## Timing random number generators

Dirk Eddelbuettel — written Dec 26, 2012 — source

Simulation studies can run for a long time, and frequently lead to benchmarking and timing studies comparing implementations and algorithms.

One sometimes overlooked aspect when comparing across languages is the difference in timing due to maily to differences in the random number generators. The implementation in R is of very high quality, but does not aim to be the fastest possible generator. For example, when we compared different approaches for a simple Gibbs sampler, it was seen that RBG accounted for a large proportion of the difference between and Rcpp implementation in C++, using R’s RNG, and a GSL-based C implementation.

Here we are comparing three approaches: Armadillo, GSL and R.

``````#include <RcppArmadillo.h>

using namespace Rcpp;

// [[Rcpp::export]]
arma::vec armaNormal(const int N) {
arma::vec x = arma::randn(N,1);
return x;
}``````

We can also use the GSL.

``````#include <RcppGSL.h>

#include <gsl/gsl_rng.h>
#include <gsl/gsl_randist.h>

using namespace Rcpp;

// [[Rcpp::depends(RcppGSL)]]
// [[Rcpp::export]]
NumericVector gslNormal(const int N) {
NumericVector x(N);
gsl_rng *r = gsl_rng_alloc(gsl_rng_mt19937);
for (int i=0; i<N; i++) {
x[i] = gsl_ran_gaussian(r,1.0);
}
gsl_rng_free(r);
return x;
}``````

Last, we look at the generator in R.

``````#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
NumericVector rcppNormal(const int N) {
return rnorm(N, 0, 1);
}``````

And we can compare the approaches.

``````library(rbenchmark)

res <- benchmark(rcppNormal(1e6),
gslNormal(1e6),
armaNormal(1e6),
order="relative")
res[,1:4]``````
```               test replications elapsed relative
3 armaNormal(1e+06)          100   8.355    1.000
1 rcppNormal(1e+06)          100  12.117    1.450
2  gslNormal(1e+06)          100  14.906    1.784
```

Here we find the speed of the Rcpp generator to be slightly ahead of the GSL generator, but both are dominated by the one in Armadillo.

Of course, speed is not the only consideration. Rcpp still allows us to generate the same random numbers as R itself which is valuable.

tags: rng  sugar  gsl