http://looxu.blogspot.com/2010/12/ruby.html

semaphore-ac.rb

#!/usr/bin/ruby

require "thread"

class Semaphore
  def initialize (limit)
    @mutex = Mutex.new
    @count = limit
  end

  def enter
    sleep(0.1) until @mutex.synchronize { @count > 0 and @count -= 1 }
  end

  def leave
    @mutex.synchronize { @count += 1 }
  end
end

def hoge(data)
  # うまくいってるか、分かりやすくするためにランダム時間待つ
  w = 3 + (rand * 5).to_i
  puts("[#{data}] begin (wait #{w} sec)")
  sleep(w)
  puts("[#{data}] end")
end

# 最大3スレッドまで実行
sem = Semaphore.new(3)

(1..10).to_a.each do
  |alpha|
  sem.enter
  Thread.new(alpha) do
    |alpha|
    begin
      hoge(alpha)
    ensure
      sem.leave
    end
  end
end

sleep 1 while Thread.list.size > 1