llama.cpp supports speculative decoding, a technique that can significantly accelerate token generation by predicting multiple tokens ahead of the main model.
[Speculative decoding](https://en.wikipedia.org/wiki/Transformer_(deep_learning)#Speculative_decoding) leverages the fact that computing n tokens in a batch (as in prompt processing) is more efficient than computing n sequentially (as in response generation). By generating draft tokens quickly and then verifying them with the target model in a single batch, this approach can achieve substantial speedups when the draft predictions are frequently correct.
The `llama-server` application supports several implementations of speculative decoding. An implementation with draft model can be mixed with an implementation without draft model.
A much smaller model (called the _draft model_) generates drafts.
A draft model is the most used approach in speculative decoding.
### n-gram Cache (`ngram-cache`)
An n-gram is a sequence of n tokens. The n-gram cache implementation maintains statistics about short n-gram sequences.
A draft is computed using probabilities derived from these statistics. External statistics can also be loaded from files for improved accuracy.
See:
-#5479, #6828, #6848
### n-gram Map (`ngram-simple`, `ngram-map-*`)
These implementations search the token history for patterns and use matching sequences as draft candidates.
They require no additional model but rely on patterns that have already appeared in the generated text.
An example to use this approach can be the rewriting of source code by a LLM.
#### n-gram Map (`ngram-simple`)
This implementation looks for the last n-gram in history that matches the current n-gram and creates a draft using the m tokens following the matched n-gram. It is the simplest self-speculative approach with minimal overhead.
This implementation looks for the current n-gram of size n (called the _key_) in the token history. If the key n-gram is followed by the same m tokens (called the _mgram_) multiple times, it creates a draft using these m tokens. This approach requires a minimum number of occurrences (argument `--spec-ngram-min-hits`, default is 1) before generating drafts.
This experimental implementation looks for the current n-gram of size n (called the _key_) in the token history. For each key, up to four _values_ (n-grams of size m, called _mgrams_) are tracked. An internal statistic counts the occurrences of each mgram after the key n-gram. If one mgram is significantly more frequent than the others, it is used as the draft.
The number of accepted tokens is stored for each used n-gram.
**Example:** Server options to be used if there are a lot of longer repetitions.
- Iterating over a block of text/code (e.g. in llama.vim)
- Reasoning models (when they have to repeat their thinking in the final answer)
- Summarization
Example Video:
- See #19164
### Differences between ngram-simple, ngram-map and ngram-mod
- ngram-simple looks for a previous matching n-gram and inserts the following m-gram.
- ngram-map-k looks for a previous matching n-gram and inserts the following m-gram but uses an internal hash-map of n-grams in the current context window.
- ngram-mod uses a hash pool which is shared across all server slots. The hash pool is a map from n-gram hash to the next token (not the next m-gram as in ngram-map).
