From: kjana@... (YANAGAWA Kazuhisa)
Date: 2001-08-08T19:54:17+09:00
Subject: [ruby-talk:19370] Re: Trying to get to grips with Ruby threads

In message <997216964.15259.0.nnrp-10.9e980a61@news.demon.co.uk>
peterhi@shake.demon.co.uk writes:

> > Sleeping does not release the mutex... so thread #1 is always going to
> > be waiting for thread #0 to stop sleeping, as thread #0 owns the mutex.
> 
> I thought that as well. However the cv.signal is before the sleep and so the
> mutex should be available. Or so I think.

ConditionVariable#signal only signals some condition is met --- Mutex
is not released.

On ConditionVariable#wait, the caller thread releases Mutex and passes
execution to another thread which may awake the waiting thread by
calling ConditionVariable#signal.


Indeed, truely critical region in your code is only getting an
existent job from the bag of jobs.  In this case you don't have to use
ConditionVariables, like following:


    require "thread"
    
    mutex = Mutex.new
    work = (1..100).collect { |i| "job #{i}" }
    
    threads = []
    2.times do
      |i|
      threads << Thread.new(i) do
        |id|
        job = nil
        while true
          mutex.synchronize do
            if not work.empty?
              job = work.pop
            else
              job = :nojobs
            end
          end
          break if job == :nojobs
    
          puts "Thread #{id} gets #{job}"
          if id == 0
            sleep(5)
          end
        end
      end
    end
    
    threads.each { |t| t.join }


But more likely to use Queue which implements a synchronized queue.


-- 
kjana@os.xaxon.ne.jp                                 August 8, 2001
Haste makes waste.