I assume you only measured JRuby at low n, where the startup and warmup time would overshadow overall performance. JRuby is usually significantly faster than C

I assume you only measured JRuby at low n, where the startup and warmup time would overshadow overall performance. JRuby is usually significantly faster than C Ruby. In this case, it's nearly twice as fast: [] ~/projects/jruby $ time jruby primes.rb Number of primes in the interval [0,67108864]: 3957809 real 6m26.536s user 6m43.472s sys 0m4.967s [] ~/projects/jruby $ time rvm ruby-2.2 do ruby primes.rb Number of primes in the interval [0,67108864]: 3957809 real 11m17.610s user 11m16.564s sys 0m0.738s