## An introduction to functional programming for scalable statistical computing and machine learning

Functional programming (FP) languages are great for statistical computing, computational statistics, and machine learning. They are particularly well-suited to scalable computation, where this could either mean scaling up to distributed algorithms for very big data, or running algorithms for more moderately sized data sets very fast in parallel on GPUs. However, people unfamiliar with FP often find FP languages quite intimidating, due to the fairly steep initial learning curve. This issue is exacerbated by the fact that there is very little introductory documentation available for people new to FP who are interested in applications to statistical computing and machine learning (ML).

So for some time I’ve been meaning to put together materials for a short course (suitable for self-study) that will get people started with FP in few different languages, with a very basic problem from statistical computing used as the running example, together with a catalogue of resources for further learning, in order to provide people with the information they need to keep going once they have got over the initial hurdle. But as with many things, it never got high enough up my priority list to actually sit down and do it. Fortunately, StatML invited me to deliver some training in advanced statistical computing, so this gave me the perfect motivation to actually assemble something. The in-person training has been delayed (due to the UCU strike), but the materials are all prepared and publicly available, and suitable for self-study now.

The course gives a very quick introduction to the ideas of FP, followed by very quick hands-on introductions to my favourite FP languages/libraries: Scala, Haskell, JAX and Dex. There is also a brief introduction to splittable random number generators which are becoming increasingly popular for the development of functional parallel Monte Carlo algorithms.

If you’ve been vaguely interested in FP for statistical computing and ML but not sure how to get started, hopefully this solves the problem.

## A scalable particle filter in Scala

### Introduction

Many modern algorithms in computational Bayesian statistics have at their heart a particle filter or some other sequential Monte Carlo (SMC) procedure. In this blog I’ve discussed particle MCMC algorithms which use a particle filter in the inner-loop in order to compute a (noisy, unbiased) estimate of the marginal likelihood of the data. These algorithms are often very computationally intensive, either because the forward model used to propagate the particles is expensive, or because the likelihood associated with each particle/observation is expensive (or both). In this case it is desirable to parallelise the particle filter to run on all available cores of a machine, or in some cases, it would even be desirable to distribute the the particle filter computation across a cluster of machines.

Parallelisation is difficult when using the conventional imperative programming languages typically used in scientific and statistical computing, but is much easier using modern functional languages such as Scala. In fact, in languages such as Scala it is possible to describe algorithms at a higher level of abstraction, so that exactly the same algorithm can run in serial, run in parallel across all available cores on a single machine, or run in parallel across a cluster of machines, all without changing any code. Doing so renders parallelisation a non-issue. In this post I’ll talk through how to do this for a simple bootstrap particle filter, but the same principle applies for a large range of statistical computing algorithms.

trait GenericColl[C[_]] {
def map[A, B](ca: C[A])(f: A => B): C[B]
def reduce[A](ca: C[A])(f: (A, A) => A): A
def flatMap[A, B, D[B] <: GenTraversable[B]](ca: C[A])(f: A => D[B]): C[B]
def zip[A, B](ca: C[A])(cb: C[B]): C[(A, B)]
def length[A](ca: C[A]): Int
}


In the typeclass we just list the methods that we expect our generic collection to provide, but do not say anything about how they are implemented. For example, we know that operations such as map and reduce can be executed in parallel, but this is a separate concern. We can now write code that can be used for any collection conforming to the requirements of this typeclass. The full code for this example is provided in the associated github repo for this blog, and includes the obvious syntax for this typeclass, and typeclass instances for the Scala collections Vector and ParVector, that we will exploit later in the example.

### SIR step for a bootstrap filter

We can now write some code for a single observation update of a bootstrap particle filter.

def update[S: State, O: Observation, C[_]: GenericColl](
dataLik: (S, O) => LogLik, stepFun: S => S
)(x: C[S], o: O): (LogLik, C[S]) = {
val xp = x map (stepFun(_))
val lw = xp map (dataLik(_, o))
val max = lw reduce (math.max(_, _))
val rw = lw map (lwi => math.exp(lwi - max))
val srw = rw reduce (_ + _)
val l = rw.length
val z = rw zip xp
val rx = z flatMap (p => Vector.fill(Poisson(p._1 * l / srw).draw)(p._2))
(max + math.log(srw / l), rx)
}


This is a very simple bootstrap filter, using Poisson resampling for simplicity and data locality, but does include use of the log-sum-exp trick to prevent over/underflow of raw weight calculations, and tracks the marginal (log-)likelihood of the observation. With this function we can now pass in a “prior” particle distribution in any collection conforming to our typeclass, together with a propagator function, an observation (log-)likelihood, and an observation, and it will return back a new collection of particles of exactly the same type that was provided for input. Note that all of the operations we require can be accomplished with the standard monadic collection operations declared in our typeclass.

### Filtering as a functional fold

Once we have a function for executing one step of a particle filter, we can produce a function for particle filtering as a functional fold over a sequence of observations:

def pFilter[S: State, O: Observation, C[_]: GenericColl, D[O] <: GenTraversable[O]](
x0: C[S], data: D[O], dataLik: (S, O) => LogLik, stepFun: S => S
): (LogLik, C[S]) = {
val updater = update[S, O, C](dataLik, stepFun) _
data.foldLeft((0.0, x0))((prev, o) => {
val next = updater(prev._2, o)
(prev._1 + next._1, next._2)
})
}


Folding data structures is a fundamental concept in functional programming, and is exactly what is required for any kind of filtering problem. Note that Brian Beckman has recently written a series of articles on Kalman filtering as a functional fold.

### Marginal likelihoods and parameter estimation

So far we haven’t said anything about parameters or parameter estimation, but this is appropriate, since parametrisation is a separate concern from filtering. However, once we have a function for particle filtering, we can produce a function concerned with evaluating marginal likelihoods trivially:

def pfMll[S: State, P: Parameter, O: Observation,
C[_]: GenericColl, D[O] <: GenTraversable[O]](
simX0: P => C[S], stepFun: P => S => S,
dataLik: P => (S, O) => LogLik, data: D[O]
): (P => LogLik) = (th: P) =>
pFilter(simX0(th), data, dataLik(th), stepFun(th))._1


Note that this higher-order function does not return a value, but instead a function which will accept a parameter as input and return a (log-)likelihood as output. This can then be used for parameter estimation purposes, perhaps being used in a PMMH pMCMC algorithm, or something else. Again, this is a separate concern.

### Example

Here I’ll just give a completely trivial toy example, purely to show how the functions work. For avoidance of doubt, I know that there are many better/simpler/easier ways to tackle this problem! Here we will just look at inferring the auto-regression parameter of a linear Gaussian AR(1)-plus-noise model using the functions we have developed.

First we can simulate some synthetic data from this model, using a value of 0.8 for the auto-regression parameter:

val inNoise = Gaussian(0.0, 1.0).sample(99)
val state = DenseVector(inNoise.scanLeft(0.0)((s, i) => 0.8 * s + i).toArray)
val noise = DenseVector(Gaussian(0.0, 2.0).sample(100).toArray)
val data = (state + noise).toArray.toList


Now assuming that we don’t know the auto-regression parameter, we can construct a function to evaluate the likelihood of different parameter values as follows:

val mll = pfMll(
(th: Double) => Gaussian(0.0, 10.0).sample(10000).toVector.par,
(th: Double) => (s: Double) => Gaussian(th * s, 1.0).draw,
(th: Double) => (s: Double, o: Double) => Gaussian(s, 2.0).logPdf(o),
data
)


Note that the 4 characters “.par” at the end of line 2 are the only difference between running this code serially or in parallel! Now we can run this code by calling the returned function with different values. So, hopefully mll(0.8) will return a larger log-likelihood than (say) mll(0.6) or mll(0.9). The example code in the github repo plots the results of calling mll() for a range of values (note that if that was the genuine use-case, then it would be much better to parallellise the parameter range than the particle filter, due to providing better parallelisation granularity, but many other examples require parallelisation of the particle filter itself). In this particular example, both the forward model and the likelihood are very cheap operations, so there is little to be gained from parallelisation. Nevertheless, I still get a speedup of more than a factor of two using the parallel version on my laptop.

### Conclusion

In this post we have shown how typeclasses can be used in Scala to write code that is parallelisation-agnostic. Code written in this way can be run on one or many cores as desired. We’ve illustrated the concept with a scalable particle filter, but nothing about the approach is specific to that application. It would be easy to build up a library of statistical routines this way, all of which can effectively exploit available parallel hardware. Further, although we haven’t demonstrated it here, it is trivial to extend this idea to allow code to be distribution over a cluster of parallel machines if necessary. For example, if an Apache Spark cluster is available, it is easy to make a Spark RDD instance for our generic collection typeclass, that will then allow us to run our (unmodified) particle filter code over a Spark cluster. This emphasises the fact that Spark can be useful for distributing computation as well as just processing “big data”. I’ll say more about Spark in subsequent posts.

## Statistical computing languages at the RSS

On Friday the Royal Statistical Society hosted a meeting on Statistical computing languages, organised by my colleague Colin Gillespie. Four languages were presented at the meeting: Python, Scala, Matlab and Julia. I presented the talk on Scala. The slides I presented are available, in addition to the code examples and instructions on how to run them, in a public github repo I have created. The talk mainly covered material I have discussed in various previous posts on this blog. What was new was the emphasis on how easy it is to use and run Scala code, and the inclusion of complete examples and instructions on how to run them on any platform with a JVM installed. I also discussed some of the current limitations of Scala as an environment for interactive statistical data analysis and visualisation, and how these limitations could be overcome with a little directed effort. Colin suggested that all of the speakers covered a couple of examples (linear regression and a Monte Carlo integral) in “their” languages, and he provided an R solution. It is interesting to see the examples in the five different languages side by side for comparison purposes. Colin is collecting together all of the material relating to the meeting in a combined github repo, which should fill out over the next few days.

For other Scala posts on this blog, see all of my posts with a “scala” tag.

## Parallel Monte Carlo using Scala

### Introduction

In previous posts I have discussed general issues regarding parallel MCMC and examined in detail parallel Monte Carlo on a multicore laptop. In those posts I used the C programming language in conjunction with the MPI parallel library in order to illustrate the concepts. In this post I want to take the example from the second post and re-examine it using the Scala programming language.

The toy problem considered in the parallel Monte Carlo post used $10^9$ $U(0,1)$ random quantities to construct a Monte Carlo estimate of the integral

$\displaystyle I=\int_0^1\exp\{-u^2\}du$.

A very simple serial program to implement this algorithm is given below:

import java.util.concurrent.ThreadLocalRandom
import scala.math.exp
import scala.annotation.tailrec

object MonteCarlo {

@tailrec
def sum(its: Long,acc: Double): Double = {
if (its==0)
(acc)
else {
sum(its-1,acc+exp(-u*u))
}
}

def main(args: Array[String]) = {
println("Hello")
val iters=1000000000
val result=sum(iters,0.0)
println(result/iters)
println("Goodbye")
}

}


Note that ThreadLocalRandom is a parallel random number generator introduced into recent versions of the Java programming language, which can be easily utilised from Scala code. Assuming that Scala is installed, this can be compiled and run with commands like

scalac monte-carlo.scala
time scala MonteCarlo


This program works, and the timings (in seconds) for three runs are 57.79, 57.77 and 57.55 on the same laptop considered in the previous post. The first thing to note is that this Scala code is actually slightly faster than the corresponding C+MPI code in the single processor special case! Now that we have a good working implementation we should think how to parallelise it…

### Parallel implementation

Before constructing a parallel implementation, we will first construct a slightly re-factored serial version that will be easier to parallelise. The simplest way to introduce parallelisation into Scala code is to parallelise a map over a collection. We therefore need a collection and a map to apply to it. Here we will just divide our $10^9$ iterations into $N=4$ separate computations, and use a map to compute the required Monte Carlo sums.

import java.util.concurrent.ThreadLocalRandom
import scala.math.exp
import scala.annotation.tailrec

object MonteCarlo {

@tailrec
def sum(its: Long,acc: Double): Double = {
if (its==0)
(acc)
else {
sum(its-1,acc+exp(-u*u))
}
}

def main(args: Array[String]) = {
println("Hello")
val N=4
val iters=1000000000
val its=iters/N
val sums=(1 to N).toList map {x => sum(its,0.0)}
val result=sums.reduce(_+_)
println(result/iters)
println("Goodbye")
}

}


Running this new code confirms that it works and gives similar estimates for the Monte Carlo integral as the previous version. The timings for 3 runs on my laptop were 57.57, 57.67 and 57.80, similar to the previous version of the code. So far so good. But how do we make it parallel? Like this:

import java.util.concurrent.ThreadLocalRandom
import scala.math.exp
import scala.annotation.tailrec

object MonteCarlo {

@tailrec
def sum(its: Long,acc: Double): Double = {
if (its==0)
(acc)
else {
sum(its-1,acc+exp(-u*u))
}
}

def main(args: Array[String]) = {
println("Hello")
val N=4
val iters=1000000000
val its=iters/N
val sums=(1 to N).toList.par map {x => sum(its,0.0)}
val result=sums.reduce(_+_)
println(result/iters)
println("Goodbye")
}

}


That’s it! It’s now parallel. Studying the above code reveals that the only difference from the previous version is the introduction of the 4 characters .par in line 22 of the code. R programmers will find this very much analagous to using lapply() versus mclapply() in R code. The function par converts the collection (here an immutable List) to a parallel collection (here an immutable parallel List), and then subsequent maps, filters, etc., can be computed in parallel on appropriate multicore architectures. Timings for 3 runs on my laptop were 20.74, 20.82 and 20.88. Note that these timings are faster than the timings for N=4 processors for the corresponding C+MPI code…

### Varying the size of the parallel collection

We can trivially modify the previous code to make the size of the parallel collection, N, a command line argument:

import java.util.concurrent.ThreadLocalRandom
import scala.math.exp
import scala.annotation.tailrec

object MonteCarlo {

@tailrec
def sum(its: Long,acc: Double): Double = {
if (its==0)
(acc)
else {
sum(its-1,acc+exp(-u*u))
}
}

def main(args: Array[String]) = {
println("Hello")
val N=args(0).toInt
val iters=1000000000
val its=iters/N
val sums=(1 to N).toList.par map {x => sum(its,0.0)}
val result=sums.reduce(_+_)
println(result/iters)
println("Goodbye")
}

}


We can now run this code with varying sizes of N in order to see how the runtime of the code changes as the size of the parallel collection increases. Timings on my laptop are summarised in the table below.

 N     T1     T2     T3
1   57.67  57.62  57.83
2   32.20  33.24  32.76
3   26.63  26.60  26.63
4   20.99  20.92  20.75
5   20.13  18.70  18.76
6   16.57  16.52  16.59
7   15.72  14.92  15.27
8   13.56  13.51  13.32
9   18.30  18.13  18.12
10   17.25  17.33  17.22
11   17.04  16.99  17.09
12   15.95  15.85  15.91

16   16.62  16.68  16.74
32   15.41  15.54  15.42
64   15.03  15.03  15.28


So we see that the timings decrease steadily until the size of the parallel collection hits 8 (the number of processors my hyper-threaded quad-core presents via Linux), and then increases very slightly, but not much as the size of the collection increases. This is better than the case of C+MPI where performance degrades noticeably if too many processes are requested. Here, the Scala compiler and JVM runtime manage an appropriate number of threads for the collection irrespective of the actual size of the collection. Also note that all of the timings are faster than the corresponding C+MPI code discussed in the previous post.

However, the notion that the size of the collection is irrelevant is only true up to a point. Probably the most natural way to code this algorithm would be as:

import java.util.concurrent.ThreadLocalRandom
import scala.math.exp

object MonteCarlo {

def main(args: Array[String]) = {
println("Hello")
val iters=1000000000
val sums=(1 to iters).toList map {x => ThreadLocalRandom.current().nextDouble()} map {x => exp(-x*x)}
val result=sums.reduce(_+_)
println(result/iters)
println("Goodbye")
}

}


or as the parallel equivalent

import java.util.concurrent.ThreadLocalRandom
import scala.math.exp

object MonteCarlo {

def main(args: Array[String]) = {
println("Hello")
val iters=1000000000
val sums=(1 to iters).toList.par map {x => ThreadLocalRandom.current().nextDouble()} map {x => exp(-x*x)}
val result=sums.reduce(_+_)
println(result/iters)
println("Goodbye")
}

}


Although these algorithms are in many ways cleaner and more natural, they will bomb out with a lack of heap space unless you have a huge amount of RAM, as they rely on having all $10^9$ realisations in RAM simultaneously. The lesson here is that even though functional languages make it very easy to write clean, efficient parallel code, we must still be careful not to fill up the heap with gigantic (immutable) data structures…

## Scala as a platform for statistical computing and data science

There has been a lot of discussion on-line recently about languages for data analysis, statistical computing, and data science more generally. I don’t really want to go into the detail of why I believe that all of the common choices are fundamentally and unfixably flawed – language wars are so unseemly. Instead I want to explain why I’ve been using the Scala programming language recently and why, despite being far from perfect, I personally consider it to be a good language to form a platform for efficient and scalable statistical computing. Obviously, language choice is to some extent a personal preference, implicitly taking into account subjective trade-offs between features different individuals consider to be important. So I’ll start by listing some language/library/ecosystem features that I think are important, and then explain why.

## A feature wish list

It should:

• be a general purpose language with a sizable user community and an array of general purpose libraries, including good GUI libraries, networking and web frameworks
• be free, open-source and platform independent
• be fast and efficient
• have a good, well-designed library for scientific computing, including non-uniform random number generation and linear algebra
• have a strong type system, and be statically typed with good compile-time type checking and type safety
• have reasonable type inference
• have a REPL for interactive use
• have good tool support (including build tools, doc tools, testing tools, and an intelligent IDE)
• have excellent support for functional programming, including support for immutability and immutable data structures and “monadic” design
• allow imperative programming for those (rare) occasions where it makes sense
• be designed with concurrency and parallelism in mind, having excellent language and library support for building really scalable concurrent and parallel applications

The not-very-surprising punch-line is that Scala ticks all of those boxes and that I don’t know of any other languages that do. But before expanding on the above, it is worth noting a couple of (perhaps surprising) omissions. For example:

• have excellent data viz capability built-in
• have vast numbers of statistical routines in the standard library

The above are points (and there are other similar points) where other languages (for example, R), currently score better than Scala. It is not that these things are not important – indeed, they are highly desirable. But I consider them to be of lesser importance as they are much easier to fix, given a suitable platform, than fixing an unsuitable language and platform. Visualisation is not trivial, but it is not fantastically difficult in a language with excellent GUI libraries. Similarly, most statistical routines are quite straightforward to implement for anyone with reasonable expertise in scientific and statistical computing and numerical linear algebra. These are things that are relatively easy for a community to contribute to. Building a great programming language, on the other hand, is really, really, difficult.

I will now expand briefly on each point in turn.

#### be a general purpose language with a sizable user community and an array of general purpose libraries, including good GUI libraries, networking and web frameworks

History has demonstrated, time and time again, that domain specific languages (DSLs) are synonymous with idiosyncratic, inconsistent languages that are terrible for anything other than what they were specifically designed for. They can often be great for precisely the thing that they were designed for, but people always want to do other things, and that is when the problems start. For the avoidance of controversy I won’t go into details, but the whole Python versus R thing is a perfect illustration of this general versus specific trade-off. Similarly, although there has been some buzz around another new language recently, which is faster than R and Python, my feeling is that the last thing the world needs right now is Just Unother Language for Indexed Arrays…

In this day-and-age it is vital that statistical code can use a variety of libraries, and communicate with well-designed network libraries and web frameworks, as statistical analysis does not exist in a vacuum. Scala certainly fits the bill here, being used in a large number of important high-profile systems, ensuring a lively, well-motivated ecosystem. There are numerous well-maintained libraries for almost any task. Picking on web frameworks, for example, there are a number of excellent libraries, including Lift and Play. Scala also has the advantage of offering seamless Java integration, for those (increasingly rare) occasions when a native Scala library for the task at hand doesn’t exist.

#### be free, open-source and platform independent

This hardly needs expanding upon, other than to observe that there are a few well-known commercial software solutions for scientific, statistical and mathematical computing. There are all kinds of problems with using closed proprietary systems, including transparency and reproducibility, but also platform and scalability problems. eg. running code requiring a license server in the cloud. The academic statistical community has largely moved away from commercial software, and I don’t think there is any going back. Scala is open source and runs on the JVM, which is about as platform independent as it is possible to get.

#### be fast and efficient

Speed and efficiency continue to be important, despite increasing processor speeds. Computationally intensive algorithms are being pushed to ever larger and more complex models and data sets. Compute cycles and memory efficiency really matter, and can’t be ignored. This doesn’t mean that we all have to code in C/C++/Fortran, but we can’t afford to code in languages which are orders of magnitude slower. This will always be a problem. Scala code generally runs well within a factor of 2 of comparable native code – see my Gibbs sampler post for a simple example including timings.

#### have a good, well-designed library for scientific computing, including non-uniform random number generation and linear algebra

I hesitated about including this in my list of essentials, because it is certainly something that can, in principle, be added to a language at a later date. However, building such libraries is far from trivial, and they need to be well-designed, comprehensive and efficient. For Scala, Breeze is rapidly becoming the standard scientific library, including special functions, non-uniform random number generation and numerical linear algebra. For a data library, there is Saddle, and for a scalable analytics library there is Spark. These libraries certainly don’t cover everything that can be found in R/CRAN, but they provide a fairly solid foundation on which to build.

#### have a strong type system, and be statically typed with good compile-time type checking and type safety

I love dynamic languages – they are fun and exciting. It is fun to quickly throw together a few functions in a scripting language without worrying about declaring the types of anything. And it is exciting to see the myriad of strange and unanticipated ways your code can crash-and-burn at runtime! ðŸ˜‰ But this excitement soon wears off, and you end up adding lots of boilerplate argument checking code that would not only be much cleaner and simpler in a statically typed language, but would be checked at compile-time, making the static code faster and more efficient. For messing about prototyping, dynamic languages are attractive, but as a solid platform for statistical computing, they really don’t make sense. Scala has a strong type system offering a high degree of compile-time checking, making it a safe and efficient language.

#### have reasonable type inference

A common issue with statically typed languages is that they lead to verbose code containing many redundant type declarations that the compiler ought to be able to check. This doesn’t just mean more typing – it leads to verbose code that can hide the program logic. Languages with type inference offer the best of both worlds – the safety of static typing without the verbosity. Scala does a satisfactory job here.

#### have a REPL for interactive use

One thing that dynamic languages have taught us is that it is actually incredibly useful to have a REPL for interactive analysis. This is true generally, but especially so for statistical computing, where human intervention is often desirable. Again, Scala has a nice REPL.

#### have good tool support (including build tools, doc tools, testing tools, and an intelligent IDE)

Tools matter. Scala has an excellent build tool in the SBT. It has code documentation in the form of scaladoc (similar to javadoc). It has a unit testing framework, and a reasonably intelligent IDE in the form of the Scala IDE (based on Eclipse).

#### have excellent support for functional programming, including support for immutability and immutable data structures and “monadic” design

I, like many others, am gradually coming to realise that functional programming offers many advantages over other programming styles. In particular, it provides best route to building scalable software, in terms of both program complexity and data size/complexity. Scala has good support for functional programming, including immutable named values, immutable data structures and for-comprehensions. And if off-the-shelf Scala isn’t sufficiently functional already, libraries such as scalaz make it even more so.

#### allow imperative programming for those (rare) occasions where it makes sense

Although most algorithms in scientific computing are typically conceived of and implemented in an imperative style, I’m increasingly convinced that most can be recast in a pure functional way without significant loss of efficiency, and with significant benefits. That said, there really are some problems that are more efficient to implement in an imperative framework. It is therefore important that the language is not so “pure” functional that this is forbidden. Again, Scala fits the bill.

#### be designed with concurrency and parallelism in mind, having excellent language and library support for building really scalable concurrent and parallel applications

These days scalability typically means exploiting concurrency and parallelism. In an imperative world this is hard, and libraries such as MPI prove that it is difficult to bolt parallelism on top of a language post-hoc. Check-points, communication overhead, deadlocks and race conditions make it very difficult to build codes that scale well to more than a few processors. Concurrency is more straightforward in functional languages, and this is one of the reasons for the recent resurgence of functional languages and programming. Scala has good concurrency support built-in, and libraries such as Akka make it relatively easy to build truly scalable software.

## Summary

The Scala programming language ticks many boxes when it comes to forming a nice solid foundation for building a platform for efficient scalable statistical computing. Although I still use R and Python almost every day, I’m increasingly using Scala for serious algorithm development. In the short term I can interface to my Scala code from R using jvmr, but in the longer term I hope that Scala will become a complete framework for statistics and data science. In a subsequent post I will attempt to give a very brief introduction to Scala and the Breeze numerical library.

## Introduction

In the previous post I showed that it is possible to couple parallel tempered MCMC chains in order to improve mixing. Such methods can be used when the target of interest is a Bayesian posterior distribution that is difficult to sample. There are (at least) a couple of obvious ways that one can temper a Bayesian posterior distribution. Perhaps the most obvious way is a simple flattening, so that if

$\pi(\theta|y) \propto \pi(\theta)\pi(y|\theta)$

is the posterior distribution, then for $t\in [0,1]$ we define

$\pi_t(\theta|y) \propto \pi(\theta|y)^t \propto [ \pi(\theta)\pi(y|\theta) ]^t.$

This corresponds with the tempering that is often used in statistical physics applications. We recover the posterior of interest for $t=1$ and tend to a flat distribution as $t\longrightarrow 0$. However, for Bayesian posterior distributions, there is a different way of tempering that is often more natural and useful, and that is to temper using the power posterior, defined by

$\pi_t(\theta|y) \propto \pi(\theta)\pi(y|\theta)^t.$

Here we again recover the posterior for $t=1$, but get the prior for $t=0$. Thus, the family of distributions forms a natural bridge or path from the prior to the posterior distributions. The power posterior is a special case of the more general concept of a geometric path from distribution $f(\theta)$ (at $t=0$) to $g(\theta)$ (at $t=1$) defined by

$h_t(\theta) \propto f(\theta)^{1-t}g(\theta)^t,$

where, in our case, $f(\cdot)$ is the prior and $g(\cdot)$ is the posterior.

So, given a posterior distribution that is difficult to sample, choose a temperature schedule

$0=t_0

and run a parallel tempering scheme as outlined in the previous post. The idea is that for small values of $t$ mixing will be good, as prior-like distributions are usually well-behaved, and the mixing of these "high temperature" chains can help to improve the mixing of the "low temperature" chains that are more like the posterior (note that $t$ is really an inverse temperature parameter the way I’ve defined it here…).

## Marginal likelihood and normalising constants

The marginal likelihood of a Bayesian model is

$\pi(y) = \int_\Theta \pi(\theta)\pi(y|\theta)d\theta.$

This quantity is of interest for many reasons, including calculation of the Bayes factor between two competing models. Note that this quantity has several different names in different fields. In particular, it is often known as the evidence, due to its role in Bayes factors. It is also worth noting that it is the normalising constant of the Bayesian posterior distribution. Although it is very easy to describe and define, it is notoriously difficult to compute reliably for complex models.

The normalising constant is conceptually very easy to estimate. From the above integral representation, it is clear that

$\pi(y) = E_\pi [ \pi(y|\theta) ]$

where the expectation is taken with respect to the prior. So, given samples from the prior, $\theta_1,\theta_2,\ldots,\theta_n$, we can construct the Monte Carlo estimate

$\displaystyle \widehat{\pi}(y) = \frac{1}{n}\sum_{i=1}^n \pi(y|\theta_i)$

and this will be a consistent estimator of the true evidence under fairly mild regularity conditions. Unfortunately, in practice it is likely to be a very poor estimator if the posterior and prior are not very similar. Now, we could also use Bayes theorem to re-write the integral as an expectation with respect to the posterior, so we could then use samples from the posterior to estimate the evidence. This leads to the harmonic mean estimator of the evidence, which has been described as the worst Monte Carlo method ever! Now it turns out that there are many different ways one can construct estimators of the evidence using samples from the prior and the posterior, some of which are considerably better than the two I’ve outlined. This is the subject of the bridge sampling paper of Meng and Wong. However, the reality is that no method will work well if the prior and posterior are very different.

If we have tempered chains, then we have a sequence of chains targeting distributions which, by construction, are not too different, and so we can use the output from tempered chains in order to construct estimators of the evidence that are more numerically stable. If we call the evidence of the $i$th chain $z_i$, so that $z_0=1$ and $z_N=\pi(y)$, then we can write the evidence in telescoping fashion as

$\displaystyle \pi(y)=z_N = \frac{z_N}{z_0} = \frac{z_1}{z_0}\times \frac{z_2}{z_1}\times \cdots \times \frac{z_N}{z_{N-1}}.$

Now the $i$th term in this product is $z_{i+1}/z_{i}$, which can be estimated using the output from the $i$th and/or $(i+1)$th chain(s). Again, this can be done in a variety of ways, using your favourite bridge sampling estimator, but the point is that the estimator should be reasonably good due to the fact that the $i$th and $(i+1)$th targets are very similar. For the power posterior, the simplest method is to write

$\displaystyle \frac{z_{i+1}}{z_i} = \frac{\displaystyle \int \pi(\theta)\pi(y|\theta)^{t_{i+1}}d\theta}{z_i} = \int \pi(y|\theta)^{t_{i+1}-t_i}\times \frac{\pi(y|\theta)^{t_i}\pi(\theta)}{z_i}d\theta$

$\displaystyle \mbox{}\qquad = E_i\left[\pi(y|\theta)^{t_{i+1}-t_i}\right],$

where the expectation is with respect to the $i$th target, and hence can be estimated in the usual way using samples from the $i$th chain.

For numerical stability, in practice we compute the log of the evidence as

$\displaystyle \log\pi(y) = \sum_{i=0}^{N-1} \log\frac{z_{i+1}}{z_i} = \sum_{i=0}^{N-1} \log E_i\left[\pi(y|\theta)^{t_{i+1}-t_i}\right]$

$\displaystyle = \sum_{i=0}^{N-1} \log E_i\left[\exp\{(t_{i+1}-t_i)\log\pi(y|\theta)\}\right].\qquad(\dagger)$

The above expression is exact, and is the obvious formula to use for computation. However, it is clear that if $t_i$ and $t_{i+1}$ are sufficiently close, it will be approximately OK to switch the expectation and exponential, giving

$\displaystyle \log\pi(y) \approx \sum_{i=0}^{N-1}(t_{i+1}-t_i)E_i\left[\log\pi(y|\theta)\right].$

In the continuous limit, this gives rise to the well-known path sampling identity,

$\displaystyle \log\pi(y) = \int_0^1 E_t\left[\log\pi(y|\theta)\right]dt.$

So, an alternative approach to computing the evidence is to use the samples to approximately numerically integrate the above integral, say, using the trapezium rule. However, it isn’t completely clear (to me) that this is better than using $(\dagger)$ directly, since there there is no numerical integration error to worry about.

## Numerical illustration

We can illustrate these ideas using the simple double potential well example from the previous post. Now that example doesn’t really correspond to a Bayesian posterior, and is tempered directly, rather than as a power posterior, but essentially the same ideas follow for general parallel tempered distributions. In general, we can use the sample to estimate the ratio of the last and first normalising constants, $z_N/z_0$. Here it isn’t obvious why we’d want to know that, but we’ll compute it anyway to illustrate the method. As before, we expand as a telescopic product, where the $i$th term is now

$\displaystyle \frac{z_{i+1}}{z_i} = E_i\left[\exp\{-(\gamma_{i+1}-\gamma_i)(x^2-1)^2\}\right].$

A Monte Carlo estimate of each of these terms is formed using the samples from the $i$th chain, and the logs of these are then summed to give $\log(z_N/z_0)$. A complete R script to run the Metropolis coupled sampler and compute the evidence is given below.

U=function(gam,x)
{
gam*(x*x-1)*(x*x-1)
}

temps=2^(0:3)
iters=1e5

chains=function(pot=U, tune=0.1, init=1)
{
x=rep(init,length(temps))
xmat=matrix(0,iters,length(temps))
for (i in 1:iters) {
can=x+rnorm(length(temps),0,tune)
logA=unlist(Map(pot,temps,x))-unlist(Map(pot,temps,can))
accept=(log(runif(length(temps)))<logA)
x[accept]=can[accept]
swap=sample(1:length(temps),2)
logA=pot(temps[swap[1]],x[swap[1]])+pot(temps[swap[2]],x[swap[2]])-
pot(temps[swap[1]],x[swap[2]])-pot(temps[swap[2]],x[swap[1]])
if (log(runif(1))<logA)
x[swap]=rev(x[swap])
xmat[i,]=x
}
colnames(xmat)=paste("gamma=",temps,sep="")
xmat
}

mat=chains()
mat=mat[,1:(length(temps)-1)]
diffs=diff(temps)
mat=(mat*mat-1)^2
mat=-t(diffs*t(mat))
mat=exp(mat)
logEvidence=sum(log(colMeans(mat)))
message(paste("The log of the ratio of the last and first normalising constants is",logEvidence))


It turns out that these double well potential densities are tractable, and so the normalising constants can be computed exactly. So, with a little help from Wolfram Alpha, I compute log of the ratio of the last and first normalising constants to be approximately -1.12. Hopefully the above script will output something a bit like that…

## Introduction

Parallel tempering is a method for getting Metropolis-Hastings based MCMC algorithms to work better on multi-modal distributions. Although the idea has been around for more than 20 years, and works well on many problems, it still isn’t routinely used in applications. I think this is partly because relatively few people understand how it works, and partly due to the perceived difficulty of implementation. I hope to show here that it is both very easy to understand and to implement. It is also rather easy to implement in parallel on multi-core systems, though I won’t get into that in this post.

## Sampling a double-well potential

To illustrate the ideas, we need a toy multi-modal distribution to sample. There are obviously many possibilities here, but I rather like to use a double potential well distribution. The simplest version of this assumes a potential function of the form

$U(x) = \gamma (x^2-1)^2$

for some given potential barrier height $\gamma$. The potential function $U(x)$ corresponds to the probability density function

$\pi(x) \propto \exp\{-U(x)\}.$

There is a physical explanation for the terminology, via Langevin diffusions, but that isn’t really important here. It is fine to just think of potentials as being a (negative) log-density scale. On this scale, high potential barrier heights correspond to regions of very low probability density. We can set up a double well potential and plot it for the case $\gamma=4$ in R with the following code

U=function(gam,x)
{
gam*(x*x-1)*(x*x-1)
}

curried=function(gam)
{
message(paste("Returning a function for gamma =",gam))
function(x) U(gam,x)
}
U4=curried(4)

op=par(mfrow=c(2,1))
curve(U4(x),-2,2,main="Potential function, U(x)")
curve(exp(-U4(x)),-2,2,main="Unnormalised density function, exp(-U(x))")
par(op)


Incidentally, the function curried(), which curries the potential function, did not include the message() statement when I first wrote it. It mostly worked fine, but some of the code below didn’t behave as I expected. I inserted the message() statement to figure out what was going on, and the code started behaving perfectly – a beautiful example of a Heisenbug! The reason is that the message statement is not redundant – it forces evaluation of the gam variable, which is necessary in some cases, due to the lazy evaluation model that R uses for function arguments. If you don’t like the message() statement, replacing it with a simple gam works just as well.

Anyway, the point is that we have defined a multi-modal density, and that a Metropolis-Hastings algorithm initialised in one of the modes will have a hard time jumping to the other mode, and the difficulty of this jump will increase as we increase the value of $\gamma$.

We can write a simple function for a Metropolis algorithm targeting a particular potential function as follows.

chain=function(target,tune=0.1,init=1)
{
x=init
xvec=numeric(iters)
for (i in 1:iters) {
can=x+rnorm(1,0,tune)
logA=target(x)-target(can)
if (log(runif(1))<logA)
x=can
xvec[i]=x
}
xvec
}


We can use this code to run some chains for a few different values of $\gamma$ as follows.

temps=2^(0:3)
iters=1e5

mat=sapply(lapply(temps,curried),chain)
colnames(mat)=paste("gamma=",temps,sep="")

require(smfsb)
mcmcSummary(mat,rows=length(temps))


We see that as $\gamma$ increases, the chain jumps between modes less frequently. Indeed, for $\gamma=8$, the chain fails to jump to the second mode at all during this particular run of 100,000 iterations. That’s a problem if we are really interested in sampling from distributions like this. Of course, for this particular problem, there are all kinds of ways to fix this sampler, but the point is to try and develop methods that will work in high-dimensional situations where we cannot just “look” at what is going wrong.

Before we see how to couple the chains and improve the mixing, it is useful to think how to re-write this sampler. Above we sampled each chain in turn for different barrier heights. To couple the chains, we need to sample them together, sampling each iteration for all of the chains in turn. This is very easy to do. The code below isn’t especially efficient, but it is written to look very similar to the single chain code above.

chains=function(pot=U, tune=0.1, init=1)
{
x=rep(init,length(temps))
xmat=matrix(0,iters,length(temps))
for (i in 1:iters) {
can=x+rnorm(length(temps),0,tune)
logA=unlist(Map(pot,temps,x))-unlist(Map(pot,temps,can))
accept=(log(runif(length(temps)))<logA)
x[accept]=can[accept]
xmat[i,]=x
}
colnames(xmat)=paste("gamma=",temps,sep="")
xmat
}

mcmcSummary(chains(),rows=length(temps))


This code should behave identically to the previous code, simulating independent parallel MCMC chains. However, the code is now in the form that is very easy to modify to couple the chains together in an attempt to improve mixing.

## Coupling parallel chains

In the above example the chains we are simulating are all independent of one another. Some mix reasonably well, and some mix very badly. But the distributions are all related to one another, changing gradually as the barrier height changes. The idea behind coupling the chains is to try and swap states between the chains to use the chains which are mixing well to improve the mixing of the chains which aren’t. In particular, suppose interest is in the target of the worst mixing chain. The other chains can be constructed as “tempered” versions of the target of interest, here by raising it to a power between 0 and 1, with 0 corresponding to a complete flattening of the distribution, and 1 corresponding to the desired target. The use of parallel chains to improve mixing in this way is usually referred to as parallel tempering, but also sometimes as $(\text{MC})^3$. In the context of Bayesian inference, tempering using the “power posterior” can often be more natural and useful (to be discussed in a subsequent post).

So, how do we swap states between the chains without affecting the target distributions? As always, just use a Metropolis-Hastings update… To keep things simple, let’s just suppose that we have two (independent, parallel) chains, one with target $f(x)$ and the other with target $g(y)$. We can consider these chains to be evolving together, with joint target $\pi(x,y)=f(x)g(y)$. The updates chosen to update the within-chain states will obviously preserve this joint target. Now we consider how to swap states between the two chains without destroying the target. We simply propose a swap of $x$ and $y$. That is, we propose to move from $(x,y)$ to $(x^\star,y^\star)$, where $x^\star=y$ and $y^\star=x$. We are proposing this move as a standard Metropolis-Hastings update of the joint chain. We may therefore wonder about exactly what the proposal density for this move is. In fact, it is a point mass at the swapped values, and therefore has density

$q((x^\star,y^\star)|(x,y)) = \delta(x^\star-y)\delta(y^\star-x),$

but it really doesn’t matter, as it is clearly a symmetric proposal, and hence will drop out of the M-H ratio. Our acceptance probability is therefore $\min\{1,A\}$, where

$\displaystyle A = \frac{\pi(x^\star,y^\star)}{\pi(x,y)} = \frac{\pi(y,x)}{\pi(x,y)} = \frac{f(y)g(x)}{f(x)g(y)}.$

So, if we use this acceptance probability whenever we propose a swap of the states between two chains, then we will preserve the joint target, and hence the marginal targets and asymptotic independence of the target. However, the chains themselves will no longer be independent of one another. They will be coupled – Metropolis coupled. This is very easy to implement. We can just add a few lines to our previous chains() function as follows

chains=function(pot=U, tune=0.1, init=1)
{
x=rep(init,length(temps))
xmat=matrix(0,iters,length(temps))
for (i in 1:iters) {
can=x+rnorm(length(temps),0,tune)
logA=unlist(Map(pot,temps,x))-unlist(Map(pot,temps,can))
accept=(log(runif(length(temps)))<logA)
x[accept]=can[accept]
# now the coupling update
swap=sample(1:length(temps),2)
logA=pot(temps[swap[1]],x[swap[1]])+pot(temps[swap[2]],x[swap[2]])-
pot(temps[swap[1]],x[swap[2]])-pot(temps[swap[2]],x[swap[1]])
if (log(runif(1))<logA)
x[swap]=rev(x[swap])
# end of the coupling update
xmat[i,]=x
}
colnames(xmat)=paste("gamma=",temps,sep="")
xmat
}


This can be used as before, but now gives results as illustrated in the following plots.

We see immediately from the plots that whilst the individual target distributions remain unchanged, the mixing of the chains is greatly improved (though still far from perfect). Note that in the code above I just picked two chains at random to propose a state swap. In practice people typically only propose to swap states between chains which are adjacent – i.e. most similar, since proposed swaps between chains with very different targets are unlikely to be accepted. Since implementation of either strategy is very easy, I would recommend trying both to see which works best.

## Parallel implementation

It should be clear that there are opportunities for parallelising the above algorithm to make effective use of modern multi-core hardware. An approach using OpenMP with C++ is discussed in this blog post. Also note that if the state space of the chains is large, it can be much more efficient to swap temperatures between the chains rather than states – so long as you are careful about keeping track of stuff – this is explored in detail in Altekar et al (’04).

## Complete R script

For convenience, a complete R script to run all of the examples in this post is given below.

# temper.R
# functions for messing around with tempering MCMC

U=function(gam,x)
{
gam*(x*x-1)*(x*x-1)
}

curried=function(gam)
{
#gam
message(paste("Returning a function for gamma =",gam))
function(x) U(gam,x)
}
U4=curried(4)

op=par(mfrow=c(2,1))
curve(U4(x),-2,2,main="Potential function, U(x)")
curve(exp(-U4(x)),-2,2,main="Unnormalised density function, exp(-U(x))")
par(op)

# global settings
temps=2^(0:3)
iters=1e5

# First look at some independent chains
chain=function(target,tune=0.1,init=1)
{
x=init
xvec=numeric(iters)
for (i in 1:iters) {
can=x+rnorm(1,0,tune)
logA=target(x)-target(can)
if (log(runif(1))<logA)
x=can
xvec[i]=x
}
xvec
}

mat=sapply(lapply(temps,curried),chain)
colnames(mat)=paste("gamma=",temps,sep="")

require(smfsb)
mcmcSummary(mat,rows=length(temps))

# Next, let's generate 5 chains at once...
chains=function(pot=U, tune=0.1, init=1)
{
x=rep(init,length(temps))
xmat=matrix(0,iters,length(temps))
for (i in 1:iters) {
can=x+rnorm(length(temps),0,tune)
logA=unlist(Map(pot,temps,x))-unlist(Map(pot,temps,can))
accept=(log(runif(length(temps)))<logA)
x[accept]=can[accept]
xmat[i,]=x
}
colnames(xmat)=paste("gamma=",temps,sep="")
xmat
}

mcmcSummary(chains(),rows=length(temps))

# Next let's couple the chains...
chains=function(pot=U, tune=0.1, init=1)
{
x=rep(init,length(temps))
xmat=matrix(0,iters,length(temps))
for (i in 1:iters) {
can=x+rnorm(length(temps),0,tune)
logA=unlist(Map(pot,temps,x))-unlist(Map(pot,temps,can))
accept=(log(runif(length(temps)))<logA)
x[accept]=can[accept]
# now the coupling update
swap=sample(1:length(temps),2)
logA=pot(temps[swap[1]],x[swap[1]])+pot(temps[swap[2]],x[swap[2]])-
pot(temps[swap[1]],x[swap[2]])-pot(temps[swap[2]],x[swap[1]])
if (log(runif(1))<logA)
x[swap]=rev(x[swap])
# end of the coupling update
xmat[i,]=x
}
colnames(xmat)=paste("gamma=",temps,sep="")
xmat
}

mcmcSummary(chains(),rows=length(temps))

# eof


## Introduction to Approximate Bayesian Computation (ABC)

Many of the posts in this blog have been concerned with using MCMC based methods for Bayesian inference. These methods are typically “exact” in the sense that they have the exact posterior distribution of interest as their target equilibrium distribution, but are obviously “approximate”, in that for any finite amount of computing time, we can only generate a finite sample of correlated realisations from a Markov chain that we hope is close to equilibrium.

Approximate Bayesian Computation (ABC) methods go a step further, and generate samples from a distribution which is not the true posterior distribution of interest, but a distribution which is hoped to be close to the real posterior distribution of interest. There are many variants on ABC, and I won’t get around to explaining all of them in this blog. The wikipedia page on ABC is a good starting point for further reading. In this post I’ll explain the most basic rejection sampling version of ABC, and in a subsequent post, I’ll explain a sequential refinement, often referred to as ABC-SMC. As usual, I’ll use R code to illustrate the ideas.

#### Basic idea

There is a close connection between “likelihood free” MCMC methods and those of approximate Bayesian computation (ABC). To keep things simple, consider the case of a perfectly observed system, so that there is no latent variable layer. Then there are model parameters $\theta$ described by a prior $\pi(\theta)$, and a forwards-simulation model for the data $x$, defined by $\pi(x|\theta)$. It is clear that a simple algorithm for simulating from the desired posterior $\pi(\theta|x)$ can be obtained as follows. First simulate from the joint distribution $\pi(\theta,x)$ by simulating $\theta^\star\sim\pi(\theta)$ and then $x^\star\sim \pi(x|\theta^\star)$. This gives a sample $(\theta^\star,x^\star)$ from the joint distribution. A simple rejection algorithm which rejects the proposed pair unless $x^\star$ matches the true data $x$ clearly gives a sample from the required posterior distribution.

#### Exact rejection sampling

• 1. Sample $\theta^\star \sim \pi(\theta^\star)$
• 2. Sample $x^\star\sim \pi(x^\star|\theta^\star)$
• 3. If $x^\star=x$, keep $\theta^\star$ as a sample from $\pi(\theta|x)$, otherwise reject.

This algorithm is exact, and for discrete $x$ will have a non-zero acceptance rate. However, in most interesting problems the rejection rate will be intolerably high. In particular, the acceptance rate will typically be zero for continuous valued $x$.

#### ABC rejection sampling

The ABC “approximation” is to accept values provided that $x^\star$ is “sufficiently close” to $x$. In the first instance, we can formulate this as follows.

• 1. Sample $\theta^\star \sim \pi(\theta^\star)$
• 2. Sample $x^\star\sim \pi(x^\star|\theta^\star)$
• 3. If $\Vert x^\star-x\Vert< \epsilon$, keep $\theta^\star$ as a sample from $\pi(\theta|x)$, otherwise reject.

Euclidean distance is usually chosen as the norm, though any norm can be used. This procedure is “honest”, in the sense that it produces exact realisations from

$\theta^\star\big|\Vert x^\star-x\Vert < \epsilon.$

For suitable small choice of $\epsilon$, this will closely approximate the true posterior. However, smaller choices of $\epsilon$ will lead to higher rejection rates. This will be a particular problem in the context of high-dimensional $x$, where it is often unrealistic to expect a close match between all components of $x$ and the simulated data $x^\star$, even for a good choice of $\theta^\star$. In this case, it makes more sense to look for good agreement between particular aspects of $x$, such as the mean, or variance, or auto-correlation, depending on the exact problem and context.

In the simplest case, this is done by forming a (vector of) summary statistic(s), $s(x^\star)$ (ideally a sufficient statistic), and accepting provided that $\Vert s(x^\star)-s(x)\Vert<\epsilon$ for some suitable choice of metric and $\epsilon$. We will return to this issue in a subsequent post.

#### Inference for an intractable Markov process

I’ll illustrate ABC in the context of parameter inference for a Markov process with an intractable transition kernel: the discrete stochastic Lotka-Volterra model. A function for simulating exact realisations from the intractable kernel is included in the smfsb CRAN package discussed in a previous post. Using pMCMC to solve the parameter inference problem is discussed in another post. It may be helpful to skim through those posts quickly to become familiar with this problem before proceeding.

So, for a given proposed set of parameters, realisations from the process can be sampled using the functions simTs and stepLV (from the smfsb package). We will use the sample data set LVperfect (from the LVdata dataset) as our “true”, or “target” data, and try to find parameters for the process which are consistent with this data. A fairly minimal R script for this problem is given below.

require(smfsb)
data(LVdata)

N=1e5
message(paste("N =",N))
prior=cbind(th1=exp(runif(N,-6,2)),th2=exp(runif(N,-6,2)),th3=exp(runif(N,-6,2)))
rows=lapply(1:N,function(i){prior[i,]})
message("starting simulation")
samples=lapply(rows,function(th){simTs(c(50,100),0,30,2,stepLVc,th)})
message("finished simulation")

distance<-function(ts)
{
diff=ts-LVperfect
sum(diff*diff)
}

message("computing distances")
dist=lapply(samples,distance)
message("distances computed")

dist=sapply(dist,c)
cutoff=quantile(dist,1000/N)
post=prior[dist<cutoff,]

op=par(mfrow=c(2,3))
apply(post,2,hist,30)
apply(log(post),2,hist,30)
par(op)


This script should take 5-10 minutes to run on a decent laptop, and will result in histograms of the posterior marginals for the components of $\theta$ and $\log(\theta)$. Note that I have deliberately adopted a functional programming style, making use of the lapply function for the most computationally intensive steps. The reason for this will soon become apparent. Note that rather than pre-specifying a cutoff $\epsilon$, I’ve instead picked a quantile of the distance distribution. This is common practice in scenarios where the distance is difficult to have good intuition about. In fact here I’ve gone a step further and chosen a quantile to give a final sample of size 1000. Obviously then in this case I could have just selected out the top 1000 directly, but I wanted to illustrate the quantile based approach.

One problem with the above script is that all proposed samples are stored in memory at once. This is problematic for problems involving large numbers of samples. However, it is convenient to do simulations in large batches, both for computation of quantiles, and also for efficient parallelisation. The script below illustrates how to implement a batch parallelisation strategy for this problem. Samples are generated in batches of size 10^4, and only the best fitting samples are stored before the next batch is processed. This strategy can be used to get a good sized sample based on a more stringent acceptance criterion at the cost of addition simulation time. Note that the parallelisation code will only work with recent versions of R, and works by replacing calls to lapply with the parallel version, mclapply. You should notice an appreciable speed-up on a multicore machine.

require(smfsb)
require(parallel)
options(mc.cores=4)
data(LVdata)

N=1e5
bs=1e4
batches=N/bs
message(paste("N =",N," | bs =",bs," | batches =",batches))

distance<-function(ts)
{
diff=ts[,1]-LVprey
sum(diff*diff)
}

post=NULL
for (i in 1:batches) {
message(paste("batch",i,"of",batches))
prior=cbind(th1=exp(runif(bs,-6,2)),th2=exp(runif(bs,-6,2)),th3=exp(runif(bs,-6,2)))
rows=lapply(1:bs,function(i){prior[i,]})
samples=mclapply(rows,function(th){simTs(c(50,100),0,30,2,stepLVc,th)})
dist=mclapply(samples,distance)
dist=sapply(dist,c)
cutoff=quantile(dist,1000/N)
post=rbind(post,prior[dist<cutoff,])
}
message(paste("Finished. Kept",dim(post)[1],"simulations"))

op=par(mfrow=c(2,3))
apply(post,2,hist,30)
apply(log(post),2,hist,30)
par(op)


Note that there is an additional approximation here, since the top 100 samples from each of 10 batches of simulations won’t correspond exactly to the top 1000 samples overall, but given all of the other approximations going on in ABC, this one is likely to be the least of your worries.

Now, if you compare the approximate posteriors obtained here with the “true” posteriors obtained in an earlier post using pMCMC, you will see that these posteriors are really quite poor. However, this isn’t a very fair comparison, since we’ve only done 10^5 simulations. Jacking N up to 10^7 gives the ABC posterior below.

This is a bit better, but really not great. There are two basic problems with the simplistic ABC strategy adopted here, one related to the dimensionality of the data and the other the dimensionality of the parameter space. The most basic problem that we have here is the dimensionality of the data. We have 16 (bivariate) observations, so we want our stochastic simulation to shoot at a point in a 16- or 32-dimensional space. That’s a tough call. The standard way to address this problem is to reduce the dimension of the data by introducing a few carefully chosen summary statistics and then just attempting to hit those. I’ll illustrate this in a subsequent post. The other problem is that often the prior and posterior over the parameters are quite different, and this problem too is exacerbated as the dimension of the parameter space increases. The standard way to deal with this is to sequentially adapt from the prior through a sequence of ABC posteriors. I’ll examine this in a future post as well, once I’ve also posted an introduction to the use of sequential Monte Carlo (SMC) samplers for static problems.

For further reading, I suggest browsing the reference list of the Wikipedia page for ABC. Also look through the list of software on that page. In particular, note that there is a CRAN package, abc, providing R support for ABC. There is a vignette for this package which should be sufficient to get started.

## MCMC on the Raspberry Pi

I’ve recently taken delivery of a Raspberry Pi mini computer. For anyone who doesn’t know, this is a low cost, low power machine, costing around 20 GBP (25 USD) and consuming around 2.5 Watts of power (it is powered by micro-USB). This amazing little device can run linux very adequately, and so naturally I’ve been interested to see if I can get MCMC codes to run on it, and to see how fast they run.

Now, I’m fairly sure that the majority of readers of this blog won’t want to be swamped with lots of Raspberry Pi related posts, so I’ve re-kindled my old personal blog for this purpose. Apart from this post, I’ll try not to write about my experiences with the Pi here on my main blog. Consequently, if you are interested in my ramblings about the Pi, you may wish to consider subscribing to my personal blog in addition to this one. Of course I’m not guaranteeing that the occasional Raspberry-flavoured post won’t find its way onto this blog, but I’ll try only to do so if it has strong relevance to statistical computing or one of the other core topics of this blog.

In order to get started with MCMC on the Pi, I’ve taken the C code gibbs.c for a simple Gibbs sampler described in a previous post (on this blog) and run it on a couple of laptops I have available, in addition to the Pi, and looked at timings. The full details of the experiment are recorded in this post over on my other blog, to which interested parties are referred. Here I will just give the “executive summary”.

The code runs fine on the Pi (running Raspbian), at around half the speed of my Intel Atom based netbook (running Ubuntu). My netbook in turn runs at around one fifth the speed of my Intel i7 based laptop. So the code runs at around one tenth of the speed of the fastest machine I have conveniently available.

As discussed over on my other blog, although the Pi is relatively slow, its low cost and low power consumption mean that is has a bang-for-buck comparable with high-end laptops and desktops. Further, a small cluster of Pis (known as a bramble) seems like a good, low cost way to learn about parallel and distributed statistical computing.

## Parallel particle filtering and pMCMC using R and multicore

### Introduction

In a previous post I showed how to construct a PMMH pMCMC algorithm for parameter estimation with partially observed Markov processes. The inner loop of a pMCMC algorithm consists of running a particle filter to construct an unbiased estimate of marginal likelihood. This inner loop is the place where the code spends almost all of its time, and so speeding up the particle filter will result in dramatic speedup of the pMCMC algorithm. This is fortunate, since as previously discussed, MCMC algorithms are difficult to parallelise other than on a per iteration basis. Here, each iteration can be speeded up if we can effectively parallelise a particle filter. Particle filters are much easier to parallelise than MCMC algorithms, and so it is tempting to try and exploit this within R. In fact, although it is the case that it is possible to effectively parallelise particle filters in efficient languages using low-level parallelisation tools (say, using C with MPI, or Java concurrency tools), it is not so easy to speed up R-based particle filters using R’s high-level parallelisation constructs, as we shall see.

### Particle filters

In the previous post we looked at the function pfMLLik within the CRAN package smfsb. As a reminder, the source code is

pfMLLik <- function (n, simx0, t0, stepFun, dataLik, data)
{
times = c(t0, as.numeric(rownames(data)))
deltas = diff(times)
return(function(...) {
xmat = simx0(n, t0, ...)
ll = 0
for (i in 1:length(deltas)) {
xmat = t(apply(xmat, 1, stepFun, t0 = times[i], deltat = deltas[i], ...))
w = apply(xmat, 1, dataLik, t = times[i + 1], y = data[i,], log = FALSE, ...)
if (max(w) < 1e-20) {
warning("Particle filter bombed")
return(-1e+99)
}
ll = ll + log(mean(w))
rows = sample(1:n, n, replace = TRUE, prob = w)
xmat = xmat[rows, ]
}
ll
})
}


The function itself doesn’t actually run a particle filter, but instead returns a function closure which does (see the previous post for a discussion of lexical scope and function closures in R). There are obviously several different steps within the particle filter, and several of these are amenable to parallelisation. However, for complex models, forward simulation from the model will be the rate-limiting step, where the vast majority of CPU cycles will be spent. Line 9 in the above code is where forward simulation takes place, and in particular, the key function call is the apply call:

apply(xmat, 1, stepFun, t0 = times[i], deltat = deltas[i], ...)


This call applies the forward simulation algorithm stepFun to each row of the matrix xmat independently. Since there are no dependencies between the function calls, this is in principle very straightforward to parallelise on multicore hardware.

### Multicore support in R

I’m writing this post on a laptop with an Intel i7 quad core chip, running the 64 bit version of Ubuntu 11.10. R has support for multicore processing on this platform – it is just a simple matter of installing the relevant packages. However, things are changing rapidly regarding multicore support in R right now, so YMMV. Ubuntu 11.10 has R 2.13 by default, but the multicore support is slightly different in the recently released R 2.14. I’m still using R 2.13. I may update this post (or comment) when I move to R 2.14. The main difference is that the package multicore has been replaced by the package parallel. There are a few other minor changes, but it should be easy to adapt what is presented here to 2.14.

There is a new O’Reilly book called Parallel R. I’ve got a copy of it. It does cover the new parallel package in R 2.14, as well as other parallel R topics, but the book is a bit light weight, to say the least, and I reviewed it on this blog. Please read my review for further details before you buy it.

If you haven’t used multicore in R previously, then

install.packages(c("multicore","doMC"))


should get you started (again, I’m assuming that your R version is strictly < 2.14). You can test it has worked with:

library(multicore)
multicore:::detectCores()


When I do this, I get the answer 8 (I have 4 cores, each of which is hyper-threaded). To begin with, I want to tell R to use just 4 process threads, and I can do this with

library(doMC)
registerDoMC(4)


Replacing the second line with registerDoMC() will set things up to use all detected cores (in my case, 8). There are a couple of different strategies we could use to parallelise this. One strategy for parallelising the apply call discussed above is to be to replace it with a foreach / %dopar% loop. This is best illustrated by example. Start with line 9 from the function pfMLLik:

xmat = t(apply(xmat, 1, stepFun, t0 = times[i], deltat = deltas[i], ...))


We can produce a parallelised version by replacing this line with the following block of code:

res=foreach(j=1:dim(xmat)[1]) %dopar% {
stepFun(xmat[j,], t0 = times[i], deltat = deltas[i], ...)
}
xmat=t(sapply(res,cbind))


Each iteration of the foreach loop is executed independently (possibly using multiple cores), and the result of each iteration is returned as a list, and captured in res. This list of return vectors is then coerced back into a matrix with the final line.

In fact, we can improve on this by using the .combine argument to foreach, which describes how to combine the results from each iteration. Here we can just use rbind to combine the results into a matrix, using:

xmat=foreach(j=1:dim(xmat)[1], .combine="rbind") %dopar% {
stepFun(xmat[j,], t0 = times[i], deltat = deltas[i], ...)
}


This code is much neater, and in principle ought to be a bit faster, though I haven’t noticed much difference in practice.

In fact, it is not necessary to use the foreach construct at all. The multicore package provides the mclapply function, which is a multicore version of lapply. To use mclapply (or, indeed, lapply) here, we first need to split our matrix into a list of rows, which we can do using the split command. So in fact, our apply call can be replaced with the single line:

xmat=t(sapply(mclapply(split(xmat,row(xmat)), stepFun, t0=times[i], deltat=deltas[i], ...),cbind))


This is actually a much cleaner solution than the method using foreach, but it does require grokking a bit more R. Note that mclapply uses a different method to specify the number of threads to use than foreach/doMC. Here you can either use the named argument to mclapply, mc.cores, or use options(), eg. options(cores=4).

As well as being much cleaner, I find that the mclapply approach is much faster than the foreach/dopar approach for this problem. I’m guessing that this is because foreach doesn’t pre-schedule tasks by default, whereas mclapply does, but I haven’t had a chance to dig into this in detail yet.

### A parallelised particle filter

We can now splice the parallelised forward simulation step (using mclapply) back into our particle filter function to get:

require(multicore)
pfMLLik <- function (n, simx0, t0, stepFun, dataLik, data)
{
times = c(t0, as.numeric(rownames(data)))
deltas = diff(times)
return(function(...) {
xmat = simx0(n, t0, ...)
ll = 0
for (i in 1:length(deltas)) {
xmat=t(sapply(mclapply(split(xmat,row(xmat)), stepFun, t0=times[i], deltat=deltas[i], ...),cbind))
w = apply(xmat, 1, dataLik, t = times[i + 1], y = data[i,], log = FALSE, ...)
if (max(w) < 1e-20) {
warning("Particle filter bombed")
return(-1e+99)
}
ll = ll + log(mean(w))
rows = sample(1:n, n, replace = TRUE, prob = w)
xmat = xmat[rows, ]
}
ll
})
}


This can be used in place of the version supplied with the smfsb package for slow simulation algorithms running on modern multicore machines.

There is an issue regarding Monte Carlo simulations such as this and the multicore package (whether you use mclapply or foreach/dopar) in that it adopts a “different seeds” approach to parallel random number generation, rather than a true parallel random number generator. This probably isn’t worth worrying too much about now, since it is fixed in the new parallel package in R 2.14, but is something to be aware of. I discuss parallel random number generation issues in Wilkinson (2005).

### Multiple particle filters and pMCMC

Let’s look again at the main loop of the pMCMC algorithm discussed in the previous post:

for (i in 1:iters) {
message(paste(i,""),appendLF=FALSE)
for (j in 1:thin) {
thprop=th*exp(rnorm(p,0,tune))
llprop=mLLik(thprop)
if (log(runif(1)) < llprop - ll) {
th=thprop
ll=llprop
}
}
thmat[i,]=th
}


It is clear that the main computational bottleneck of this code is the call to mLLik on line 5, as this is the call which runs the particle filter. The purpose of making the call is to obtain an unbiased estimate of marginal likelihood. However, there are plenty of other ways that we can obtain such estimates than by running a single particle filter. In particular, we could run multiple particle filters and average the results. So, let’s look at how to do this in the multicore setting. Let’s start by thinking about running 4 particle filters. We could just replace the line

llprop=mLLik(thprop)


with the code

llprop=0.25*foreach(i=1:4, .combine="+") %dopar% {
mLLik(thprop)
}


Now, there are at least 2 issues with this. The first is that we are now just running 4 particle filters rather than 1, and so even with perfect parallelisation, it will run no quicker than the code we started with. However, the idea is that by running 4 particle filters we ought to be able to get away with each particle filter using fewer particles, though it isn’t trivial to figure out exactly how many. For example, averaging the results from 4 particle filters, each of which uses 25 particles is not as good as running a single particle filter with 100 particles. In practice, some trial and error is likely to be required. The second problem is that we have computed the mean of the log of the likelihoods, and not the likelihoods themselves. This will almost certainly work fine in practice, as the resulting estimate will in most cases be very close to unbiased, but it will not be exactly unbiased, as so will not lead to an “exact” approximate algorithm. In principle, this can be fixed by instead using

res=foreach(i=1:4) %dopar% {
mLLik(thprop)
}
llprop=log(mean(sapply(res,exp)))


but in practice this is likely to be subject to numerical underflow problems, as it involves manipulating raw likelihood values, which is generally a bad idea. It is possible to compute the log of the mean of the likelihoods in a more numerically stable way, but that is left as an exercise for the reader, as this post is way too long already… However, one additional tip worth mentioning is that the foreach package includes a convenience function called times for situations like the above, where the argument is not varying over calls. So the above code can be replaced with

res=times(4) %dopar% mLLik(thprop)
llprop=log(mean(sapply(res,exp)))


which is a bit cleaner and more readable.

Using this approach to parallelisation, there is now a much better chance of getting some speedup on multicore architectures, as the granularity of the tasks being parallelised is now much larger. Consider the example from the previous post, where at each iteration we ran a particle filter with 100 particles. If we now re-run that example, but instead use 4 particle filters each using 25 particles, we do get a slight speedup. However, on my laptop, the speedup is only around a factor of 1.6 using 4 cores, and as already discussed, 4 filters each with 25 particles isn’t actually quite as good as a single filter with 100 particles anyway. So, the benefits are rather modest here, but will be much better with less trivial examples (slower simulators). For completeness, a complete runnable demo script is included after the references. Also, it is probably worth emphasising that if your pMCMC algorithm has a short burn-in period, you may well get much better overall speed-ups by just running parallel MCMC chains. Depressing, perhaps, but true.

### References

• McCallum, E., Weston, S. (2011) Parallel R, O’Reilly.
• Wilkinson, D. J. (2005) Parallel Bayesian Computation, Chapter 16 in E. J. Kontoghiorghes (ed.) Handbook of Parallel Computing and Statistics, Marcel Dekker/CRC Press, 481-512.
• Wilkinson, D. J. (2011) Stochastic Modelling for Systems Biology, second edition, Boca Raton, Florida: Chapman & Hall/CRC Press.
• ### Demo script

require(smfsb)
data(LVdata)

require(multicore)
require(doMC)
registerDoMC(4)

# set up data likelihood
noiseSD=10
dataLik <- function(x,t,y,log=TRUE,...)
{
ll=sum(dnorm(y,x,noiseSD,log=TRUE))
if (log)
return(ll)
else
return(exp(ll))
}
# now define a sampler for the prior on the initial state
simx0 <- function(N,t0,...)
{
mat=cbind(rpois(N,50),rpois(N,100))
colnames(mat)=c("x1","x2")
mat
}
# convert the time series to a timed data matrix
LVdata=as.timedData(LVnoise10)
# create marginal log-likelihood functions, based on a particle filter

# use 25 particles instead of 100
mLLik=pfMLLik(25,simx0,0,stepLVc,dataLik,LVdata)

iters=1000
tune=0.01
thin=10
th=c(th1 = 1, th2 = 0.005, th3 = 0.6)
p=length(th)
ll=-1e99
thmat=matrix(0,nrow=iters,ncol=p)
colnames(thmat)=names(th)
# Main pMCMC loop
for (i in 1:iters) {
message(paste(i,""),appendLF=FALSE)
for (j in 1:thin) {
thprop=th*exp(rnorm(p,0,tune))
res=times(4) %dopar% mLLik(thprop)
llprop=log(mean(sapply(res,exp)))
if (log(runif(1)) < llprop - ll) {
th=thprop
ll=llprop
}
}
thmat[i,]=th
}
message("Done!")
# Compute and plot some basic summaries
mcmcSummary(thmat)