Revolutionizing Transformers: DeepMind’s PEER Layer and the Power of a Million Experts

The feedforward (FFW) layers in standard transformer architectures experience a linear increase in computational costs and activation memory as the hidden layer width expands. To address this issue, sparse mixture-of-experts (MoE) architectures have emerged, effectively decoupling model size from computational cost. A recent discovery, the fine-grained MoE scaling law, shows that higher granularity leads to better performance. However, existing MoE models are limited by computational and optimization challenges, restricting the number of experts they can employ.

In a new paper Mixture of A Million Experts, a Google DeepMind research team introduces parameter efficient expert retrieval (PEER), a innovative layer design leverages the product key technique for sparse retrieval from an extensive pool of tiny experts (over a million). Its impressive performance-compute trade-off unlocks the potential for further scaling transformer models while maintaining computational efficiency.

The team highlights their main contributions as follows:

1. Exploration of Extreme MoE Setting: Departing from the conventional focus on a small number of large experts, this work investigates the under-explored scenario of numerous tiny experts.
2. Learned Index Structure for Routing: For the first time, the study demonstrates that a learned index structure (Kraska et al., 2018) can efficiently route to over a million experts.
3. New Layer Design: By combining product key routing with single-neuron experts, the PEER layer expands layer capacity without significant computational overheads. Empirical results show its superior efficiency compared to dense FFW, coarse-grained MoEs, and Product Key Memory (PKM) layers.
4. Comprehensive Ablation Studies: The researchers explore various design choices of PEER, such as the number of experts, active parameters, number of heads, and query batch normalization, focusing on their impact on language modeling tasks.

A PEER layer is formally defined as a function consisting of three components: a pool of experts, each sharing the same signature; a corresponding set of product keys; and a query network that maps the input vector to a query vector.

A PEER layer can be inserted into the middle of a transformer backbone or used to replace FFW layers. Given the state vector from the previous layer, a query network maps it to a query vector. This vector is then compared with the product keys to compute the router scores and retrieve the top experts. After the retrieved experts make their predictions, their outputs are linearly combined using softmax-normalized router scores as weights.

In their empirical study, the researchers conducted isoFLOP analysis on language modeling tasks, comparing PEER with various baselines. The results demonstrate that PEER layers outperform dense FFWs and coarse-grained MoEs in terms of performance-compute trade-off.

The paper Mixture of A Million Experts is on arXiv.

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Author: Hecate He | Editor: Chain Zhang