On the Power of Pre-training for Generalization in RLProvable Benefits and Hardness

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arXiv:2210.10464v2 [cs.LG] 29 Jun 2023
On the Power of Pre-training for Generalization in RL:
Provable Benefits and Hardness
Haotian Ye * 1 Xiaoyu Chen * 2 Liwei Wang 2 3 Simon S. Du 4
Abstract
Generalization in Reinforcement Learning (RL)
aims to train an agent during training that gen-
eralizes to the target environment. In this work,
we first point out that RL generalization is fun-
damentally different from the generalization in
supervised learning, and fine-tuning on the tar-
get environment is necessary for good test per-
formance. Therefore, we seek to answer the fol-
lowing question: how much can we expect pre-
training over training environments to be help-
ful for efficient and effective fine-tuning? On
one hand, we give a surprising result showing
that asymptotically, the improvement from pre-
training is at most a constant factor. On the
other hand, we show that pre-training can be in-
deed helpful in the non-asymptotic regime by de-
signing a policy collection-elimination (PCE) al-
gorithm and proving a distribution-dependent re-
gret bound that is independent of the state-action
space. We hope our theoretical results can pro-
vide insight towards understanding pre-training
and generalization in RL.
1. Introduction
Reinforcement learning (RL) is concerned with sequen-
tial decision making problems in which the agent inter-
acts with the environment aiming to maximize its cu-
mulative reward. This framework has achieved tremen-
dous successes in various fields such as game play-
ing (Mnih et al.,2013;Silver et al.,2017;Vinyals et al.,
2019), resource management (Mao et al.,2016), recom-
*Equal contribution 1Yuanpei College, Peking University,
China 2National Key Laboratory of General Artificial Intelligence,
School of Intelligence Science and Technology, Peking Univer-
sity, China 3Center for Data Science, Peking University, China
4University of Washington, United States. Correspondence to:
Simon S. Du <ssdu@cs.washington.edu>, Liwei Wang <wan-
glw@pku.edu.cn>.
Proceedings of the 40 th International Conference on Machine
Learning, Honolulu, Hawaii, USA. PMLR 202, 2023. Copyright
2023 by the author(s).
mendation systems (Shani et al.,2005;Zheng et al.,2018)
and online advertising (Cai et al.,2017). However, many
empirical applications of RL algorithms are typically re-
stricted to the single environment setting. That is, the RL
policy is learned and evaluated in the exactly same envi-
ronment. This learning paradigm can lead to the issue of
overfitting in RL (Sutton,1995;Farebrother et al.,2018),
and may have degenerate performance when the agent is
deployed to an unseen (but similar) environment.
The ability to generalize to test environments is impor-
tant to the success of reinforcement learning algorithms,
especially in the real applications such as autonomous
driving (Shalev-Shwartz et al.,2016;Sallab et al.,2017),
robotics (Kober et al.,2013;Kormushev et al.,2013) and
health care (Yu et al.,2021). In these real-world tasks,
the environment can be dynamic, open-ended and always
changing. We hope the agent can learn meaningful skills
in the training stage and be robust to the variation dur-
ing the test stage. Furthermore, in applications such as
robotics where we have a simulator to efficiently and safely
generate unlimited data, we can firstly train the agent in
the randomized simulator models and then generalize it to
the real environment (Rusu et al.,2017;Peng et al.,2018;
Andrychowicz et al.,2020). An RL algorithm with good
generalization ability can greatly reduce the demand of real-
world data and improve test-time performance.
Generalization in supervised learning has been
widely studied for decades (Mitchell et al.,1986;
Bousquet & Elisseeff,2002;Kawaguchi et al.,2017).
For a typical supervised learning task such as classification,
given a hypothesis space Hand a loss function , the
agent aims to find an optimal solution in the average
manner. That is, we hope the solution is near-optimal
compared with the optimal hypothesis hin expectation
over the data distribution, which is formally defined as
h= arg minhH E(h(X), Y ). From this perspective,
generalization in RL is fundamentally different. Once
the agent is deployed in the test environment Msampled
from distribution D, it is expected to achieve comparable
performance with the optimal policy in M. In other words,
we hope the learned policy can perform near-optimal
compared with the optimal value V
Min instance for the
sampled test environment M.
1
On the Power of Pre-training for Generalization in RL: Provable Benefits and Hardness
Unfortunately, as discussed in many previous
works (Malik et al.,2021;Ghosh et al.,2021), the
instance-optimal solution in the target environment can
be statistically intractable without additional assumptions.
We formulate this intractability into a lower bound (Propo-
sition 3.2) to show that it is impractical to directly obtain a
near-optimal policy for the test environment Mwith high
probability. This motivates us to ask: in what settings can
the generalization problem in RL be tractable?
Targeting on RL generalization, the agent is often allowed
to further interact with the test environment to improve its
policy. For example, many previous results in robotics have
demonstrated that fine-tuning in the test environment can
greatly improve the test performance for sim-to-real trans-
fer (Rusu et al.,2017;James et al.,2019;Rajeswaran et al.,
2016). Therefore, one possible way to formulate general-
ization is to allow further interaction with the target en-
vironment during the test stage. Specifically, suppose the
agent interacts with MDP M ∼ Din the test stage, and we
measure the performance of the fine-tuning algorithm Aus-
ing the expected regret in Kepisodes, i.e. RegK(D,A) =
EM∼DhPK
k=1 Vπ(M)
MVπk
Mi. In this setting, can the
information obtained from pre-training 1help reduce the
regret suffered during the test stage?
In addition, when the test-time fine-tuning is not al-
lowed, what can we expect the pre-training to be help-
ful? As discussed above, we can no longer demand
instance-optimality in this setting, but can only step back
and pursue a near-optimal policy in expectation. Specifi-
cally, our goal is to perform near-optimal in terms of the
optimal policy with maximum value in expectation, i.e.
π(D) = arg maxπΠEM∼DVπ
M. Here Vπ
Mis the value
function of the policy πin MDP M. We seek to answer:
is it possible to design a sample-efficient training algo-
rithm that returns a ǫ-optimal policy πin expectation, i.e.
EM∼DhVπ(D)
MVπ
Miǫ?
Main contributions. In this paper, we theoretically study
RL generalization in the above two settings. We show that:
When fine-tuning is allowed, we study the benefit of
pre-training for the test-time performance. Since all
information we can gain from training is no more than
the distribution Ditself, we start with a somewhat sur-
prising theorem showing the limitation of this bene-
fit: there exists hard cases where, even if the agent
has exactly learned the environment distribution Din
the training stage, it cannot improve the test-time re-
gret up to a universal factor in the asymptotic setting
(K→ ∞). In other words, knowing the distribution
1We call the training stage “pre-training” when interactions
with the test environment are allowed.
Dcannot provide more information in consideration
of the regret asymptotically. Our theorem is proved
by using Radon transform and Lebesgue integral anal-
ysis to give a global level information limit, which we
believe are novel techniques for RL communities.
Inspired by this asymptotic hardness, we focus on the
non-asymptotic setting where Kis fixed, and study
whether and how much we can reduce the regret.
We propose an efficient pre-training and test-time
fine-tuning algorithm called PCE (Policy Collection-
Elimination). By maintaining a minimum policy set
that generalizes well, it achieves a regret upper bound
˜
OpC(D)Kin the test stage, where C(D)is a com-
plexity measure of the distribution D. This bound re-
moves the polynomial dependence on the cardinality
of state-action space by leveraging the information ob-
tained from pre-training. We give a fine-grained anal-
ysis on the value of C(D)and show that our bound can
be significantly smaller than state-action space depen-
dent bound in many settings. We also give a lower
bound to show that this measure is a tight and bidirec-
tional control of the regret.
• When the agent cannot interact with the test envi-
ronment, we propose an efficient algorithm called
OMERM (Optimistic Model-based Empirical Risk
Minimization) to find a near-optimal policy in ex-
pectation. This algorithm is guaranteed to return a
ǫ-optimal policy with Olog NΠ
ǫ/(12H)2sam-
pled MDP tasks in the training stage where NΠ
ǫ/(12H)
is the complexity of the policy class. This rate matches
the traditional generalization rate in many supervised
learning results (Mohri et al.,2018;Kawaguchi et al.,
2017).
2. Related Works
Generalization and Multi-task RL. Many empirical
works study how to improve generalization for deep
RL algorithms (Packer et al.,2018;Zhang et al.,2020;
Ghosh et al.,2021). We refer readers to a recent survey
(Kirk et al.,2021) for more discussion on empirical results.
Our paper is more closely related to the recent works to-
wards understanding RL generalization from the theoretical
perspective.Wang et al. (2019) focused on a special class
of reparameterizable RL problems, and derive generaliza-
tion bounds based on Rademacher complexity and the PAC-
Bayes bound. The most related work is a recent paper
of Malik et al. (2021), which also provided lower bounds
showing that instance-optimal solution is statistically diffi-
cult for RL generalization when we cannot access the sam-
pled test environment. Further, they proposed efficient al-
gorithms which is guaranteed to return a near-optimal pol-
2
On the Power of Pre-training for Generalization in RL: Provable Benefits and Hardness
icy for deterministic MDPs. However, their work is dif-
ferent from ours since they studied a restricted setting un-
der structural assumptions of the environments, such as
all MDPs share a common optimal policy, and their al-
gorithm requires access to a query model. Our paper is
also related to recent works studying multi-task learning in
RL (Brunskill & Li,2013;Tirinzoni et al.,2020;Hu et al.,
2021;Zhang & Wang,2021;Lu et al.,2021), in which they
studied how to transfer the knowledge learned from previ-
ous tasks to new tasks. Their problem formulation is differ-
ent from ours since they study the multi-task setting where
the MDP is selected from a given MDP set without prob-
ability mechanism. In addition, they typically assume that
all the tasks have similar transition dynamics or share com-
mon representations.
Provably Efficient Exploration in RL. Recent years have
witnessed many theoretical results studying provably effi-
cient exploration in RL (Osband et al.,2013;Azar et al.,
2017;Osband & Van Roy,2017;Jin et al.,2018;2020b;
Wang et al.,2020;Zhang et al.,2021) with the minimax re-
gret for tabular MDPs with non-stationary transition being
˜
O(HSAK). These results indicate that polynomial de-
pendence on the whole state-action space is unavoidable
without additional assumptions. Their formulation corre-
sponds to the single-task setting where the agent only in-
teracts with a single environment aiming to maximize its
cumulative rewards without pre-training. The regret de-
fined in the fine-tuning setting coincides with the concept
of Bayesian regret in the previous literature (Osband et al.,
2013;Osband & Van Roy,2017;O’Donoghue,2021). The
best-known Bayesian regret for tabular RL is ˜
O(HSAK)
when applied to our setting (O’Donoghue,2021).
Latent MDPs. This work is also related to previous results
on latent MDPs (Kwon et al.,2021b;a), which is a special
class of Partially Observable MDPs (Azizzadenesheli et al.,
2016;Guo et al.,2016;Jin et al.,2020a). In latent MDPs,
the MDP representing the dynamics of the environment is
sampled from an unknown distribution at the start of each
episode. Their works are different from ours since they
focus on tackling the challenge of partial observability.
3. Preliminary and Framework
Notations Throughout the paper, we use [N]to denote
the set {1,···, N}where NN+. For an event E, let
I[E]be the indicator function of event E, i.e. I[E] = 1 if
and only if Eis true. For any domain , we use C(Ω)
to denote the continuous function on . We use O(·)to
denote the standard big Onotation, and ˜
O(·)to denote the
big Onotation with log(·)term omitted.
3.1. Episodic MDPs
An episodic MDP Mis specified as a tuple
(S,A,PM, RM, H), where S,Aare the state and ac-
tion space with cardinality Sand Arespectively, and His
the steps in one episode. PM,h :S × A7→ ∆(S)is the
transition such that PM,h(s|s, a)denotes the probability
to transit to state sif action ais taken in state sin step h.
RM,h :S × A 7→ ∆(R)is the reward function such that
RM,h(s, a)is the distribution of reward with non-negative
mean rM,h(s, a)when action ais taken in state sat step
h. In order to compare with traditional generalization, we
make the following assumption:
Assumption 3.1. The total mean reward is bounded by 1,
i.e. ∀M ∈ ,PH
h=1 rM,h(sh, ah)1for all trajectory
(s1, a1,···, sH, aH)with positive probability in M; The
reward mechanism RM(s, a)is 1-subgaussian, i.e.
EXRM,h(s,a)[exp(λ[XrM,h(s, a)])] exp λ2
2
for all λR.
The total reward assumption follows the previous works
on horizon-free RL (Ren et al.,2021;Zhang et al.,2021;
Li et al.,2022) and covers the traditional setting where
rM,h(s, a)[0,1] by scaling H, and it is more natural
in environments with sparse rewards (Vecerik et al.,2017;
Riedmiller et al.,2018). In addition, it allows us to com-
pare with supervised learning bound where H= 1 and
the loss is bounded by [0,1]. The subgaussian assumption
is more common in practice and is widely used in ban-
dits (Lattimore & Szepesv´ari,2020). It also covers tradi-
tional RL setting where RM,h(s, a)∆([0,1]), and al-
lows us to study MDP environment with a wider range. For
the convenience of explanation, we assume the agent al-
ways starts from the same state s1. It is straightforward to
recover the initial state distribution µfrom this setting by
adding an initial state s0with transition µ(Du et al.,2019;
Chen et al.,2021).
Policy and Value Function. A policy πis set of Hfunc-
tions where each maps a state to an action distribution, i.e.
π={πh}H
h=1, πh:S 7→ ∆(A)and πcan be stochastic.
We denote the set of all policies described above as Π. We
define NΠ
ǫas the ǫ-covering number of the policy space
Πw.r.t. distance d(π1, π2) = maxs∈S,h[H]kπ1
h(·|s)
π2
h(·|s)k1. Given πand h[H], we define the Q-function
Qπ
M,h :S × A 7→ R+, where
Qπ
M,h(s, a) = rM,h(s, a)+ X
s∈S
PM,h(s|s, a)Vπ
M,h+1(s),
and the V-function Vπ
M,h :S 7→ R+, where
Vπ
M,h(s) = Eaπh(·|s)Qπ
M,h(s, a)
3
On the Power of Pre-training for Generalization in RL: Provable Benefits and Hardness
for hHand Vπ
M,H+1(s) = 0. We abbreviate
Vπ
M,1(s1)as Vπ
M, which can be interpreted as the value
when executing policy πin M. Following the notations
in previous works, we use PhV(s, a)as the shorthand of
Ps∈S Ph(s|s, a)V(s)in our analysis.
3.2. RL Generalization Formulation
We mainly study the setting where all MDP instances we
face in training and testing stages are i.i.d. sampled from
a distribution Dsupported on a (possibly infinite) count-
able set . For an MDP M ∈ , we use P(M)to
denote the probability of sampling Maccording to dis-
tribution D. For an MDP set ˜
, we similarly de-
fine P(˜
Ω) = PM∈˜
P(M). We assume that S,A, H is
shared by all MDPs, while the transition and reward are dif-
ferent. When interacting with a sampled instance M, one
does not know which instance it is, but can only identify its
model through interactions.
In the training (pre-training) stage, the agent can sample
i.i.d. MDP instances from the unknown distribution D. The
overall goal is to perform well in the test stage with the in-
formation learned in the training stage. Define the optimal
policy as
π(M) = arg max
πΠ
Vπ
M, π(D) = arg max
πΠ
EM∼DVπ
M.
We say a policy πis ǫ-optimal in expectation, if
EM∼DVπ(D)
MVπ
Mǫ.
We say a policy πis ǫ-optimal in instance, if
EM∼DVπ(M)
MVπ
Mǫ.
Without Test-time Interaction. When the interaction
with the test environment is unavailable, optimality in in-
stance can be statistically intractable, and we can only pur-
sue optimality in expectation. We formulate this difficulty
into the following proposition.
Proposition 3.2. There exists an MDP support , such that
for any distribution Dwith positive p.d.f. p(r),ǫ0>0,
and for any deployed policy ˆπ,
EMDVπ(M)
MVˆπ
Mǫ0.
Proposition 3.2 is proved by constructing as a set of
MDPs with opposed optimal action, and the complete proof
can be found in Appendix A. When is discrete, there ex-
ists hard instances where the proposition holds for ǫ01
2.
This implies that without test-time interactions or special
knowledge on the structure of and D, it is impractical
to be near optimal in instance. This intractability arises
from the demand on instance optimal policy, which is never
asked in supervised learning.
With Test-time Interaction. To pursue the optimality in
instance, we study the problem of RL generalization with
test-time interaction. When our algorithm is allowed to in-
teract with the target MDP MDfor Kepisodes in the
test stage, we want to reduce the regret, which is defined as
RegK(D,A),EMDRegK(M,A),
RegK(M,A),
K
X
k=1
[Vπ(M)
MVπk
M],
where πkis the policy that Adeploys in episode k. Here
Mis unknown and unchanged during all Kepisodes.
The choice of Bayesian regret is more natural in gener-
alization, and can better evaluate the performance of an
algorithm in practice. From the standard Regret-to-PAC
technique (Jin et al.,2018;Dann et al.,2017), an algorithm
with ˜
O(K)regret can be transformed to an algorithm
that returns an ǫ-optimal policy with ˜
O(12)trajectories.
Therefore, we believe regret can also be a good criterion to
measure the sample efficiency of fine-tuning algorithms in
the test stage.
4. Results for the Setting with Test-time
Interaction
In this section, we study the setting where the agent is al-
lowed to interact with the sampled test MDP M. When
there is no pre-training stage, the typical regret bound in the
test stage is ˜
O(SAHK)(Zhang et al.,2021). For gener-
alization in RL, we mainly care about the performance in
the test stage, and hope the agent can reduce test regret
by leveraging the information learned in the pre-training
stage. Obviously, when is the set of all tabular MDPs
and the distribution Dis uniform over , pre-training can
do nothing on improving the test regret, since it provides no
extra information for the test stage. Therefore, we seek a
distribution-dependent improvement that is better than tra-
ditional upper bound in most of benign settings.
4.1. Hardness in the Asymptotic Setting
We start by understanding how much information the pre-
training stage can provide at most, regardless of the test
time episode K. One natural focus is on the MDP distri-
bution D, which is a sufficient statistic of the possible envi-
ronment that the agent will encounter in the test stage. We
strengthen the algorithm by directly telling it the accurate
distribution D, and analyze how much this extra informa-
tion can help to improve the regret. Specifically, we ask:
Is there a distribution based multiplied factor C(D)that is
small when Denjoys some benign properties (e.g. Dis
sharp and concentrated), such that when knowing D, there
exists an algorithm that can reduce the regret by a factor of
C(D)for all K?
4
On the Power of Pre-training for Generalization in RL: Provable Benefits and Hardness
Perhaps surprisingly, our answer towards this question is
negative for all large enough Kin the asymptotic case.
As is formulated in Theorem 4.1, the importance of Dis
constrained by a universal factor c0asymptotically. Here
c0=1
16 holds universally and does not depend on D. This
theorem implies that no matter what distribution Dis, for
sufficiently large K, any algorithm can only reduce the to-
tal regret by at most a constant factor with the extra knowl-
edge of D, making a distribution-dependent improvement
impossible for universal K.
Theorem 4.1. There exists an MDP instance set , a uni-
versal constant c0=1
16 , and an algorithm ˆ
Athat only
inputs the episode K, such that for any distribution Dwith
positive p.d.f. p(r)C(Ω) (which ˆ
Adoes NOT know), any
algorithm Athat inputs Dand the episode K,
1. is not degraded, i.e.,
lim
K→∞ RegK(D,A(D, K)) = +.
2. Knowing the distribution is useless up to a constant,
i.e.
lim inf
K→∞
RegK(D,A(D, K))
RegK(D,ˆ
A(K)) c0.
In Theorem 4.1, Point (1) avoids any trivial support
where πthat is optimal for all M , in which case
the distribution is of course useless since ˆ
Acan be optimal
by simply following πeven it does not know D. Note that
our bounds hold for any distribution D, which indicates that
even a very sharp distribution cannot provide useful infor-
mation in the asymptotic case where K→ ∞. The value of
c0depends on the coefficient of previous upper and lower
bound, and we conjecture that it could be arbitrarily close
to 1.
We defer the complete proof to Appendix B, and briefly
sketch the intuition here. The key observation is that the
information provided in the training stage (prior) is fixed,
while the required information gradually increase as Kin-
creases. When K= 1, the agent can clearly benefit from
the knowledge of D. Without this knowledge, all it can do
is a random guess since it has never interacted with Mbe-
fore. However, when Kis large, the algorithm can interact
with Mmany times and learn Mmore accurately, while
the prior Dwill become relatively less informative. As a
result, the benefits of knowing Dvanishes eventually.
Theorem 4.1 lower bounds the improvement of regret by
a constant. As is commonly known, the regret bound
can be converted into a PAC-RL bound (Jin et al.,2018;
Dann et al.,2017). This implies that when δ, ǫ 0,
in terms of pursuing a ǫ-optimal policy to π(M), pre-
training cannot help reduce the sample complexity. Despite
negative, we point out that this theorem only describe the
asymptotic setting where K→ ∞, but it imposes no con-
straint when Kis fixed.
4.2. Improvement in the Non-asymptotic Setting
In the last subsection, we provide a lower bound showing
that the information obtained from the training stage can
be useless if we require an universal improvement with re-
spect to all episode K. However, a near-optimal regret in
the non-asymptotic and non-universal setting is also desir-
able in many applications in practice. In this section, we
fix the value of Kand seek to design an algorithm such
that it can leverage the pre-training information and reduce
K-episode test regret. To avoid redundant explanation for
single MDP learning, we formulate the following oracles.
Definition 4.2. (Policy learning oracle) We define
Ol(M, ǫ, log(1)) as the policy learning oracle which can
return a policy πthat is ǫ-optimal w.r.t. MDP Mwith prob-
ability at least 1δ, i.e. V
M(s1)Vπ
M(s1)ǫ. The ran-
domness of the policy πis due to the randomness of both
the oracle algorithm and the environment.
Definition 4.3. (Policy evaluation oracle) We define
Oe(M, π, ǫ, log(1/δ)) as the policy evaluation oracle
which can return a value vthat is ǫ-close to the value func-
tion Vπ
M(s1)with probability at least 1δ, i.e. |v
Vπ
M(s1)| ≤ ǫ. The randomness of the value vis due to
the randomness of both the oracle and the environment.
Both oracles can be efficiently implemented using the
previous algorithms for single-task MDPs. Specifi-
cally, we can implement the policy learning oracle us-
ing algorithms such as UCBVI (Azar et al.,2017), LSVI-
UCB (Jin et al.,2020b) and GOLF (Jin et al.,2021) with
polynomial sample complexities. The policy evaluation
oracle can be achieved by the standard Monte Carlo
method (Sutton & Barto,2018).
4.2.1. ALGORITHM
There are two major difficulties in designing the algorithm.
First, what do we want to learn during the pre-training pro-
cess and how to learn it? One idea is to directly learn the
whole distribution D, which is all that we can obtain for
the test stage. However, this requires ˜
O(||22)samples
for a required accuracy δ, and is unacceptable when ||is
large or even infinite. Second, how to design the test-stage
algorithm to leverage the learned information effectively?
If we cannot effectively use the information from the pre-
training, the regret or samples required in the test stage can
be ˜
O(poly(S, A)) in the worst case.
To tackle the above difficulties, we formulate this problem
as a policy candidate collection-elimination process. Our
intuition is to find a minimum policy set that can gener-
alize to most MDPs sampled from D. In the pre-training
5
摘要:

arXiv:2210.10464v2[cs.LG]29Jun2023OnthePowerofPre-trainingforGeneralizationinRL:ProvableBenefitsandHardnessHaotianYe*1XiaoyuChen*2LiweiWang23SimonS.Du4AbstractGeneralizationinReinforcementLearning(RL)aimstotrainanagentduringtrainingthatgen-eralizestothetargetenvironment.Inthiswork,wefirstpointoutthatR...

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