Learning from preference labels plays a crucial role in fine-tuning large language models. There are several distinct approaches for preference fine-tuning, including supervised learning, on-policy reinforcement learning (RL), and contrastive learning. Different methods come with different implementation tradeoffs and perfor- mance differences, and existing empirical findings present different conclusions, for instance, some results show that online RL is quite important to attain good fine-tuning results, while others find (offline) contrastive or even purely supervised methods sufficient. This raises a natural question: what kind of approaches are important for fine-tuning with preference data and why? In this paper, we answer this question by performing a rigorous analysis of a number of fine-tuning techniques on didactic and full-scale LLM problems. Our main finding is that, in general, approaches that use on-policy sampling or attempt to minimize the likelihood on certain responses (i.e., employ a negative gradient) outperform offline and maximum likelihood objectives. We conceptualize our insights and unify methods that use on-policy sampling or negative gradient under the notion of mode-seeking objectives, extended to categorical distributions. Mode-seeking objectives are able to alter probability mass on specific bins of a categorical distribution at a fast rate compared to maximum likelihood, allowing them to relocate masses across bins more effectively. Our analysis prescribes actionable insights for preference fine-tuning of LLMs and informs how data should be collected for maximal improvement.
We considered three problems for our experiments: (1) a didactic bandit problems that are easy to simulate, (2) Synthetic LLM fine-tuning problems, where ground-truth reward functions exist and are easy to track (though we still train the policy against a proxy reward model), and (3) Full-scale LLM fine-tuning on AlpacaFarm and UltraFeedback. We use these problems to study the impact of on-policy sampling and negative gradients on performance and behavior of algorithms.
The performance of preference-finetuning algorithms can be affected by several factors: (1) the coverage of the preference dataset, (2) the reference policy initialization, and (3) the underlying ground truth reward distribution. To study this impact, we consider two orthogonal axes: [C1] the geometric alignment between the ground-truth reward function and the reference policy initialization and [C2] the coverage of the preference data used to train the surrogate reward model relative to the reference policy. Condition [C1] is reminiscent of concentrability coefficient in RL and condition [C2] fundamentally imposes a condition on statistical errors in the reward model on different responses.
Understanding the behavior of various approaches as a function of these factors will allow us to better understand the performance of various approaches on downstream fine-tuning in terms of problem geometry [C1] and statistical learning considerations [C2]. More concretely, in the synthetic LLM setting we consider the following cases
We consider three different types of problem scenarios: Min Length, Mode Length, and Skew Length These scenarios are designed to test the performance of different fine-tuning algorithms under different conditions.
To study the impact of on-policy sampling, we vary the extent to which updates are made on data from the current policy. Below, we introduce Algorithm 1, which is a encapsulates how fine-tuning algorithms updates on preference data.
One way in which we can control the amount of on-policy sampling in Algorithm 1 is by by varying the total number of samples |𝒟| = 𝐵/𝐶 × 𝐶 = 𝐵 used for a given training iteration but keeping minibatch size 𝑀 used for the gradient update fixed, assuming the algorithm performs exactly one pass over all this sampled data. One result from the paper is shown below:
Performance on AlpacaFarm with varying batch sizes. We see that decreasing the batch size improves performance, which leads to more on-policy sampling leads to better performance on the AlpacaFarm domain. Empirically, we see benefits of this sampling when the reference policy is far from the optimal policy.
Takeaway: On-policy sampling generally improves performance and efficiency, especially in cases when the peak of reward appears farther from the reference policy, even when the reward model is learned from the same preference dataset that methods use without on-policy learning also use. In some cases, sample reuse can reduce the dependency on on-policy sampling of data, but it presents a tradeoff by reducing exploration of the response space.
To understand the role of negative gradient, we will compare contrastive algorithms (e.g., DPO or IPO) with maximum likelihood supervised methods (e.g., Best-of-N) in a fully offline setting, where no new on-policy samples are collected. In the paper, we compare various algorithms and also explore mechanisms behind the behaviors of methods. Our main finding is that negative gradients often speed up convergence of algorithms, often allowing them to reach a better solution. For example, in one of the settings, we compare supervised Best-of-N to supervised Best-of-N with a negative gradient explicitly added and observe that the latter is able to quickly learn to find a better solution. Likewise we also find DPO attains better results.
Negative gradients in AlpacaFarm (left) and UltraFeedback (right). For these domains, we consider the increase in average gold reward compared to the SFT model for different offline approaches. Algorithms with a negative gradient such as DPO outperform approaches such as Pref-FT, which do not utilize any negative gradient term in their objective.
DPO vs Pref-FT Reward Margin. The reward margin between the preferred and dispreferred responses for contrastive algorithms such as DPO is higher than maximum likelihood algorithms such as Pref-FT.
Takeaway: A negative gradient improves over offline supervised methods when the peak in the reward appears in less likely regions of the reference policy. It can increase the likelihood of the preferred response, when the dispreferred response is sufficiently different from the prefered response, model capacity is large, and the reference initialization is chosen appropriately. If not, the margin between log likelihoods of preferred and dispreferred will still be larger when a negative gradient is used, but the recovered probability mass will go into increasing likelihoods of other responses, not the preferred response necessarily.
On-policy sampling and offline negative gradients present complementary benefits, in that the best offline loss function with negative gradients can be used to train on on-policy data, improving over on-policy RL or supervised learning. Conceptually, while sampling responses on-policy provides coverage of the response space, an effective negative gradient loss provides a stronger learning signal given a set of samples. It can also result in computational benefits.
Complementary benefit of on-policy sampling and negative gradients on the synthetic LLM length experiments. Online DPO performs the best where optimal policy and reference policy lies far from each other (min length and skew length), and all algorithms perform similarly when these two policies are close (mode length).
Takeaway: We observe that the best offline loss function with negative gradients can be used to train on on-policy data, improving over on-policy RL or supervised learning. In our experiments, we find this loss function to be a contrastive loss akin to DPO or IPO. Running these contrastive methods on on-policy data helps improve performance. This can also result in computational benefits, improving over wall-clock time.
With empirical results showing the benefits of on-policy sampling and negative gradient for preference fine-tuning of LLMs, we tie these two approaches together under the notion of mode-seeking objectives in contrast to mode-covering objectives such as (weighted / filtered) maximum likelihood.
In the paper, we argue (in Lemmas 6.1 and 6.2) that (1) on-policy algorithms are mode-seeking as they optimize the regularized reverse-KL objective and (2) if the negative responses are chosen appropriately, then the contrastive update accelerates the rate of increase of probability mass on the preferred responses and exhibits mode-seeking behavior. For more details, formal definitions, and the proof of these theoretical results, please refer to the paper.
We then also analyze the benefits of mode-seeking behavior formally via a case study on reverse (mode-seeking) and forward KL (mode-covering) divergences. Prior work typically shows that the benefits of mode-seeking behavior of this sort are more apparent when the model 𝑝(𝑥) is unable to realize the target distribution 𝑞(𝑥), such that minimizing either KL would give rise to difference solutions. Unlike this prior argument, our theoretical argument in Theorem 6.5 shows that even when the 𝑝(𝑥) can fully represent the target distribution 𝑞(𝑥), when training with gradient descent, reverse KL is able to quickly re-distribute probability mass to only a subset of the required categories likely in target distribution within a few gradient update steps. This is especially important when early stopping is used and the loss cannot be minimized to exactly 0 on the training data -- in such cases, we would expect the reverse KL divergence to quickly redistribute probability mass in a way that is more effective. We illustrate a toy version of this idea using a numerical toy experiment as seen below.
Empirical Example contrasting mode-seeking (Reverse KL) and mode-covering (forward KL) objectives.
Takeaway: We conceptually unify on-policy sampling and negative gradients under the notion of mode-seeking objectives, extended to categorical distributions. An analysis of the behavior of some representative mode-seeking and mode-covering objectives corroborates our empirical observations in this paper.
@inproceedings{ tajwar2024preference, title={Preference Fine-Tuning of {LLM}s Should Leverage Suboptimal, On-Policy Data}, author={Fahim Tajwar and Anikait Singh and Archit Sharma and Rafael Rafailov and Jeff Schneider and Tengyang Xie and Stefano Ermon and Chelsea Finn and Aviral Kumar}, booktitle={Forty-first International Conference on Machine Learning}, year={2024}, url={https://openreview.net/forum?id=bWNPx6t0sF} }