# 🧁 Low Precision LayerNorm#

Natural Language Processing, Math Equivalent

Low Precision LayerNorm forces torch.nn.LayerNorm modules to run in float16 or bfloat16 precision, improving utilization. This should not affect final convergence values, but in rare cases may cause loss spikes. This algorithm is an alternative to Fused LayerNorm, and should give very similar performance.

## How to Use#

Low Precision LayerNorm wraps torch.nn.LayerNorm, forcing the module to run in a lower precision if you have autocast enabled. If you are running in Automatic Mixed Precision (amp) mode, Low Precision LayerNorm will run in torch.float16. If you are running in bf16 mode, Low Precision LayerNorm will run in torch.bfloat16.

This algorithm will have no effect if you are running in fp32 or fp16 mode.

### Functional Interface#

# Apply surgery on the model to swap-in the Low Precision LayerNorm using the Composer functional API

import composer.functional as cf

cf.apply_low_precision_layernorm(model, precision='amp')

loss_fn = F.cross_entropy
model.train()

y_hat = model(X)
loss = loss_fn(y_hat, y)
loss.backward()
opt.step()


### Composer Trainer#

from composer.trainer import Trainer
from composer.algorithms import LowPrecisionLayerNorm

trainer = Trainer(model=model,
max_duration='1ep',
algorithms=[LowPrecisionLayerNorm()])

trainer.fit()


### Implementation Details#

Low Precision LayerNorm is implemented by performing model surgery, which looks for instances of torch.nn.LayerNorm and replaces them with composer.algorithms.low_precision_layernorm.low_precision_layernorm.LPLayerNorm, which is a thin wrapper around torch.nn.LayerNorm that manually turns autocast off and specifices the input dtype to lower precision. In bf16 mode on PyTorch versions prior to 1.13, Low Precision LayerNorm will fall back to Fused LayerNorm, replacing instances of torch.nn.LayerNorm with apex.normalization.fused_layer_norm.

## Suggested Hyperparameters#

Low Precision LayerNorm utilizes the existing normalized_shape and d_eps from the original model. The functional version of Low Precision LayerNorm allows users to specify the precision mode, which should be set to the Composer precision format of your model. When using the algorithm through the Composer trainer, Low Precision LayerNorm will use the trainer’s precision mode.

## Technical Details#

Low Precision LayerNorm is meant to replace our Fused LayerNorm algorithm. The two algorithms achieve very similar throughput. Fused LayerNorm also runs in low precision, but it is a more complex algorithm, since it uses a custom kernel. Since the custom kernel provides no additional speedup, we have replaced it with this simpler algorithm.

✅ Low Precision LayerNorm Improves Training Speed

In our experiments, Low Preicision LayerNorm improves the attainable tradeoffs between training speed and the final quality of the trained model. We recommend using Low Precision LayerNorm.

Algorithm class: composer.algorithms.LowPrecisionLayerNorm