# Using the Rcpp Timer

January 6, 2013
By

(This article was first published on Rcpp Gallery, and kindly contributed to R-bloggers)

Sine the 0.10.2 release, Rcpp contains an internal class Timer which can be used for fine-grained benchmarking. Romain motivated Timer in a post to the mailing * list where Timer is used to measure the different components of the costs of random number generation.

A slightly modified version of that example follows below.

#include <Rcpp.h>
#include <Rcpp/Benchmark/Timer.h>

using namespace Rcpp;

// [[Rcpp::export]]
NumericVector useTimer() {
int n = 1000000;

// start the timer
Timer timer;
for(int i=0; i<n; i++) {
GetRNGstate();
PutRNGstate();
}
timer.step("get/put") ;

for(int i=0; i<n; i++) {
GetRNGstate();
rnorm(10, 0.0, 1.0);
PutRNGstate();
}
timer.step("g/p+rnorm()");

for(int i=0; i<n; i++) {
// empty loop
}
timer.step( "empty loop" ) ;

NumericVector res(timer);
for (int i=0; i<res.size(); i++) {
res[i] = res[i] / n;
}
return res;
}


We get the following result, each expressing the cost per iteration in nanoseconds:

useTimer()

    get/put g/p+rnorm()  empty loop
1.634e+03   2.573e+03   2.620e-04


The interesting revelation is that repeatedly calling GetRNGstate() and PutRNGstate() can amount to about 60% of the cost of RNG draws. Luckily, we usually only have to call these helper functions once per subroutine called from R (rather than repeatedly as shown here) so this is not really a permanent cost to bear when running simulations with R.

It also show the usefulness of a fine-grained timer at the code level.