Using RcppNT2 to Compute the Variance

Introduction

The Numerical Template Toolbox (NT²) collection of header-only C++ libraries that make it possible to explicitly request the use of SIMD instructions when possible, while falling back to regular scalar operations when not. NT² itself is powered by Boost, alongside two proposed Boost libraries – Boost.Dispatch, which provides a mechanism for efficient tag-based dispatch for functions, and Boost.SIMD, which provides a framework for the implementation of algorithms that take advantage of SIMD instructions. RcppNT2 wraps and exposes these libraries for use with R.

If you haven’t already, read the RcppNT2 introduction article to get acquainted with the RcppNT2 package.

Computing the Variance

As you may or may not know, there are a number of algorithms for computing the variance, each making different tradeoffs in algorithmic complexity versus numerical stability. We’ll focus on implementing a two-pass algorithm, whereby we compute the mean in a first pass, and later the sum of squares in a second pass.

First, let’s look at the R code one might write to compute the variance.

set.seed(123)
x <- rnorm(16)
myVar <- function(x) {
  sum((x - mean(x))^2) / (length(x) - 1)
}
stopifnot(all.equal(var(x), myVar(x)))
c(var(x), myVar(x))

[1] 0.833939 0.833939

We can decompose the operation into a few distinct steps:

1. Compute the mean for our vector ‘x’,
2. Compute the squared deviations from the mean,
3. Sum the deviations about the mean,
4. Divide the summation by the length minus one.

Naively, we could imagine writing a ‘simdTransform()’ for step 2, and an ‘simdReduce()’ for step 3. However, this is a bit inefficient as the transform would require allocating a whole new vector, with the same length as our initial vector. When neither ‘simdTransform()’ nor ‘simdReduce()’ seem to be a good fit, we can fall back to ‘simdFor()’. We can pass a stateful functor to handle accumulation of the transformed results.

Let’s write a class that encapsulates this ‘sum of squares’ operation.

// [[Rcpp::depends(RcppNT2)]]
#include <RcppNT2.h>
using namespace RcppNT2;

#include <Rcpp.h>
using namespace Rcpp;

class SumOfSquaresAccumulator
{
public:
   
   // Since the 'transform()' operation requires knowledge
   // of the mean, we ensure our class must be constructed
   // with a mean value. We will also hold the final result
   // within the 'result_' variable, and a SIMD 'pack_' to
   // hold the results for SIMD computations.
   explicit SumOfSquaresAccumulator(double mean)
      : mean_(mean), result_(0.0), pack_(0.0)
   {}
   
   // We need to provide two call operators: one to handle
   // SIMD packs, and one to handle scalar values. We do this
   // as we want to accumulate SIMD results in a SIMD data
   // structure, and scalar results in a scalar data object.
   //
   // We _could_ just accumulate all our results in a single
   // 'double', but this is actually less efficient -- it
   // pays to use packed structures when possible.
   void operator()(double data)
   {
      result_ += nt2::sqr(data - mean_);
   }
   
   void operator()(const boost::simd::pack<double>& data)
   {
      pack_ += nt2::sqr(data - mean_);
   }
   
   // We can use 'value()' to extract the result of the 
   // computation, after providing this object to
   // 'simdFor()'.
   double value() const
   {
      return result_ + boost::simd::sum(pack_);
   }
   
private:
   double mean_;
   double result_;
   boost::simd::pack<double> pack_;
};

Now that we have our accumulator class defined, we can use it to compute the variance. We’ll call our function ‘simdVar()’, and export it using Rcpp attributes in the usual way.

// [[Rcpp::export]]
double simdVar(NumericVector data)
{
   using namespace RcppNT2;
   
   // First, compute the mean as we'll need that for
   // computation of the sum of squares.
   double total = simdReduce(data.begin(), data.end(), 0.0, functor::plus());
   
   std::size_t n = data.size();
   double mean = total / n;
   
   // Use our accumulator to compute the sum of squares.
   SumOfSquaresAccumulator accumulator(mean);
   simdFor(data.begin(), data.end(), accumulator);
   double ssq = accumulator.value();
   
   // Divide by 'n - 1', and we're done!
   return ssq / (n - 1);
}

Let’s confirm that this works as expected…

x <- as.numeric(1:10)
c(var(x), simdVar(x))

[1] 9.166667 9.166667

stopifnot(all.equal(var(x), simdVar(x)))

And let’s benchmark, to see how performance compares.

library(microbenchmark)

# A helper function for printing microbenchmark output
printBm <- function(bm) {
  summary <- summary(bm)
  print(summary[, 1:7], row.names = FALSE)
}

small <- as.numeric(1:16)
stopifnot(all.equal(var(small), simdVar(small)))
bm <- microbenchmark(var(small), simdVar(small))
printBm(bm)

           expr    min      lq     mean median      uq    max
     var(small) 11.784 14.3395 16.37862 15.096 15.7225 40.346
 simdVar(small)  1.506  1.7045  2.06541  1.947  2.1055 10.935

large <- rnorm(1E6)
stopifnot(all.equal(var(large), simdVar(large)))
bm <- microbenchmark(var(large), simdVar(large))
printBm(bm)

           expr      min       lq      mean    median       uq      max
     var(large) 3046.597 3194.231 3278.7417 3301.6205 3323.581 3809.090
 simdVar(large)  579.784  594.887  607.0411  607.9845  614.386  712.038

As we can see, taking advantage of SIMD instructions has improved the runtime quite drastically.

However, we should note that this is not an entirely fair comparison with Rs implementation of the variance. In particular, we do not have a nice mechanism for handling missing values; if your data does have any NA or NaN values, they will simply be propagated (and not necessarily in the same way that R propagates missingness). If you’re interested, a separate example is provided as part of the RcppNT2 package, and you can browse the standalone source file here.

This article provides just a taste of how RcppNT2 can be used. If you’re interested in learning more, please check out the RcppNT2 website.