1.rb

def f
  sleep 1
end

BEGIN {
  using Module.new {
    refine(Kernel) do
      def sleep *; end
    end
  }
}

100.times { f }

2.rb

def f
  sleep 1
end

100.times.map { Thread.new { f } }.each(&:join)

3.rb

def f
  sleep 1
end

100.times.map { fork { f } }.compact.each { Process.waitpid(_1) }