Discrepancy in LayerNorm Calculations?

Cover image: Automatically generated by GPT-4 after reading this article.

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Today, I decided to manually calculate the values of LayerNorm.

We all know that the formula for LayerNorm is as follows:

$$\text{LayerNorm}(x) = \frac{x - \mu}{\sqrt{\text{Var[}x\text{]} + \epsilon}} \times \gamma + \beta$$

Where $\mu$ is the mean of $x$, and $\text{Var}$ is the variance of $x$.

With this information, let's calculate it ourselves, ignoring $\gamma$ and $\beta$ for simplicity:

import torch

x = torch.rand(16, 768)
mu = x.mean(dim=-1, keepdim=True)
var = x.var(dim=-1, keepdim=True)
eps = 1e-5
y = (x - mu) / (var + eps).sqrt()

This yields the following values:

# tensor([[ 0.1219, -0.0222, -1.4742,  ...,  0.1738, -0.6124, -0.3001],
#         [-1.6009, -1.5814,  1.5357,  ...,  0.1917,  1.3787, -0.2772],
#         [ 0.3738,  1.0520,  0.4403,  ...,  1.1353, -0.7488, -0.9137],
#         ...,
#         [ 0.8823, -1.5427,  0.4725,  ..., -1.2544, -1.5354, -0.4305],
#         [ 1.4548,  0.3059, -0.6732,  ..., -0.7109,  0.4908, -1.2447],
#         [-0.4067,  0.5974, -0.9113,  ..., -0.2511, -0.2279, -0.9675]])

Next, let's compare these results with PyTorch's torch.nn.LayerNorm:

layer_norm = torch.nn.LayerNorm(768, elementwise_affine=False, bias=False)

y_ln = layer_norm(x)

This yields:

# tensor([[ 0.1220, -0.0222, -1.4752,  ...,  0.1739, -0.6128, -0.3003],
#         [-1.6020, -1.5824,  1.5367,  ...,  0.1918,  1.3796, -0.2774],
#         [ 0.3741,  1.0527,  0.4406,  ...,  1.1360, -0.7493, -0.9143],
#         ...,
#         [ 0.8829, -1.5437,  0.4728,  ..., -1.2552, -1.5364, -0.4308],
#         [ 1.4557,  0.3061, -0.6736,  ..., -0.7113,  0.4911, -1.2455],
#         [-0.4069,  0.5978, -0.9119,  ..., -0.2513, -0.2281, -0.9681]])

When we compare the two, why are they different?

Unbiased Estimation

After a quick search, I found that torch.var has a parameter called correction, which defaults to 1, meaning it uses an unbiased estimate.

This means that it divides by N-1 instead of N, whereas torch.nn.LayerNorm uses N.

So let's modify the torch.var function by setting correction=0:

var = x.var(dim=-1, correction=0, keepdim=True)

tip

correction is an alias for unbiased, introduced in PyTorch 2.0.0.

In earlier versions, you would set it using unbiased=False:

var = x.var(dim=-1, unbiased=False, keepdim=True)

Now, let's compare the results again:

# tensor([[ 0.1220, -0.0222, -1.4752,  ...,  0.1739, -0.6128, -0.3003],
#         [-1.6020, -1.5824,  1.5367,  ...,  0.1918,  1.3796, -0.2774],
#         [ 0.3741,  1.0527,  0.4406,  ...,  1.1360, -0.7493, -0.9143],
#         ...,
#         [ 0.8829, -1.5437,  0.4728,  ..., -1.2552, -1.5364, -0.4308],
#         [ 1.4557,  0.3061, -0.6736,  ..., -0.7113,  0.4911, -1.2455],
#         [-0.4069,  0.5978, -0.9119,  ..., -0.2513, -0.2281, -0.9681]])

Now the numbers match perfectly!

Why Doesn't LayerNorm Use Unbiased Estimation?

To summarize briefly, it's about stability and simplification of calculations.

If you're interested in a deeper dive, here are some key points:

• Stability in Small Batch Calculations

  LayerNorm is typically applied to the features of individual samples (e.g., each neuron or feature) rather than across the entire batch. The number of features per sample is usually much larger than the number of samples. Thus, using population variance provides a more stable and accurate estimate, especially with small sample sizes.

• Reduced Importance of Unbiased Estimation

  The unbiased nature of sample variance (i.e., dividing by n-1 instead of n) is crucial when estimating population parameters from a sample. However, in deep learning regularization and normalization, like in LayerNorm, the impact of this bias is relatively minor. This is because these computations are used to normalize activations rather than to estimate overall statistics. Using population variance simplifies calculations with minimal effect on training outcomes.

• Stability in Gradient Calculations

  Stable gradients are vital during backpropagation. Using population variance leads to smoother and more stable gradient calculations, avoiding additional noise that might arise from small sample sizes. This contributes to better convergence and training performance.

• Simplified Calculations

  From a computational perspective, calculating population variance is slightly simpler than sample variance because it omits a subtraction operation (i.e., dividing by n rather than n-1). While this isn't a decisive factor, it's a consideration in the design process.

Conclusion

This article was inspired by a sudden curiosity.

I hope this explanation helps clarify the issue for you.