Prioritized Generative Replay

Authors: Renhao Wang, Kevin Frans, Pieter Abbeel, Sergey Levine, Alexei A Efros
Primary area: reinforcement learning
Keywords: online learning, model-based reinforcement learning, generative modeling, synthetic data, continual learning

Abstract:
Sample-efficient online reinforcement learning often uses replay buffers to store experience for reuse when updating the value function. 
However, uniform replay is inefficient, since certain classes of transitions can be more relevant to learning. While prioritization of more useful samples is helpful, this strategy can also lead to overfitting, as useful samples are likely to be more rare. 
In this work, we instead propose a prioritized, parametric version of an agent's memory, using generative models to capture online experience. This paradigm enables (1) densification of past experience, with new generations that benefit from the generative model's generalization capacity and (2) guidance via a family of ``relevance functions'' that push these generations towards more useful parts of an agent's acquired history. We show this recipe can be instantiated using conditional diffusion models and simple relevance functions such as curiosity- or value-based metrics. 
Our approach consistently improves performance and sample efficiency in both state- and pixel-based domains. We expose the mechanisms underlying these gains, showing how guidance promotes diversity in our generated transitions and reduces overfitting. We also showcase how our approach can train policies with even higher update-to-data ratios than before, opening up avenues to better scale online RL agents.

Decision: accept (oral)
Reviewer scores: 8, 8, 6, 8

Reviews and discussion:
<thread>
creation date:
1730403208342

thread_type:
official_review

<thread_object>
creator: reviewer_8yrp

<content>
summary:
This paper introduces a framework called Prioritized Generative Replay (PGR), a novel approach to enhance sample efficiency in online reinforcement learning (RL). Traditionally, replay buffers store experienced transitions and replay them uniformly or with prioritization based on metrics like TD-error. However, the authors point out that uniform replay can be inefficient, and prioritization can lead to overfitting. PGR addresses these issues by using a conditional generative model to create a parametric replay buffer. 

The paper claims that this allows for two key advantages:
1) Densification: The generative model can create new, plausible transitions beyond those directly experienced, enriching the training data, especially in sparsely explored regions.
2) Guidance: By conditioning the generative model on "relevance functions," the generated transitions can be steered towards areas more critical for learning, such as states with high novelty or uncertainty.

The authors also explore various relevance functions, including return, TD-error, and curiosity. They find that curiosity, based on the prediction error of a learned dynamics model, performs best. This is attributed to its ability to promote diversity in the generated transitions, thus reducing overfitting. They also show that their approach consistently improves performance and sample efficiency in both state- and pixel based domains.

strengths:
1) PGR offers a fresh perspective on replay buffers by combining generative modeling with guided replay. Framing this problem as a conditional generation problem with diffusion models is novel.
2) Diffusion model typically uses one single set of HPs requires no additional tuning I'd assume. This works well for PGR
3) Empirical results on various benchmarks demonstrate that PGR consistently outperforms existing model-free and model-based RL algorithms, as well as a generative replay baseline without guidance. Also has been shown to work in both state-based and pixel-based environments. 
4) PGR is shown to scale well with larger policy networks and higher synthetic-to-real data ratios (important ablation that I wanted to see), potentially enabling more data-efficient training of large-scale RL agents. Really important result for scaling to many real use cases.
5) The authors also provide insights into why PGR works, particularly highlighting the role of curiosity in promoting diversity and reducing overfitting.

weaknesses:
1) The curiosity-based relevance function relies on a learned dynamics model, which might be challenging to train accurately in complex environments.
2) Increasing Synthetic Data ratio does not benefit PGR and the unconditional baseline (SynthER) equally. PGR scales better at r=0.75 than SYNTHER but neither benefits from 0.875. We would think the trend would be consistent? whats the intution behind this? Also this figure 7 could be improved with the variation in r being shown
3) (Minor) writing issues throughout the paper with some missing words etc. Please re-read the paper and make the necessary changes.

questions:
1) How robust is PGR to errors in the learned dynamics model? Are there ways to mitigate the impact of inaccurate dynamics predictions on the curiosity-based relevance function?
2) Could PGR be extended to offline RL settings? If so, what modifications would be necessary?
3) How does PGR's performance compare against PER baselines which use approximate parametric models of prior experience?
4) Are there any other relevance functions thats been tried out? As thats core to the working of PGR.
</content>
</thread_object>

</thread>
<thread>
creation date:
1730689988449

thread_type:
official_review

<thread_object>
creator: reviewer_ir9q

<content>
summary:
The paper proposes to use conditional diffusion models to improve the experience replay for an RL learning agent. The method proposed improves performance by improving the diversity of the samples in the experience replay buffer and reducing overfitting.

strengths:
1. The paper is well written and provides a clear explanation of their method.
2. The research problem addressed in the paper is well laid out and is an important one to improve the performance of RL methods.

weaknesses:
1. While the method shows improved performance, it is a bit simple as it combines existing elements in diffusion models and RL to propose the solution.
2. It would be useful to compare the effect of different kinds of exploration bonuses.

questions:
1. Is the method compatible with different kinds of exploration bonuses? If so, how do you think they would compare?
2. How do you think the method would do when simply having diverse samples does not imply usefulness? An example is the noisy tv problem.
3. How sensitive is the algo towards the frequency of the inner loop in Algo 1?
4. Can multiple relevance functions be combined?
</content>
</thread_object>

</thread>
<thread>
creation date:
1730694412680

thread_type:
official_review

<thread_object>
creator: reviewer_yyu8

<content>
summary:
This paper proposes a conditional diffusion model as a synthetic replay buffer to combat early overfitting in online reinforcement learning and to diversify experiences for the learning model. This is achieved with a relevance function that selects samples that are rare but important for learning based on the temporal difference error, the value of a learned Q-function and a modified intrinsic curiosity module.

strengths:
One of the strength of this paper are the clear and concise language as well as good structured presentation of the proposed method.
It is quite logical to improve on the already existing prioritized experience replay method and implement it in the generative domain. The method is explained well and should be quite easily reproducable.
Overall the research could be a valuable contribution to the reinforcement learning community.

weaknesses:
A topic i feel like missed somewhat are the different ways to approach generative replay such as mentions of other generative models (e.g. variational auto encoders, gaussian mixture models) and why they were not used.
One thing i found rather off putting and this is very nitpicky is that the Tables 1, 2 and 3 are a bit crammed and slightly off from each other.

questions:
What exploration method does the agent use?
Could the exploration method be improved instead of the sample generation to improve diversity of samples?
Would a combination of both a better exploration and this method be the optimal and a possible solution?
</content>
</thread_object>

</thread>
<thread>
creation date:
1730845947261

thread_type:
official_review

<thread_object>
creator: reviewer_1k48

<content>
summary:
The work proposes a form of sample based experience replays that leverages a generative model to provide and augment samples drawn from the replay buffer. To avoid overfitting, a set of guidance functions are used to steer the generative process toward diverse and useful samples. The generative replay mechanism is a diffusion model that is conditioned on some auxiliary information. The authors propose a few different versions of this conditioning such as intrinsic curiosity, TD error, or Q values. The idea is that using these scores, the generative model can be steered towards generating high quality samples. Given such a replay mechanism, this work evaluates model free and model-based RL agents trained via this generative replay on gym and dmc.The results show improvement on both pixel based and state based tasks. There are also ablations with larger policy networks and higher generative data rations, which show further improvements.

-------------------------------------------------
I thank the authors for a substantive rebuttal that addressed my and (as far as I can tell) other concerns. I therefore raise my score to an 8.

strengths:
* This work proposes a scalable method for training model-free or model-based agents in a variety of domains. I believe the formulation is simple enough to be integrated into and improve other approaches. 

* I also found the presentation clear and easy to read. 

* I found the scaling experiments to be very compelling, I'm a little concerned about the general thrust of driving up the syn-real data ratio as high as possible, since we do need to ground the generations in real experience. But I still think insights here are valuable.

weaknesses:
I have two points of contention with this work. 
1. From a paradigm perspective, I don't understand how this is different from prior work in model-based RL that apples intrinsic rewards to a learned dynamics model [1] or world-model [2]. These methods also utilize a generative model as a copy of the environment, then train the agent in simulation to acquire interesting data (under the intrinsic reward). It seems that this method does the same, except that instances, rather than full trajectories are generated. I do see how this is different than just applying an intrinsic bonus during training, since here the synthetic data has a chance to be more diverse. 

2. I thank the authors for providing numerous experiments, but I am not at all convinced that this method is robust to the choice of guiding function F. ICM is known to be susceptible to the noisy TV problem, where difficult-to-model environmental factors score arbitrarily high under ICM. The chosen tasks are too simple perceptually to see this problem. This in and of itself is not a problem, but it means that we need to search for another F that works for our task which is hard in practice. In the meantime, there are other intrinsic rewards that do not suffer from this pathology [3]. 



[1] Shyam, Pranav, Wojciech Jaśkowski, and Faustino Gomez. "Model-based active exploration." International conference on machine learning. PMLR, 2019.

[2] Hafner, Danijar, et al. "Dream to control: Learning behaviors by latent imagination." arXiv preprint arXiv:1912.01603 (2019).

[3] Savinov, Nikolay, et al. "Episodic curiosity through reachability." arXiv preprint arXiv:1810.02274 (2018).

questions:
I'll rephrase my above concerns as questions. 

1. How is this method novel with respect to prior work that uses intrinsic rewards on rollouts from a learned dynamics model? It seems like a very similar approach to acquiring data that scores well under a given guidance function F, where F can be ICM or another intrinsic reward. 

2. How does this method handle noisy-tvs?
</content>
</thread_object>

</thread>
<thread>
creation date:
1731997578503

thread_type:
official_comment

<thread_object>
creator: authors

<content>
title:
General Response (1/2)

comment:
We thank the reviewers for their helpful comments on our work. We are glad to see the reviewers believe our problem setting is “an important one to improve the performance of RL methods” (reviewer ir9Q) and that our work in this setting is a “valuable contribution to the RL community” (reviewer Yyu8). Thank you also for finding our approach to be "simple" (reviewers 1k48, Yyu8), "reproducible" (reviewer Yyu8), "novel" (reviewer 8yrp) and "worth integrating with other approaches" (reviewer 1k48). We are especially grateful that the reviewers recognized the importance of our scaling experiments for “enabling more data-efficient training of large-scale RL agents” (reviewer 8yrp) and the resulting insights to be "compelling and valuable" (reviewer 1k48).

We now turn to two common questions amongst the reviewers, before addressing specific comments in individual responses below.

### 1. Choice of Relevance Function $\mathcal{F}$ and the Noisy-TV Problem (Reviewers 1k48, ir9Q, 8yrp)

**Our framework is compatible with a wide variety of relevance functions, such as RND [1] or pseudo-count [2], which have all been demonstrated in prior work to be robust to the noisy-TV problem.**

We have added to the appendix additional results on our pixel-based DMC-100k benchmark, which demonstrate that PGR conditioned on more robust curiosity-based metrics continue to enjoy strong performance. In particular, we look at intrinsic rewards obtained via Random Network Distillation (RND) [1] and pseudo-counts from a context tree switching (CTS) density model [2]. For training details we refer reviewers to Appendix A.1. We reproduce the results here:

|                 | Walker-Walk            | Cheetah-Run            |
|-----------------|------------------------|------------------------|
| DrQ-v2          | 514.11 $\pm$ 81.42     | 489.30 $\pm$ 69.26     |
| SynthER         | 468.53 $\pm$ 28.65     | 465.09 $\pm$ 28.27     |
| PGR (RND)       | **602.10 $\pm$ 43.44** | 512.58 $\pm$ 23.81     |
| PGR (CTS)       | 540.78 $\pm$ 88.27     | 508.17 $\pm$ 74.03     |
| PGR (Curiosity) | 570.99 $\pm$ 41.44     | **529.70 $\pm$ 27.76** |

**But what about more complex environments which actually have noisy TVs? We investigate the setting described in Savinov et al. [3], and simultaneously demonstrate that PGR is also compatible with relevance functions defined via (online) episodic curiosity (ECO).**

In particular, we use the “noise”-randomized versions of the DMLab environment in Savinov et al. [3]. This environment features a maze navigation task, where visual inputs constantly feature uniform random RGB noise in the lower right portion of the image (i.e. a noisy TV). For a visualization of the environment and training details, we refer reviewers to Appendix A.2, and also to Tables S12 and S13 in Section S6 of Savinov et al. [3]. We reproduce the results here:

|                 | Sparse             | Very Sparse        |
|-----------------|--------------------|--------------------|
| PPO             | 7.3 $\pm$ 2.7      | 4.1 $\pm$ 2.4      |
| PPO + Curiosity | 5.6 $\pm$ 1.8      | 2.8 $\pm$ 1.0      |
| PPO + RND       | 8.2 $\pm$ 1.9      | 4.0 $\pm$ 1.1      |
| PPO + ECO       | 16.3 $\pm$ 3.6     | 12.9 $\pm$ 1.9     |
| PGR (Curiosity) | 9.3 $\pm$ 2.0      | 5.7 $\pm$ 2.2      |
| PGR (RND)       | 11.2 $\pm$ 1.0     | 8.0 $\pm$ 1.8      |
| PGR (ECO)       | **21.9 $\pm$ 2.6** | **18.7 $\pm$ 2.1** |

We see that PGR can flexibly condition on relevance functions like episodic curiosity to restore its superior performance in environments with highly stochastic transitions.

**But more crucially, generative replay with the “right” relevance function $\mathcal{F}$ is always better than without.**

Our contribution is not related to effective intrinsic exploration in stochastic environments, or even generally robust conditioning choices for generative replay. We simply seek to make the case that online generative replay is a compelling paradigm for sample-efficient online RL. We rely on the insight that prioritized replay can be elegantly connected to, and easily instantiated via, conditional generation in generative replay. Finally, our mechanistic explanations do not rely on showing that the environment is better explored, which is a common underlying narrative in intrinsic motivation literature and the noisy-TV problem. We only demonstrate the possibility that PGR improves learning dynamics and reduces the prolific problem of early overfitting in Q-learning.
</content>
</thread_object>

</thread>


Metareview:
metareview:
This paper uses generative models for samples for experience replay in RL. The paper uses conditional diffusion models, and explores different relevance functions, showing empirical improvements across experiments in online RL. 

Reviewers agree that this paper is well-written, and the research problem is important and well-addressed. The method is novel and the results are strong, as well as the insights as to why the method works. 

Reviewers had numerous concerns about the relevance function (eg robustness), which the authors addressed convincingly in their rebuttal (and reviewers have increased scores as a result). All reviewers agree that this work is above the acceptance threshold, with most agreeing it is a clear accept. Overall, this is a strong paper which will be a good addition to ICLR.