Rakefile

class ForkPool
  attr_accessor :maximum_active_forks

  def initialize
    @maximum_active_forks = 2
  end

  def add &block
    @blocks ||= []
    @blocks << block
  end

  def start
    loop do
      break unless @blocks.any?
      blocks_to_execute = []
      1.upto(maximum_active_forks) { blocks_to_execute << @blocks.shift }
      blocks_to_execute.each { |block| Thread.new { Process.wait(fork { block.call }) } }
    end
  end
end