further illustrate the common strategy of frequent restarts, we can use littler which allows us to run simple expressions directly from the command-line. Here is an example I sometimes use to time different BLAS for a simple crossprod.
r -e'N<-3*10^3; M<-matrix(rnorm(N*N),ncol=N); print(system.time(crossprod(M)))'Likewise,
r -lMatrix -e'example(spMatrix)'loads the Matrix package (via the --packages | -l switch) and runs the examples of the spMatrix function. As r always starts 'fresh', this method is also a good test during package development.
Last but not least r also work great for automated batch mode in scripts using the '#!/usr/bin/r' shebang-header. Rscript is an alternative where littler is unavailable (e.g. on Windows).