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Large language models (LLMs) are revolutionizing various domains. However, their power comes at a cost – they can be computationally expensive, especially during the inference stage, where the model applies its learned knowledge to new tasks. This is where techniques like grouped query attention (GQA, first introduced in the paper “GQA: Training Generalized Multi-Query Transformer Models from Multi-Head Checkpoints”) come in.
Before diving into grouped query attention, let's first revisit the fundamental concept of attention mechanisms in LLMs. At its core, attention allows models to selectively focus on different parts of the input sequence, enabling them to weigh the importance of each element when making predictions or generating outputs. In a typical attention mechanism, each query (e.g., a word or token representation) computes its attention scores concerning all keys (usually encoder representations), resulting in a weighted sum that captures the relevance of each key to the current query. While powerful, traditional attention mechanisms, like multi-head attention (MHA), can be computationally demanding. MHA uses multiple "heads" to attend to different aspects of the input, leading to increased memory usage and processing time.
MHA enables parallel processing, each process focusing on a different aspect of the input sequence by utilizing several "heads," each with its own set of projections for the query, key, and value elements. Each head can calculate attention scores simultaneously, leading to a richer and more comprehensive understanding of the input.
While powerful, MHA can be computationally expensive. The multiple calculations involved in each head and the need to store separate key-value pairs for each head can lead to slower inference times and higher memory usage.
Multi-query attention (MQA) offers a more streamlined way to handle attention. Here, a shared key and value pair are used for all the queries. Unlike MHA, MQA only works with the same key and value information. This reduces the number of calculations needed and potentially improves efficiency.
While MQA is faster, it might not capture as much detail from different aspects as MHA. The shared information source can limit performance.
GQA emerges as an innovative extension of traditional attention mechanisms, aiming to address several challenges associated with processing long sequences efficiently. It acts as a bridge between MHA and MQA.
The key idea behind GQA is to partition queries into distinct groups or clusters and compute attention within each group independently. To be more specific, GQA takes the multiple query heads used in MHA and groups them. Each group then shares a single set of key and value heads. This reduces the number of distinct key-value pairs the model needs to process, leading to significant efficiency gains while still capturing meaningful relationships within the data.
The number of groups created plays a crucial role. A higher number of groups leads to faster inference times but can potentially impact the quality of the model's output. Conversely, fewer groups provide better performance but sacrifice speed. Finding the optimal balance between group numbers and performance requires fine-tuning the model.
Overview and comparison of the Multi-head attention, Grouped-query attention, and Multi-query attention.
Several techniques underpin the implementation and effectiveness of grouped query attention:
In the GQA paper, the experiments show that GQA gains a favorable tradeoff compared to the MHA models by achieving higher quality than the similar speed MHA and faster inference than the extra-large MHA model with high performance.