# Ergodicity of Combocontinuous Adaptive MCMC algorithms

This preprint by Jeff Rosenthal and Jinyoung Yang (currently available from Jeffs webpage) might also be called “Easily verifiable adaptive MCMC”. Jeff Rosenthal gave a tutorial on adaptive MCMC during MCMSki 2016 mentioning this work.  Adaptive MCMC is based on the idea that one can use the information gathered from sampling a distribution using MCMC to improve the efficiency of the sampling process.

If two conditions, diminishing adaptation and containment are satisfied, an adaptive MCMC algorithm is valid in the sense of asymptotically consistent. Diminishing adaptation means that two consecutive Markov Kernels in the algorithm will be asymptotically equal. In other words, we either stop adaptation at some point or we know that the adaptation algorithm converges.
Containment means the number of repeated applications of all used Markov Kernels to get close to the target measure is bounded. Concretely, let $\gamma$ be a Markov kernel index, $P_\gamma^m(x,\cdot)$ be the distribution resulting from m-fold application of kernel $P_\gamma$ starting from $x$ . In other words start MCMC at point x with kernel $P_\gamma$, let it run for m iterations and consider the induced distribution for the last point. Let $\pi$ be the target distribution. Then containment requires that
$\{M_\epsilon(X_n, \Gamma_n)\}_{n=1}^\infty$  is bounded in probability for all $\epsilon > 0$. Here $M_\epsilon(x, \gamma) = \inf \{ m \geq 1 : \| P_\gamma^m(x,\cdot) - \pi(\cdot) \|_\textrm{TV}\}$ and $\|\cdot\|_\textrm{TV}\}$ is a worst case distance between distributions (total variation distance).

The paper is concerned with trying to find conditions for containment in adaptive MCMC that are more easily verified than those from earlier papers. First however it gives a kind of blueprint for adaptive algorithms that satisfy containment.

### A blueprint for consistent adaptive MCMC

Nameley, let $\mathbb{R}^d$ be the support of the target distribution and $K \subseteq \mathbb{R}^d$ some large bounded region, $D > 0$ some large constant. The blueprint, Bounded Adaptive Metropolis, is the following:

Start the algorithm at some $X_0 \in K$ and fix a $d \times d$ covariance matrix $\Sigma_*$. At iteration n generate a proposal $Y_{n+1}$ by

(1)$Y_{n+1} \sim \mathcal{N}(X_n, \Sigma_*)~\textrm{if}~X_n \notin K$
(2)$Y_{n+1} \sim \mathcal{N}(X_n, \Sigma_{n+1})~\textrm{if}~X_n \in K$

Reject if $|Y_{n+1} - X_{n}| > D$, else accept with the usual Metropolis-Hastings acceptance probability. The $\Sigma_{n+1}$ can be chosen almost arbitrarily if the diminishing adaptation condition is met, so either the mechanism of choosing is fixed asymptotically or converges.

It would seem to me that we can actually change the distribution in (2) arbitrarily if we continue to meet diminishing adaptation. So for example we could use an independent metropolis, adaptive Langevin or other sophisticated proposal inside K, so long as condition (e) in the paper is satisfied, i.e. the adaptive proposal distribution used in (2) is continuous in $X_n$. Which leads us to the actual conditions for containment.

### General conditions for containment in adaptive MCMC

Let $\mathcal{X}$ be a general state space. For example in the Bounded Metropolis we had $\mathcal{X}=\mathbb{R}^d$. The conditions the authors give are (even more simplified by me):

(a)  The probability to move more than some finite distance D > 0 is zero: $Pr(|X_{n+1} - X_n| > D) = 0$
(b) Outside of K, the algorithm uses a fixed transition kernel P that never changes (and still respects that we can at most move D far away)
(c) The fixed kernel P is bounded above by $P(x, dy) \leq M \mu_*(dy)$ for finite constant M > 0 and all x that are outside K but no farther from it than D (call that set $K_D$) and all y that are between D and 2D distance from K (call that set $K_{2D} \backslash K_D$). Here $\mu_*$ is any distribution concentrated on $K_{2D} \backslash K_D$.
(d) The fixed kernel P is bounded below by $P^{n_0}(x, A) \geq \epsilon \nu_*(A)$ for some measure $\nu_*$ on $\mathcal{X}$, some $n_0 \in \mathbb{N}$ and some event A.
(e) Let $\gamma$ be the parameter adapted by the algorithm. The overall proposal densities $q_\gamma(x,y)$ (combining the proposal in and outside of K) are continuous in $\gamma$ for fixed (x,y) and combocontinuous in x. Practically, this would be that the fixed proposal when outside  K and the adaptive proposal when inside K are both continuous.

Here, conditions (a) and (b) are very easy to ensure even when not an expert on MCMC. Conditions (c) and (d) sound harder, but as mentioned above it seems to me that they are easy to ensure by just using a (truncated, i.e. respecting (a)) gaussian random walk proposal outside of K. Finally, (e) seems to boil down to making the adaptive proposal continuous in both $\gamma$ and x.

The proofs use a generalization of piecewise continuous functions and a generalized version of Dinis theorem to prove convergence in total variation distance.

This paper seems to me to be a long way from Roberts & Rosenthal (2007, Journal of Applied Probability) which was the first paper I read on ergodicity conditions for adaptive MCMC. It truly makes checking containment much easier. My one concern is that the exposition could be clearer for people that are not MCMC researchers. Then again, this is a contribution paper rather than a tutorial.

# A Variational Analysis of Stochastic Gradient Algorithms

This arXival by Mandt, Hoffman and Blei from February takes a look at what they call SGD with constant learning rate or constant SGD. Which is not really Stochastic Gradient Descent anymore, but rather a sampling algorithm, as we should bounce  around the mode of the posterior rather than converging to it. Consequently they interpret constant SGD as a stochastic process in discrete time with some stationary distribution. They go on to analyse it under the assumption that the stationary distribution is Gaussian (Assumption 1 and 4). Something that springs to my mind here is the following: even if we accept that the stationary distribution of the process might be Gaussian (intuitively, not rigorously), what guarantees that the stationary distribution is the posterior? Even if the posterior is also gaussian, it could have a different (co)variance. I think a few more words about this question might be good. In particular, without adding artificial noise, is it guaranteed that every point in the posteriors domain can be reached with non-zero probability? And can be reached an infinite number of times if we collect infinite samples? If not, the constant SGD process is transient and does not enjoy good stability properties.

Another delicate point hides in Assumption 1: As our stochastic gradient is the sum of independent RVs, the CLT is invoked to assume that the gradient noise is Gaussian. But the CLT might not yet take effect if the number of data points in our subsample is small, which is the very thing one would like in order to have advantages in computation time. This of course is yet another instance of the dilemma common to all scalable sampling techniques, and at this stage of research this is not a show stopper.
Now assuming that constant SGD is not transient and it has a stationary distribution that is the posterior and that the posterior or its dominant mode is approximately Gaussian, we can of course try to optimize the parameters of constant SGD to make the sample it produces as close a possible to a posterior sample. Which the authors conveniently do using an Ornstein-Uhlenbeck approximation, which has a Gaussian stationary distribution. Their approach is to minimize KL-Divergence between the OU approximation and the posterior, and derive optimal parameter settings for constant SGD in closed form. The most interesting part of the theoretical results section probably is the one on optimal preconditioning in Stochastic Gradient Fisher Scoring, as that sampler adds artificial noise. Which might solve some of the transience questions.

The presentation would gain a lot by renaming “constant SGD” for what it is – a sampling algorithm. The terminology and notation in general are sometimes a bit confusing, but nothing a revision can’t fix. In general, the paper presents an interesting approach to deriving optimal parameters for a class of algorithms that is notoriously hard to tune. This is particularly relevant because the constant step size could improve mixing considerably over Welling & Tehs SGLD. What would be interesting to see is wether the results could be generalized to the case where the posterior is not Gaussian. For practical reasons, because stochastic VB/EP works quite well in the gaussian case. For theoretical reasons, because EP now even comes with some guarantees (haven’t read that paper admittedly). Maybe a path would be to take a look at the Gaussian approximation to a multimodal posterior spanning all modes, and minimizing KL-Divergence between that and the OU process. Or maybe one can proof that constant SGD (with artificial noise?) has some fixed stationary distribution to which stochastic drift term plus noise are a Markov Kernel, which might enable a pseudo-marginal correction.

# Variational Hamiltonian Monte Carlo via Score Matching

This is an arXival (maybe by now ICML paper?) by Zhang, Shahbaba and Zhao. It suggest fitting an emulator/surrogate $q_\eta$ to the target density $\pi$ via score matching and then use the gradients of $q_\eta$ rather than those of $\pi$ for generating proposal with HMC. Their main selling point being that this decreases wall-clock time for HMC.

However, that seems to me like reselling the earlier paper by Heiko Strathmann and collaborators on Gradient-free Hamiltonian Monte Carlo with Efficient Kernel Exponential Families. Zhang et al use basically the same idea but fail to look at this earlier work. Their reasoning to use a neural network rather than a GP emulator (computation time) is a bit arbitrary. If you go for a less rich function class (neural network) then the computation time will go down of course – but you would get the same effect by using GPs with inducing points.

Very much lacking to my taste is reasoning for doing the tuning they do. Sometimes they tune HMC/Variational HMC to 85% acceptance, sometimes to 70% acceptance. Also, it seems they not adapting the mass matrix of HMC. If they would, I conjecture the relative efficiency of Standard HMC vs Variational HMC could change drastically. Details on how they tune SGLD is completely lacking.

Overall, I think it is not yet clear what can be learned from the work reported in the paper.

# Marginal Likelihood From the Metropolis–Hastings Output

This is a rather old JASA paper by Siddhartha Chib and Ivan Jeliazkov from 2001. I dug it up again as I am frantically trying to get my contribution for the CRiSM workshop on Estimating Constants in shape. My poster will be titled Flyweight evidence estimates as it looks at methods for estimating model evidence (aka marginal likelihood, normalizing constant, free energy) that are computationally very cheap when a sample from the target/posterior distribution exists.
I noticed again that the approach by Chib & Jeliazkov (2001) does not satisfy this constraint, although they claim otherwise in the paper. The estimator is based on the fact that the normalized probability of a point $\theta^*$ under the target when using Metropolis Hastings is

where $q$ is the proposal density, $\alpha$ the acceptance probability and $y$ the data. If $\bar\pi$ is the unnormalized target, we can get the evidence estimate $Z = \bar\pi (\theta^*|y)/\pi(\theta^*|y)$.
Now the integral in the numerator is cheap to estimate when we already have samples from $\pi$. The authors claim that because it is cheap to sample from  $q$, the denominator is cheap as well. I can’t help it: this statement feels a bit deceptive to me. While indeed samples from $q$ are cheep to generate, evaluating  $\alpha(\theta^*,\theta^{(j)}|y)$ for each sample $\theta^{(j)} \sim q(\theta^*,\cdot|y)$ is the opposite of cheap! It involves evaluating $\bar\pi(\theta^{(j)}|y)$ for all j, which is basically the same cost as getting samples from the target in the first place.
I already included that criticism in my PhD thesis, but when revising under time pressure I no longer thought it was valid and erroneously took it out.

# Non-asymptotic convergence analysis for the Unadjusted Langevin Algorithm

This is an important arXival by Alain Durmus and Eric Moulines. The title is slightly optimized for effect, as the paper actually contains non-asymptotic and asymptotic analysis.

The basic theme of the paper is in getting upper bounds on total variation (and more general distribution distances) between an uncorrected discretized Langevin diffusion wrt some target $\pi$ and $\pi$ itself. The discretization used is the common scheme with the scary name Euler-Maruyama:

$X_{k} \sim \mathcal{N}(\cdot|X_{k-1} + \gamma_{k}\nabla \log \pi(X_{k-1}), \sqrt{2\gamma_{k}I} = R_{\gamma_{k}}(\cdot |X_{k-1})$

Under a Foster-Lyapunov condition, $R_{\gamma_{k}}$ is a Markov kernel that admits a unique stationary distribution $\pi_{\gamma_{k}}$ that is close to the desired $\pi$ in total variation distance and gets closer when $\gamma_{k}$ decreases.

Now in the non-asymptotic case with fixed $\gamma = \gamma_k$, the authors provide bounds that explicitly depend on dimensionality of the support of the target, the number of samples drawn and the chosen step size $\gamma$. Unfortunately, these bounds contain some unknowns as well, such as the Lipschitz constant $L$ of the gradient of the logpdf $\log \pi(\cdot)$ and some suprema that I am unsure how to get explicitly.

Durmus and Moulines particularly take a look at scaling with dimension under increasingly strong conditions on $\pi$, getting exponential (in dimension) constants for the convergence when $\pi$ is superexponential outside a ball. Better convergence can be achieved when assuming $\pi$ to be log-concave or strongly log-concave. This is not surprising, nevertheless the theoretical importance of the results is clear from the fact that together with Arnak Dalalyan this is the first time that results are given for the ULA after the Roberts & Tweedie papers from 1996.

As a practitioner, I would have wished for very explicit guidance in picking  $\gamma$ or the series $\{\gamma_k\}_{k=1}^\infty$. But hopefully with Alains paper as a foundation that can be the next step. As a non-mathematician, I had some problems in following the paper and at some point I completely lost it. This surely is in part due to the quite involved content. However, one might still manage to give intuition even in this case, as Sam Livingstones recent paper on HMC shows. I hope Alain goes over it again with readability and presentation in mind so that it will get the attention it deserves. Yet another task for something that already took a lot of work…

(photo: the Lipschitz Grill diner in Berlin – I don’t
know about their food, but the name is remarkable)

# On the Geometric Ergodicity of Hamiltonian Monte Carlo

This remarkable arXival by Sam Livingstone, Betancourt, Byrne and Girolami takes a look at the conditions under which Hybrid/Hamiltonian Monte Carlo is ergodic wrt a certain distribution $\pi$, and, in a second step, under which it will converge in total variation at a geometric rate. A nice feature being that they even prove that setting the integration time dynamically can lead to the sampler yielding geometric rate for a larger class of targets. Under idealized conditions for the dynamic integration time and, admittedly, in a certain exponential family.

Whats great about this paper, apart from the results, is how it manages to give the arguments in a rather accessible manner. Considering the depth of the results that is. The ideas are that

• the gradient should at most grow linearly
• for the gradient of $\pi$ to be of any help, it cannot lead further into the tails of the distribution or vanish (both assumption 4.4.1)
• as we approach infinity, every proposed point thats closer to the origin will be accepted (assumption 4.4.2)

If the gradient grows faster than linear, numerics will punch you in the face. That happened to Heiko Strathmann and me just 5 days ago in a GradientIS-type algorithm, where an estimated gradient had length $10^{144}$ and led to Nirvana. If not the estimation is the problem but the actual gradient, of course you have the same effect. The second assumption is also very intuitive: if the gradient leads into the tails, then it hurts rather than helps convergence. And your posterior is improper. The final assumption basically requires that your chance to improve by proposing a point closer to the origin goes to $1$ the further you move away from it.

A helpful guidance they provide is the following. For a class of exponential family distributions given by $exp(-\alpha|x|^\beta)~\forall \alpha,\beta >0$,  whenever the tails of the distribution are no heavier than Laplace ($\beta \geq 1$) and no thinner than Gaussian ($\beta \leq 2$), HMC will be geometrically ergodic. In all other cases, it won’t.

Another thing that caught my eye was their closed form expression for the proposal after $L$ leapfrog steps with stepsize $\epsilon$, eq. (13):

Which gives me hope that one should be able to compute the proposal probability after any number of leapfrog steps as long as $\nabla \log \pi$ is invertible and thus a Jacobian exists to account for the transformation of $p_0$ hidden in the $x_{i\epsilon}$. A quick check with $\pi$ given by a bayesian posterior shows that this might not be trivial however.

# A comparison of generalized hybrid Monte Carlo methods with and without momentum flip

This experimental paper from 2009 by Akhmatskaya, Bou-Rabee and Reich in the Journal of Computational Physics takes a look at what in statistics people would call HMC with partial momentum refreshment.The paper got my attention mainly because I met Elena Akhmatskaya at MCMSki.

The ‘generalized’ in the paper refers to the fact that the momenta are not resampled at each new HMC step, but rather according towhere $p_i \sim \mathcal{N}(0,\Sigma)$ is the old momentum, and we mix it with another Gaussian RV of the same distribution $u_i$ for some parameter $\phi$. We recover HMC for $\phi=\pi/2$, as in that case the momentum is always refreshed.

Also, they are looking at a so-called shadow HMC variant, which really samples from a Hamiltonian system that is not the original one, but close to it. Correction for sampling from the wrong distribution is done by importance weighting (when she was telling me that I thought “I knew IS could work in high dimensions!”, as they sample from up to 10,000 dimensions and do better than HMC!).

Now the main gist of the paper is that they where trying to get rid of momentum flips in the case of rejection of the proposal, as it was previously thought that momentum flips lead to a worse sampler. The intuition would be that a momentum flip leads you back to where you started. However, the finding is that to the contrary, their version without flip (that still satisfies a detailed balance criterion) does worse in actual estimation tasks than the conservative Generalized Shadow HMC (GSHMC) sampler. I like when things stay easy. But as this paper does not describe the actual GSHMC method in detail, I will have to go back to the original paper, as my interest is sparked.