Mixture-of-Depths: Dynamic Compute Allocation in Transformers

These April 4, 2024 Google Deepmind paper introduces the **Mixture-of-Depths (MoD)** transformer architecture, a method that improves efficiency by learning to dynamically allocate compute to only the necessary tokens within a sequence. This is achieved by setting a static capacity, C (or k), which **limits the total number of tokens** that can participate in the expensive self-attention and Multi-Layer Perceptron (MLP) computations at any given layer. The sources explain that this capacity limitation is key to compute reduction, citing that if capacity is halved, the self-attention operation becomes only **25% as intensive** due to the squared relationship of the tokens involved. Beyond compute savings, the constraint forces the network to **learn which tokens matter**, which, in turn, allows MoD models to match or exceed the performance of baseline transformers while using fewer FLOPs per forward pass. Crucially, the MoD method uses an expert-choice routing scheme and a defined capacity to ensure a **static computation graph**, which is vital for maintaining high hardware efficiency during training and inference, and also anticipates potential reductions in Key-Value (KV) cache memory.Source:https://arxiv.org/pdf/2404.02258

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