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Draft-Target Speculative Decoding
Breaking the Autoregressive Speed Barrier
Autoregressive decoding generates one token per forward pass. Because modern GPUs have massive parallel compute, running a single token through a 70B parameter model severely underutilizes the GPU's execution units.
The Two-Phase Speculative Mechanism
- Speculative Drafting: A small, ultra-fast Draft Model (e.g. Llama 3.2 1B), a multi-head prediction head (Medusa / Hydra), or a feature-level recurrent draft engine (EAGLE-3) generates (K) candidate tokens in a tree structure.
- Target Verification: The large Target Model (e.g. Llama 3.3 70B) evaluates all (K+1) candidate tokens in a single parallel forward pass using tree-attention masks.
- Acceptance Criterion: Using modified rejection sampling, the harness accepts tokens along the tree path up to the first disagreement. Accepted tokens are emitted simultaneously, yielding a 2.5x to 3.8x wall-clock speedup with mathematically identical output distribution!
Next-Gen Speculative Architectures: EAGLE-3 & Medusa
| Architecture | Mechanism | Draft Overhead | Acceptance Rate |
|---|---|---|---|
| EAGLE-3 | Feature-level recurrence; drafts at the second-to-last hidden layer | ~10% VRAM of standard draft model | Up to 82% token acceptance |
| Medusa Heads | Multi-head linear decoders atop target model; zero auxiliary draft model | 0 auxiliary model parameters (heads only) | 2.2x – 2.8x speedup |
| Independent Draft | Separate small model (e.g., Llama 1B drafting for 70B) | Requires dedicated GPU memory for draft KV cache | 65% – 75% acceptance |
⌨ HANDS-ON LABConfigure Speculative Decoding Engine
⭐ +250 XPPair a 1B draft model with a 70B target model and measure candidate acceptance rate.
1Initialize dual-model speculative decoding harness.
2Run benchmark request and compute acceptance rate.
OBJECTIVE 1 / 2 — type "hint" if stuck
SYNAPSE VERIFICATION
QUERY 1 // 1
Why does speculative decoding produce the exact same mathematical probability distribution as standard sampling?
Because rejection sampling adjusts candidate logits so the joint probability strictly equals the target model's distribution
Because it uses quantum computing
Because the draft model is an exact clone of the target model
It does not; speculative decoding is always lossy