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Title: Entropy-SGD optimizes the prior of a PAC-Bayes bound: Data-dependent PAC-Bayes priors via differential privacy

Paper content:
We show that Entropy-SGD (Chaudhari et al., 2017), when viewed as a learning algorithm, optimizes a PAC-Bayes bound on the risk of a Gibbs (posterior) classifier, i.e., a randomized classifier obtained by a risk-sensitive perturbation of the weights of a learned classifier.
Entropy-SGD works by optimizing the bound’s prior, violating the hypothesis of the PAC-Bayes theorem that the prior is chosen independently of the data.
Indeed, available implementations of Entropy-SGD rapidly obtain zero training error on random labels and the same holds of the Gibbs posterior.
In order to obtain a valid generalization bound, we show that an ε-differentially private prior yields a valid PAC-Bayes bound, a straightforward consequence of results connecting generalization with differential privacy.
Using stochastic gradient Langevin dynamics (SGLD) to approximate the well-known exponential release mechanism, we observe that generalization error on MNIST (measured on held out data) falls within the (empirically nonvacuous) bounds computed under the assumption that SGLD produces perfect samples.
In particular, Entropy-SGLD can be configured to yield relatively tight generalization bounds and still fit real labels, although these same settings do not obtain state-of-the-art performance.
Optimization is central to much of machine learning, but generalization is the ultimate goal.
Despite this, the generalization properties of many optimization-based learning algorithms are poorly understood.
The standard example is stochastic gradient descent (SGD), one of the workhorses of deep learning, which has good generalization performance in many settings, even under overparametrization BID36 , but rapidly overfits in others BID44 .
Can we develop high performance learning algorithms with provably strong generalization guarantees?
Or is their a limit?In
this work, we study an optimization algorithm called Entropy-SGD BID10 , which was designed to outperform SGD in terms of generalization error when optimizing an empirical risk. Entropy-SGD
minimizes an objective f : R p → R indirectly by performing (approximate) stochastic gradient ascent on the so-called local entropy DISPLAYFORM0 where C is a constant and N denotes a zero-mean isotropic multivariate normal distribution on R p .Our first contribution
is connecting Entropy-SGD to results in statistical learning theory, showing that maximizing the local entropy corresponds to minimizing a PAC-Bayes bound BID31 on the risk of the so-called Gibbs posterior. The distribution of w
+ ξ is the PAC-Bayesian "prior", and so optimizing the local entropy optimizes the bound's prior. This connection between
local entropy and PAC-Bayes follows from a result due to Catoni (2007, Lem. 1.1.3) in the case of bounded risk. (See Theorem 4.1.) In the
special case where
f is the empirical cross entropy, the local entropy is literally a Bayesian log marginal density. The connection between minimizing
PACBayes bounds under log loss and maximizing log marginal densities is the subject of recent work by BID19 . Similar connections have been made
by BID45 ; Zhang (2006b) ; BID20 ; BID21 .Despite the connection to PAC-Bayes
, as well as theoretical results by Chaudhari et al. suggesting that Entropy-SGD may be more stable than SGD, we demonstrate that Entropy-SGD (and its corresponding Gibbs posterior) can rapidly overfit, just like SGD. We identify two changes, motivated
by theoretical analysis, that suffice to control generalization error, and thus prevent overfitting.The first change relates to the stability of optimizing the prior mean. The PAC-Bayes theorem requires that
the prior be independent of the data, and so by optimizing the prior mean, Entropy-SGD invalidates the bound. Indeed, the bound does not hold empirically
. While a PAC-Bayes prior may not be chosen
based on the data, it can depend on the data distribution. This suggests that if the prior depends only
weakly on the data, it may be possible to derive a valid bound.We formalize this intuition using differential privacy BID13 BID17 . By modifying the cross entropy loss to be bounded
and replacing SGD with stochastic gradient Langevin dynamics (SGLD; BID43 , the data-dependent prior mean can be shown to be (ε, δ )-differentially private BID42 BID34 . We refer to the SGLD variant as Entropy-SGLD. Using
results connecting statistical validity and differential
privacy (Dwork et al., 2015b, Thm. 11) , we show that an ε-differentially private prior mean yields a valid, though looser, generalization bound using the PAC-Bayes theorem. (See Theorem 5.4.)A gap remains between pure and approximate differential
privacy. Under some technical conditions, in the limit as the number of iterations
diverges, the distribution of SGLD's output is known to converge weakly to the corresponding stationary distribution, which is the well-known exponential mechanism in differential privacy (Teh, Thiery, and Vollmer, 2016, Thm. 7) . Weak convergence, however, falls short of implying that SGLD achieves pure
ε-differential privacy. We proceed under the approximation that SGLD enjoys the same privacy as the
exponential release mechanism, and apply our ε-differentially private PAC-Bayes bound. We find that the corresponding 95% confidence intervals are reasonably tight
but still conservative in our experiments. While the validity of our bounds are subject to our approximation, the bounds
give us a view as to the limitations of combining differential privacy with PAC-Bayes bounds: when the privacy of Entropy-SGLD is tuned to contribute no more than 2ε 2 × 100 ≈ 0.2% to the generalization error, the test error of the learned network is 3-8%, which is approximately 5-10 times higher than the state of the art, which for MNIST is between 0.2-1%, although the community has almost certainly overfit its networks/learning rates/loss functions/optimizers to MNIST. We return to these points in the discussion.The second change pertains to the
stability of the stochastic gradient estimate made on each iteration of Entropy-SGD. This estimate is made using SGLD. (Hence Entropy-SGD is SGLD within SGD.) Chaudhari
et al. make a subtle but critical
modification to the noise term in SGLD
update: the noise is divided by a factor that ranges from 10 3 to 10 4 . (This factor was ostensibly tuned to produce good empirical results.) Our analysis
shows that, as a result of this modification, the Lipschitz constant
of the objective function is approximately 10 6 -10 8 times larger, and the conclusion that the Entropy-SGD objective is smoother than the original risk surface no longer stands. This change to the noise also negatively impacts the differential privacy of the
prior mean. Working backwards from the desire to obtain tight generalization bounds, we are
led to divide the SGLD noise by a factor of only 4 √ m, where m is the number of data points. (For MNIST, 4 √ m ≈ 16.) The resulting bounds are nonvacuous and tighter than those
recently published by BID18
, although it must be emphasized that the bound presented here hold subject to the approximation concerning privacy of the prior mean, which is certainly violated but to an unknown degree.We begin with a review of some related work, before introducing sufficient background so that we can make a formal connection between local entropy and PAC-Bayes bounds. We then introduce a differentially private PAC-Bayes bound. In Section 6, we present experiments
on MNIST which provide evidence for our theoretical analysis
. (Empirical validation is required in order to address the aforementioned gap between pure and approximate
differential privacy.) We close with a short discussion.
Our work reveals that Entropy-SGD can be understood as optimizing a PAC-Bayes generalization bound in terms of the bound's prior.
Because the prior must be independent of the data, the bound is invalid, and, indeed, we observe overfitting in our experiments with Entropy-SGD when the thermal noise 2/τ is set to 0.0001 as suggested by Chaudhari et al. for MNIST.
PAC-Bayes priors can, however, depend on the data distribution.
This flexibility seems wasted, since the data sample is typically viewed as one's only view onto the data distribution.
However, using differential privacy, we can span this gap.
By performing a private computation on the data, we can extract information about the underlying distribution, without undermining the statistical validity of a subsequent PAC-Bayes bound.
Our PAC-Bayes bound based on a differentially private prior is made looser by the use of a private data-dependent prior, but the gains in choosing a datadistribution-dependent prior more than make up for the expansion of the bound due to the privacy.
(The gains come from the KL term being much smaller on the account of the prior being better matched to the posterior.)
Understanding how our approach compares to local PAC-Bayes priors BID9 is an important open problem.The most elegant way to make Entropy-SGD private is to replace SGD with a sample from the Gibbs distribution (known as the exponential mechanism in the differential privacy literature).
However, generating an exact sample is intractable, and so practicioners use SGLD to generate an approximate sample, relying on the fact that SGLD converges weakly to the exponential mechanism under certain technical conditions.
Our privacy approximation allows us to proceed with a theoretical analysis by assuming that SGLD achieves the same privacy as the exponential mechanism.
On the one hand, we do not find overt evidence that our approximation is grossly violated.
On the other, we likely do not require such strong privacy in order to control generalization error.We might view our privacy-based bounds as being optimistic and representing the bounds we might be able to achieve rigorously should there be a major advance in private optimization.
(No analysis of the privacy of SGLD takes advantage of the fact that it mixes weakly.)
On the account of using private data-dependent priors, our bounds are significantly tighter than those reported by BID18 .
However, despite our bounds potentially being optimistic, the test set error we are able to achieve is still 5-10 times that of SGD.
Differential privacy may be too conservative for our purposes, leading us to underfit.
Indeed, we think it is unlikely that Entropy-SGD has strong differential privacy, yet we are able to achieve good generalization on both true and random labels under 0.01 thermal noise.
Identifying the appropriate notion of privacy/stability to combine with PAC-Bayes bounds is an important problem.Despite our progress on building learning algorithms with strong generalization performance, and identifying a path to much tighter PAC-Bayes bounds, Entropy-SGLD learns much more slowly than Entropy-SGD, the risk of Entropy-SGLD is far from state of the art, and our PAC-Bayes bounds are loose.
It seems likely that there is a fundamental tradeoff between the speed of learning, the excess risk, and the ability to produce a certificate of one's generalization error via a rigorous bound.
Characterizing the relationship between these quantities is an important open problem.A BACKGROUND: DIFFERENTIAL PRIVACY Here we formally define some of the differential privacy related terms used in the main text.
(See BID13 BID15 for more details.)
Let U,U 1 ,U 2 , . . . be independent uniform (0, 1) random variables, independent also of any random variables introduced by P and E, and let π : DISPLAYFORM0 Definition A.1.
A randomized algorithm A from R to T , denoted A : R T , is a measurable map A : [0, 1] × R → T .
Associated to A is a (measurable) collection of random variables {A r : r ∈ R} that satisfy A r = A (U, r).
When there is no risk of confusion, we will write A (r) for A r .
Definition A.2.
A randomized algorithm A : Z m T is (ε, δ )-differentially private if, for all pairs S, S ∈ Z m that differ at only one coordinate, and all measurable subsets B ⊆ T , we have P(A (S) ∈ B) ≤ e ε P(A (S ) ∈ B) + δ .We
will write ε-differentially private to mean (ε, 0)-differentially private algorithm. Definition
A.3. Let A : R
T and A : DISPLAYFORM1 Lemma A.4 (post-processing). Let A : Z
m T be (ε, δ )-differentially private and let F : T T be arbitrary. Then F •
A is (ε, δ )-differentially private.

assistant: text: We show that Entropy-SGD optimizes the prior of a PAC-Bayes bound, violating the requirement that the prior be independent of data; we use differential privacy to resolve this and improve generalization.