[Rcpp-devel] Rcpp speed

[Gabor Grothendieck]

Can anyone explain these results?   Rcpp takes 30% more time than R
with 10 elements, the same time as R with 1000 elements and 7x as long
with a million elements.  I would have expected the ratio to be
highest for `10 elements since that would be dominated by the call
sequence but its the other way around and it seems to get relatively
less efficient as the size of the vector grows.

> require(inline)
> testfun <- cxxfunction(signature(x="numeric", y="numeric"),
+ body = '
+ NumericVector xx(x);
+ NumericVector yy(y);
+ NumericVector zz = xx + yy;
+ return( zz );
+ ', plugin="Rcpp")
> x <- 1:3
> y <- 10 * x
> testfun(x, y)
[1] 11 22 33
>
> library(rbenchmark)
> x <- 1:10
> benchmark(replications = 10000, R = x + x, Rcpp = testfun(x, x))
  test replications elapsed relative user.self sys.self user.child sys.child
1    R        10000    0.17 1.000000      0.16        0         NA        NA
2 Rcpp        10000    0.22 1.294118      0.23        0         NA        NA
>
> x <- 1:1000
> benchmark(R = x + x, Rcpp = testfun(x, x))
  test replications elapsed relative user.self sys.self user.child sys.child
1    R          100    0.00        0      0.00        0         NA        NA
2 Rcpp          100    0.01        1      0.01        0         NA        NA
>
> x <- 1:1000000
> benchmark(R = x + x, Rcpp = testfun(x, x))
  test replications elapsed relative user.self sys.self user.child sys.child
1    R          100    2.34 1.000000      1.81     0.15         NA        NA
2 Rcpp          100   16.64 7.111111     12.63     1.66         NA        NA


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