#
Scaling Test-Time Compute for Longer Thinking in LLMs

Authored by: Sergio Paniego

🚨 WARNING: This notebook is resource-intensive and requires substantial computational power. If you’re running this in Colab, it will utilize an A100 GPU.

In this recipe, we’ll guide you through extending the inference time for an Instruct LLM system using test-time compute to solve more challenging problems, such as complex math problems. This approach, inspired by OpenAI o1-o3 models, demonstrates that longer reasoning time during inference can enhance model performance.

This technique builds on experiments shared in this blog post, which show that smaller models, like the 1B and 3B Llama Instruct models, can outperform much larger ones on the MATH-500 benchmark when given enough “time to think”. Recent research from DeepMind suggests that test-time compute can be scaled optimally through strategies like iterative self-refinement or using a reward model.

The blog introduces a new repository for running these experiments. In this recipe, we’ll focus on building a small chatbot that engages in longer reasoning to tackle harder problems using small open models.

##
1. Install Dependencies

Let’s start by installing the search-and-learn repository! 🚀
This repo is designed to replicate the experimental results and is not a Python pip package. However, we can still use it to generate our system. To do so, we’ll need to install it from source with the following steps:

    !git clone https://github.com/huggingface/search-and-learn

    %cd search-and-learn
    !pip install -e '.[dev]'

Log in to Hugging Face to access meta-llama/Llama-3.2-1B-Instruct, as it is a gated model! 🗝️
If you haven’t previously requested access, you’ll need to submit a request before proceeding.

    from huggingface_hub import notebook_login

    notebook_login()

##
2. Setup the Large Language Model (LLM) and the Process Reward Model (PRM) 💬

As illustrated in the diagram, the system consists of an LLM that generates intermediate answers based on user input, a PRM model that evaluates and scores these answers, and a search strategy that uses the PRM feedback to guide the subsequent steps in the search process until reaching the final answer.

Let’s begin by initializing each model. For the LLM, we’ll use the meta-llama/Llama-3.2-1B-Instruct model, and for the PRM, we’ll use the RLHFlow/Llama3.1-8B-PRM-Deepseek-Data model.

    import torch
    from vllm import LLM
    from sal.models.reward_models import RLHFFlow

    model_path="meta-llama/Llama-3.2-1B-Instruct"
    prm_path="RLHFlow/Llama3.1-8B-PRM-Deepseek-Data"

    llm = LLM(
        model=model_path,
        gpu_memory_utilization=0.5,  # Utilize 50% of GPU memory
        enable_prefix_caching=True,  # Optimize repeated prefix computations
        seed=42,                     # Set seed for reproducibility
    )

    prm = RLHFFlow(prm_path)

###
2.1 Instantiate the Question, Search Strategy, and Call the Pipeline

Now that we’ve set up the LLM and PRM, let’s proceed by defining the question, selecting a search strategy to retrieve relevant information, and calling the pipeline to process the question through the models.

1. Instantiate the Question: In this step, we define the input question that the system will answer, considering the given context.

2. Search Strategy: The system currently supports the following search strategies: `best_of_n`, `beam_search`, and `dvts` (see diagram). For this example, we’ll use `best_of_n`, but you can easily switch to any of the other strategies based on your needs. We need to define some configuration parameters for the configuration of the search strategy. You can check the full list here.

3. Call the Pipeline: With the question and search strategy in place, we’ll call the inference pipeline, processing the inputs through both the LLM and PRM to generate the final answer.

The first step is to clearly define the question that the system will answer. This ensures that we have a precise task for the model to tackle.

    question_text = 'Convert the point $(0,3)$ in rectangular coordinates to polar coordinates.  Enter your answer in the form $(r,\theta),$ where $r > 0$ and $0 \le \theta < 2 \pi.$'
    input_batch = {"problem": [question_text]}

Next, we define the configuration, including parameters like the number of candidate answers `(N)`, and choose the search strategy that will be used. The search strategy dictates how we explore the potential answers. In this case, we’ll use `best_of_n`.

With the question and configuration in place, we use the selected search strategy to generate multiple candidate answers. These candidates are evaluated based on their relevance and quality and the final answer is returned.

    from sal.config import Config
    from sal.search import beam_search, best_of_n, dvts

    config = Config()
    config.n=32 # Number of answers to generate during the search

    search_result = best_of_n(x=input_batch, config=config, llm=llm, prm=prm)

###
2.2 Display the Final Result

Once the pipeline has processed the question through the LLM and PRM, we can display the final result. This result will be the model’s output after considering the intermediate answers and scoring them using the PRM.

Here’s how to display the final answer:

    search_result['pred'][0]

The model’s output might include special tokens, such as `<|start_header_id|>` or `<|end_header_id|>`. To make the answer more readable, we can safely remove them before displaying it to the end user.

    formatted_output = search_result['pred'][0].replace("<|start_header_id|>assistant<|end_header_id|>\n\n", "").strip()
    formatted_output

After removing any special tokens, we can display the final answer to the user. Since the answer is based on markdown, it can be rendered properly by displaying it as markdown.

    from IPython.display import display, Markdown

    display(Markdown(formatted_output))

##
3. Assembling It All! 🧑‍🏭️

Now, let’s create a method that encapsulates the entire pipeline. This will allow us to easily reuse the process in future applications, making it efficient and modular.

By combining the LLM, PRM, search strategy, and result display, we can simplify the workflow and ensure that it’s reusable for other tasks or questions.

We simplify the workflow, ensuring that it’s reusable for different tasks or questions. Additionally, we’ll track the time spent on each method so that we can understand the practical implications of using each strategy and configuration.

Here’s how we can structure the method:

    import time

    def generate_with_search_and_learn(question, config, llm, prm, method='best_of_n'):
        """
        Generate an answer for a given question using the search-and-learn pipeline.

        Args:
        - question (str): The input question to generate an answer for.
        - config (Config): Configuration object containing parameters for search strategy.
        - llm (LLM): Pretrained large language model used for generating answers.
        - prm (RLHFFlow): Process reward model used for evaluating answers.
        - method (str): Search strategy to use. Options are 'best_of_n', 'beam_search', 'dvts'. Default is 'best_of_n'.

        Returns:
        - str: The formatted output after processing the question.
        """
        batch = {"problem": [question]}

        start_time = time.time()
        if method == 'best_of_n':
          result = best_of_n(x=batch, config=config, llm=llm, prm=prm)
        elif method == 'beam_search':
          result = beam_search(examples=batch, config=config, llm=llm, prm=prm)
        elif method == 'dvts':
          result = dvts(examples=batch, config=config, llm=llm, prm=prm)

        elapsed_time = time.time() - start_time
        print(f"\nFinished in {elapsed_time:.2f} seconds\n")

        tokenizer = llm.get_tokenizer()
        total_tokens = 0
        for completion in result['completions']:
            for comp in  completion:
                output_tokens = tokenizer.encode(comp)
                total_tokens += len(output_tokens)

        print(f"Total tokens in all completions: {total_tokens}")

        formatted_output = result['pred'][0].replace("<|start_header_id|>assistant<|end_header_id|>\n\n", "").strip()
        return formatted_output

###
⏳ 3.1 Comparing Thinking Time for Each Strategy

Let’s compare the thinking time of three methods: `best_of_n`, `beam_search`, and `dvts`. Each method is evaluated using the same number of answers during the search process, measuring the time spent thinking in seconds and the number of generated tokens.

In the results below, the `best_of_n` method shows the least thinking time, while the `dvts` method takes the most time. However, `best_of_n` generates more tokens due to its simpler search strategy.

Method | Number of Answers During Search | Thinking Time (Seconds) | Generated Tokens
--- | --- | --- | ---
best_of_n | 8 | 3.54 | 3087
beam_search | 8 | 10.06 | 2049
dvts | 8 | 8.46 | 2544

This comparison illustrates the trade-offs between the strategies, balancing time spent thinking and the complexity of the search process.

####
1. Best of n

We’ll begin by using the `best_of_n` strategy. Here’s how to track the thinking time for this method:

    >>> question = 'Convert the point $(0,3)$ in rectangular coordinates to polar coordinates.  Enter your answer in the form $(r,\theta),$ where $r > 0$ and $0 \le \theta < 2 \pi.$'

    >>> config.n=8

    >>> formatted_output = generate_with_search_and_learn(question=question, config=config, llm=llm, prm=prm, method='best_of_n')

    Finished in 3.54 seconds

    Total tokens in all completions: 3087

    display(Markdown(formatted_output))

####
2. Beam Search

Now, let’s try using the `beam_search` strategy.

    >>> config.n=8
    >>> # beam search specific
    >>> config.sort_completed=True
    >>> config.filter_duplicates=True

    >>> formatted_output = generate_with_search_and_learn(question=question, config=config, llm=llm, prm=prm, method='beam_search')

    Finished in 10.06 seconds

    Total tokens in all completions: 2049

    display(Markdown(formatted_output))

####
3. Diverse Verifier Tree Search (DVTS)

Finally, let’s try the `dvts` strategy.

    >>> config.n=8
    >>> # dvts specific
    >>> config.n_beams = config.n // config.beam_width

    >>> formatted_output = generate_with_search_and_learn(question=question, config=config, llm=llm, prm=prm, method='dvts')

    Finished in 8.46 seconds

    Total tokens in all completions: 2544

    display(Markdown(formatted_output))

###
🙋 3.2 Testing the System with a Simple Question

In this final example, we’ll test the system using a straightforward question to observe how it performs in simpler cases. This allows us to verify that the system works as expected even for basic queries.

Let’s try the following question:

    >>> question = 'What\'s the capital of Spain?'

    >>> config.n=32

    >>> formatted_output = generate_with_search_and_learn(question=question, config=config, llm=llm, prm=prm, method='best_of_n')

    Finished in 1.03 seconds

    Total tokens in all completions: 544

    display(Markdown(formatted_output))

Even though we set a larger number of candidate answers (`N`), the time spent thinking remains relatively small (1.03 seconds and 544 generated tokens). This demonstrates the system’s ability to efficiently handle easier problems, spending less time on them, while leveraging its enhanced capabilities for more complex questions.

🏆 We now have a fully operational pipeline that leverages test-time compute, enabling the system to “think longer” for more complicated queries, while also maintaining fast response times for straightforward questions.

This approach ensures the system can scale its thinking time based on the task’s complexity, offering an efficient and responsive solution for both simple and challenging problems.

##
4. Continuing the Journey and Resources 🧑‍🎓️

If you’re eager to continue exploring, be sure to check out the original experimental blog and all the references mentioned within it. These resources will deepen your understanding of test-time compute, its benefits, and its applications in LLMs.

Happy learning and experimenting! 🚀

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