TL;DR: I’m adding DTrace probes to Ruby, and I used those to make a 6% performance improvement to rake environment. ruby*:::function-entry { printf("-> %s::%s (%s:%d)\n", copyinstr(arg0), copyinstr(arg1), copyinstr(arg2), arg3); } $ sudo dtrace -q -s calls.d -c'rake environment' -> Module::local_constant_names (/Users/aaron/git/rails/activesupport/lib/active_support/core_ext/module/introspection.r