,
    backoff_factor=0.5,
    growth_interval=1000,
    hysteresis=2
)