spec_decode.rst

.. _spec_decode:

Speculative decoding in vLLM
============================

.. warning::
    Please note that speculative decoding in vLLM is not yet optimized and does
    not usually yield inter-token latency reductions for all prompt datasets or sampling parameters. The work
    to optimize it is ongoing and can be followed in `this issue. <https://github.com/vllm-project/vllm/issues/4630>`_

This document shows how to use `Speculative Decoding <https://x.com/karpathy/status/1697318534555336961>`_ with vLLM.
Speculative decoding is a technique which improves inter-token latency in memory-bound LLM inference.

Speculating with a draft model
------------------------------

The following code configures vLLM in an offline mode to use speculative decoding with a draft model, speculating 5 tokens at a time.

.. code-block:: python

    from vllm import LLM, SamplingParams

    prompts = [
        "The future of AI is",
    ]
    sampling_params = SamplingParams(temperature=0.8, top_p=0.95)

    llm = LLM(
        model="facebook/opt-6.7b",
        tensor_parallel_size=1,
        speculative_model="facebook/opt-125m",
        num_speculative_tokens=5,
        use_v2_block_manager=True,
    )
    outputs = llm.generate(prompts, sampling_params)

    for output in outputs:
        prompt = output.prompt
        generated_text = output.outputs[0].text
        print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")

To perform the same with an online mode launch the server:

.. code-block:: bash

    python -m vllm.entrypoints.openai.api_server --host 0.0.0.0 --port 8000 --model facebook/opt-6.7b \
    --seed 42 -tp 1 --speculative_model facebook/opt-125m --use-v2-block-manager \
    --num_speculative_tokens 5 --gpu_memory_utilization 0.8

 Then use a client:

.. code-block:: python

    from openai import OpenAI

    # Modify OpenAI's API key and API base to use vLLM's API server.
    openai_api_key = "EMPTY"
    openai_api_base = "http://localhost:8000/v1"

    client = OpenAI(
        # defaults to os.environ.get("OPENAI_API_KEY")
        api_key=openai_api_key,
        base_url=openai_api_base,
    )

    models = client.models.list()
    model = models.data[0].id

    # Completion API
    stream = False
    completion = client.completions.create(
        model=model,
        prompt="The future of AI is",
        echo=False,
        n=1,
        stream=stream,
    )

    print("Completion results:")
    if stream:
        for c in completion:
            print(c)
    else:
        print(completion)

Speculating by matching n-grams in the prompt
---------------------------------------------

The following code configures vLLM to use speculative decoding where proposals are generated by
matching n-grams in the prompt. For more information read `this thread. <https://x.com/joao_gante/status/1747322413006643259>`_

.. code-block:: python

    from vllm import LLM, SamplingParams

    prompts = [
        "The future of AI is",
    ]
    sampling_params = SamplingParams(temperature=0.8, top_p=0.95)

    llm = LLM(
        model="facebook/opt-6.7b",
        tensor_parallel_size=1,
        speculative_model="[ngram]",
        num_speculative_tokens=5,
        ngram_prompt_lookup_max=4,
        use_v2_block_manager=True,
    )
    outputs = llm.generate(prompts, sampling_params)

    for output in outputs:
        prompt = output.prompt
        generated_text = output.outputs[0].text
        print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")

Speculating using MLP speculators
---------------------------------

The following code configures vLLM to use speculative decoding where proposals are generated by
draft models that conditioning draft predictions on both context vectors and sampled tokens.
For more information see `this blog <https://pytorch.org/blog/hitchhikers-guide-speculative-decoding/>`_ or
`this technical report <https://arxiv.org/abs/2404.19124>`_.

.. code-block:: python

    from vllm import LLM, SamplingParams

    prompts = [
        "The future of AI is",
    ]
    sampling_params = SamplingParams(temperature=0.8, top_p=0.95)

    llm = LLM(
        model="meta-llama/Meta-Llama-3.1-70B-Instruct",
        tensor_parallel_size=4,
        speculative_model="ibm-fms/llama3-70b-accelerator",
        speculative_draft_tensor_parallel_size=1,
        use_v2_block_manager=True,
    )
    outputs = llm.generate(prompts, sampling_params)

    for output in outputs:
        prompt = output.prompt
        generated_text = output.outputs[0].text
        print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")

Note that these speculative models currently need to be run without tensor parallelism, although
it is possible to run the main model using tensor parallelism (see example above). Since the
speculative models are relatively small, we still see significant speedups. However, this
limitation will be fixed in a future release.

A variety of speculative models of this type are available on HF hub:

* `llama-13b-accelerator <https://huggingface.co/ibm-fms/llama-13b-accelerator>`_
* `llama3-8b-accelerator <https://huggingface.co/ibm-fms/llama3-8b-accelerator>`_
* `codellama-34b-accelerator <https://huggingface.co/ibm-fms/codellama-34b-accelerator>`_
* `llama2-70b-accelerator <https://huggingface.co/ibm-fms/llama2-70b-accelerator>`_
* `llama3-70b-accelerator <https://huggingface.co/ibm-fms/llama3-70b-accelerator>`_
* `granite-3b-code-instruct-accelerator <https://huggingface.co/ibm-granite/granite-3b-code-instruct-accelerator>`_
* `granite-8b-code-instruct-accelerator <https://huggingface.co/ibm-granite/granite-8b-code-instruct-accelerator>`_
* `granite-7b-instruct-accelerator <https://huggingface.co/ibm-granite/granite-7b-instruct-accelerator>`_
* `granite-20b-code-instruct-accelerator <https://huggingface.co/ibm-granite/granite-20b-code-instruct-accelerator>`_


Resources for vLLM contributors
-------------------------------
* `A Hacker's Guide to Speculative Decoding in vLLM <https://www.youtube.com/watch?v=9wNAgpX6z_4>`_
* `What is Lookahead Scheduling in vLLM? <https://docs.google.com/document/d/1Z9TvqzzBPnh5WHcRwjvK2UEeFeq5zMZb5mFE8jR0HCs/edit#heading=h.1fjfb0donq5a>`_
* `Information on batch expansion <https://docs.google.com/document/d/1T-JaS2T1NRfdP51qzqpyakoCXxSXTtORppiwaj5asxA/edit#heading=h.kk7dq05lc6q8>`_
* `Dynamic speculative decoding <https://github.com/vllm-project/vllm/issues/4565>`_