LLM Optimization: Layer-wise Optimal Rank Adaptation (LORA)

Tomas Vykruta
7 min readApr 30, 2023

Layer-wise Optimal Rank Adaptation (LORA) is a technique that aims to reduce the computational cost and memory footprint of large-scale Transformer models while preserving their performance. The intuition behind LORA is based on the observation that many matrices in deep neural networks, especially the attention mechanism of Transformers, can be effectively approximated by low-rank matrices. A low-rank matrix is a matrix that can be decomposed into the product of two smaller matrices. By leveraging this property, LORA reduces the number of parameters and computational complexity, making it more efficient and practical to use large models in various applications.

Here is a high-level explanation of why LORA works:

  1. Low-rank approximation: In deep neural networks, many weight matrices have a low-rank structure. This means that they can be decomposed into the product of two smaller matrices, which significantly reduces the number of parameters required to represent the original matrix. LORA focuses on approximating these weight matrices by their low-rank counterparts, thus reducing the model’s size and computational cost.
  2. Layer-wise adaptation: LORA adapts the rank of the low-rank approximation on a per-layer basis. This is based on the observation that different layers in a Transformer may have different optimal ranks. By choosing the rank for each layer individually, LORA allows for a more efficient trade-off between model size, computational cost, and performance.
  3. Fine-tuning: After the low-rank approximation is applied to the target layers, LORA fine-tunes the model to compensate for potential loss of performance. This fine-tuning step helps the model to adapt to the low-rank approximation and maintain its original performance.

LORA is particularly useful for large-scale Transformer models, as these models can have billions of parameters and require significant computational resources. By using LORA, it is possible to create more efficient models that are easier to deploy in real-world applications, such as on-device machine learning or low-latency inference tasks.

In summary, LORA leverages the low-rank structure of the weight matrices in Transformer models to reduce the number of parameters and computational cost. The layer-wise adaptation and fine-tuning steps ensure that the model’s performance is maintained while achieving these efficiency gains.

how does the layer-wise adaptation work?

The layer-wise adaptation in LORA refers to the process of selecting an optimal rank for the low-rank approximation on a per-layer basis. The intuition behind this is that different layers in a Transformer model may have different optimal ranks for the low-rank approximation, which allows for a more efficient trade-off between model size, computational cost, and performance. Here’s a step-by-step explanation of how layer-wise adaptation works:

  1. Identify target layers: First, the target layers for applying LORA are identified. These are usually the layers that contribute the most to the model’s size and computational cost, such as the query and value projection matrices in the multi-head self-attention mechanism.
  2. Compute low-rank approximation for each layer: For each target layer, a low-rank approximation is computed. This can be done using various methods, such as singular value decomposition (SVD) or other matrix factorization techniques. The objective is to find two smaller matrices whose product closely approximates the original weight matrix in the target layer.
  3. Select optimal rank: The optimal rank for each layer is chosen by considering the trade-off between the model’s performance and the reduction in parameters and computational cost. This can be done by evaluating the performance of the model on a validation set using different ranks and selecting the rank that provides the best balance between performance and efficiency. In some cases, this process can be automated using techniques like cross-validation or Bayesian optimization.
  4. Replace target layers: Once the optimal rank for each layer is determined, the original weight matrices in the target layers are replaced with their low-rank approximations. This reduces the number of parameters in the model and the computational cost associated with the target layers.
  5. Fine-tuning: After the low-rank approximation is applied to the target layers, the model is fine-tuned to adapt to the changes and recover any potential loss of performance. This can be done using standard training techniques, such as gradient descent, with a smaller learning rate to avoid large updates that could harm the performance.

By performing layer-wise adaptation, LORA can effectively balance the trade-off between model size, computational cost, and performance. It allows for more efficient models that are tailored to the specific requirements of each layer in the Transformer architecture, resulting in more efficient and practical models for real-world applications.

why was r=16 chosen for the OPT6.7B model? what does this number mean and why not use a smaller or larger number?

The r=16 parameter in the LORA configuration for the OPT6.7B model refers to the rank of the low-rank approximation. The rank is the dimensionality of the smaller matrices resulting from the low-rank decomposition. By using a lower rank, the number of parameters in the model is reduced, which leads to a decrease in computational cost and memory requirements.

Choosing the optimal rank is a trade-off between model efficiency and performance. A smaller rank results in a more compact model with fewer parameters, leading to faster inference and lower memory usage. However, if the rank is too small, the low-rank approximation may not capture the essential information in the original weight matrices, which can lead to a significant degradation in the model’s performance.

On the other hand, a larger rank provides a better approximation of the original weight matrices, which helps preserve the model’s performance. However, a larger rank also means more parameters and higher computational cost, which can negate the benefits of using LORA in the first place.

The choice of r=16 for the OPT6.7B model likely comes from experimentation and validation on a held-out dataset. The developers would have tested different values of r to find the optimal balance between model efficiency and performance. The r=16 value was probably chosen because it provided a good trade-off between maintaining the model's performance and reducing its size and computational cost.

It is essential to understand that the optimal value of r may vary depending on the specific model, dataset, and task. If you are working with a different model, you might need to perform your own experiments to find the best value of r for your use case.

Intuitively, what does r=16 means, and how it would differ from r=2?

When using LORA, the parameter r represents the rank of the low-rank approximation applied to the target layers in the Transformer model. It determines the dimensionality of the smaller matrices resulting from the low-rank decomposition. The choice of r directly affects the trade-off between model efficiency (fewer parameters and lower computational cost) and performance (accuracy and generalization).

Here’s a step-by-step explanation of what r=16 means and how it would differ from r=2:

  1. Identify target layers: First, identify the target layers to which LORA will be applied. These are usually the layers that contribute the most to the model’s size and computational cost, such as the query and value projection matrices in the multi-head self-attention mechanism.
  2. Compute low-rank approximation: For each target layer, compute a low-rank approximation using the specified rank (r). This is typically achieved using methods like singular value decomposition (SVD) or other matrix factorization techniques. The objective is to find two smaller matrices whose product closely approximates the original weight matrix in the target layer.
  • For r=16, the low-rank approximation will result in two matrices with a dimensionality of 16. The product of these matrices will approximate the original weight matrix.
  • For r=2, the low-rank approximation will result in two matrices with a dimensionality of 2. The product of these matrices will approximate the original weight matrix.
  1. Replace target layers: Replace the original weight matrices in the target layers with their low-rank approximations. This reduces the number of parameters in the model and the computational cost associated with the target layers.
  • With r=16, the model will have more parameters than with r=2, but the approximation will be closer to the original weight matrices, which can help maintain the model's performance.
  • With r=2, the model will have fewer parameters and lower computational cost, but the approximation may be less accurate, potentially leading to a significant degradation in the model's performance.
  1. Fine-tuning: After the low-rank approximation is applied to the target layers, the model is fine-tuned to adapt to the changes and recover any potential loss of performance. This can be done using standard training techniques, such as gradient descent, with a smaller learning rate to avoid large updates that could harm the performance.

To summarize, when using r=16, the low-rank approximation will have a higher rank, resulting in more parameters and higher computational cost compared to r=2. However, the approximation will likely be more accurate, preserving the model's performance better. In contrast, when using r=2, the model will be more compact, with fewer parameters and lower computational cost, but the performance may degrade due to the less accurate approximation. The optimal value of r depends on the specific model, dataset, and task, and it should be chosen based on a balance between maintaining performance and achieving efficiency gains.

What is a typical rank without Lora?

The original rank of the weight matrices in a Transformer model without LORA is equal to the dimensionality of those matrices. For the query and value projection matrices in the multi-head self-attention mechanism, their dimensions depend on the model architecture and the number of attention heads.

In a typical Transformer architecture, the input embeddings and the hidden states have a dimensionality of d_model, and the model has num_heads attention heads. Each attention head has a separate query and value projection matrix. The dimensions of the query and value projection matrices are d_model x (d_model // num_heads).

For example, if you have a Transformer model with d_model = 768 and num_heads = 12, the dimensions of the query and value projection matrices would be 768 x 64. In this case, the original rank of the weight matrices is 64 (equal to d_model // num_heads).

When applying LORA, the original rank is replaced with a lower rank (r) to achieve the low-rank approximation, which reduces the number of parameters and the computational cost. The choice of r depends on the desired trade-off between model efficiency and performance preservation.

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