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Why Have Decoder-Only Architectures Become the Standard in Large Language Models?

By Yangming Li

15 min read Approx. 2500 words Estimated 3750 tokens
Keywords: LLM, Decoder-Only Architecture, BERT, GPT, Transformer, NLP, Machine Learning, Attention Mechanisms, Causal Attention, Self-Attention, Model Architecture

Background

Currently, most leading large language models (LLMs) are based on decoder-only architectures. In the past, encoder-only and encoder-decoder structures also held prominence, yet today, the trend has shifted toward decoder-only designs. This article explores the reasons behind this shift.

Core Architectural Designs

1. Encoder-Only Architecture (BERT-style)

Encoder-only models like BERT utilize bidirectional self-attention mechanisms to process input sequences. These models excel at understanding context from both directions but cannot generate text. Key characteristics include:

  • Bidirectional attention allows tokens to attend to both past and future context
  • Masked Language Modeling (MLM) as the primary pre-training objective
  • Optimal for classification, token-level tasks, and semantic understanding
  • Examples: BERT, RoBERTa, DeBERTa

2. Encoder-Decoder Architecture (Seq2Seq)

Models like T5 and BART implement both encoding and decoding components, making them suitable for transformation tasks. Technical features include:

  • Cross-attention mechanism between encoder and decoder
  • Encoder processes input bidirectionally
  • Decoder generates output autoregressively
  • Examples: T5, BART, mT5

3. Decoder-Only Architecture (GPT-style)

Modern LLMs predominantly use this architecture, characterized by:

  • Causal (unidirectional) self-attention mechanism
  • Next-token prediction as the primary training objective
  • Unified approach to both understanding and generation
  • Examples: GPT-3, LLaMA, Claude

Technical Advantages of Decoder-Only Architectures

1. Mathematical Properties of Causal Attention

Decoder-only architectures implement causal attention, which offers several mathematical advantages:

  • Forms a lower triangular attention matrix with non-zero diagonal elements
  • Guarantees full rank properties (det(A) ≠ 0)
  • Enables stable gradient flow during training
  • Maintains information preservation through each layer

2. Computational Efficiency

The architecture provides significant computational benefits:

  • O(n) complexity for inference vs O(n²) for bidirectional attention
  • Efficient KV-cache implementation for sequential processing
  • Reduced memory footprint during inference
  • Better parallelization capabilities during training

3. Advanced Context Processing

Technical aspects of context handling include:

  • Implicit in-context learning through attention patterns
  • Efficient gradient flow through causal attention layers
  • Enhanced ability to maintain long-range dependencies
  • Superior performance in few-shot and zero-shot scenarios

4. Training Dynamics

Unique training characteristics include:

  • More stable optimization landscape due to causal structure
  • Better scaling properties with model size
  • Improved gradient flow through deeper networks
  • More efficient parameter utilization

Conclusion

Given their advantages in training efficiency, engineering implementation, and theoretical properties, decoder-only architectures have become the mainstream choice for LLM design. Particularly in generative tasks, introducing bidirectional attention offers no substantial benefits, and encoder-decoder models only outperform in certain cases due to larger parameter counts. With equivalent parameter sizes and inference costs, the decoder-only architecture stands out as the optimal choice.

References

Zhihu.com