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    <p>Ok, so the main problem with the implementation is: how to make sure no signal is lost and avoid dead locks ?</p> <p>In my experience, this is REALLY hard to achieve with condition variables and mutex, but easy with semaphores. It so happens that ruby implement an object called Queue (or SizedQueue) that should solve the problem. Here is my suggested implementation:</p> <pre><code>require 'thread' begin require 'fasttread' rescue LoadError $stderr.puts "Using the ruby-core thread implementation" end class ThreadPool class Worker def initialize(thread_queue) @mutex = Mutex.new @cv = ConditionVariable.new @queue = thread_queue @running = true @thread = Thread.new do @mutex.synchronize do while @running @cv.wait(@mutex) block = get_block if block @mutex.unlock block.call @mutex.lock reset_block end @queue &lt;&lt; self end end end end def name @thread.inspect end def get_block @block end def set_block(block) @mutex.synchronize do raise RuntimeError, "Thread already busy." if @block @block = block # Signal the thread in this class, that there's a job to be done @cv.signal end end def reset_block @block = nil end def busy? @mutex.synchronize { !@block.nil? } end def stop @mutex.synchronize do @running = false @cv.signal end @thread.join end end attr_accessor :max_size def initialize(max_size = 10) @max_size = max_size @queue = Queue.new @workers = [] end def size @workers.size end def busy? @queue.size &lt; @workers.size end def shutdown @workers.each { |w| w.stop } @workers = [] end alias :join :shutdown def process(block=nil,&amp;blk) block = blk if block_given? worker = get_worker worker.set_block(block) end private def get_worker if !@queue.empty? or @workers.size == @max_size return @queue.pop else worker = Worker.new(@queue) @workers &lt;&lt; worker worker end end end </code></pre> <p>And here is a simple test code:</p> <pre><code>tp = ThreadPool.new 500 (1..1000).each { |i| tp.process { (2..10).inject(1) { |memo,val| sleep(0.1); memo*val }; print "Computation #{i} done. Nb of tasks: #{tp.size}\n" } } tp.shutdown </code></pre>
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    1. CO1. Shouldn't the access to @workers be synchronized? 2. Why there's still need to lock and unlock in the worker thread?
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    2. COThe access to worker is always done from the same thread ... so synchronization is not needed. As for the lock in the worker thread, you need them to wake the thread up safely.
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    3. COThere's still a problem with this - there's a chance for a deadlock - when the worker thread adds itself to the queue, the ThreadPool can then take it from the queue and assign a task. In that case a signal will be sent. However, if the worker thread is not waiting on a cv, the signal will be lost.
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