Producer-Consumer problem

Hi All,

I am really struggling with a really intermittent bug. I have a thread
pool set up in a producer-consumer fashion. General semantics are:

  1. initialize thread_pool, with each thread waiting on a synchronized
    Queue.pop call to get a Proc object to run.
  2. call run_thread(&block), adding each job (block/Proc object) into a
    Queue if all threads are busy
  3. call ThreadPool#join which enqueues a ‘nil’ object for each thread,
    meaning that Queue.pop will return something that will evaluate to
    false, signaling the threads that there is no more work.
  4. If any Exceptions accumulated when the blocks were running, process

Now, that much works perfectly well. No problems. It’s only when I try
to make things a little more clever that things start going wrong. I
should point out that I did have this working for Ruby v1.8.7 it was
only when I tried to run it on v1.9.1 that things stopped working. To
me, this indicates that I had been behaving badly and the native threads
are less forgiving than the green threads. The clever part that I
mentioned was the fact that I added the ability to spawn a new process
and run the task inside that and have one of the threads in the pool
simply take care of the admin, like collecting the status when it’s all
done and uses a pipe to Marshal any exceptions to accumulate them for
the join. And for the most part this works. But sometimes the whole
thing hangs, waiting for the last thread to complete (it is always only
ever one thread that doesn’t complete when it does fail, but I haven’t
found any other consistencies in the failure conditions).

Here is the code for the run_process portion. Some of you might
recognise parts of it. I have been trying to come up with a simple case
to narrow the problem down, but that doesn’t seem to be an option.
Also, if I remove the Exception handling code things still fail like a
champion, so I presume it is something to do with the fork()/Thread
combination stopping a mutex somewhere from working correctly, but I
have no idea how.

Positive values of priority_increment cause the scheduler to

favour this subprocess less.

The default is priority is 0 and goes to a maximum of 19.

you need to have root priviledges to use negative increments.

[+priority_increment+] The amount by which to lower the process’s


[+block+] The block to run inside the new process

def run_process( priority_increment = 0, &block)
run do
read_end,write_end = IO.pipe
pid = fork() do
current_priority =
Process.getpriority(Process::PRIO_PROCESS, 0)
Process.setpriority(Process::PRIO_PROCESS, 0,
current_priority + priority_increment)
rescue SystemExit => ignore
rescue Exception => error
Marshal.dump(error, write_end)
exception_string =
pid,status = Process.waitpid2(pid)
unless exception_string.empty?
@failed << Marshal.restore(exception_string)

(And no, a SystemExit isn’t being generated, I have checked that!)

Please help me!!! I am soooo lost!


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