run this to install rbenv and ruby 1.9.3:

curl -s | sh

or by hand, with rbenv and ruby-build:

curl | sh && rbenv global 1.9.3-p125-perf

or with rvm:

rvm get head && rvm reinstall 1.9.3-perf --patch falcon,debug --force-autoconf -j 3

finally (from the docs): Putting the following in your shell config (eg. ~/.bash_profile) will make Rails even faster, but will increase its memory footprint:

echo "# export RUBY_HEAP_MIN_SLOTS=1000000 export RUBY_HEAP_SLOTS_INCREMENT=1000000 export RUBY_HEAP_SLOTS_GROWTH_FACTOR=1 export RUBY_GC_MALLOC_LIMIT=1000000000 export RUBY_HEAP_FREE_MIN=500000" >> ~/.bash_profile

mad props to fortnight labs for pointing this out

full documentation here: